Accredited official statistics

Annual epidemiological commentary: Gram-negative, MRSA, MSSA bacteraemia and C. difficile infections, up to and including financial year 2022 to 2023

Updated 26 September 2024

Applies to England

Correction notice

This is a correction notice for this statistical release, first published on 28 September 2023 and revised on 16 February 2024.

Since the initial release of these statistics, it has been identified that the hospital-onset rates in the mortality section of each data collection were calculated using national population estimates instead of the appropriate bed-days denominator. Additionally, it was identified that some figures in the data tables had not been reported correctly in other sections of the report commentary.

All identified errors have been corrected in this revised report and accompanying tables, both superseding the initial release of these statistics in September 2023. Notably, some corrections have led to a change in the direction of some reported rates from comparison periods.

See corrections to these statistics for further detail.

UK Health Security Agency and this report

These official statistics were independently reviewed by the Office for Statistics Regulation in May 2022. They comply with the standards of trustworthiness, quality and value in the Code of Practice for Statistics and should be labelled ‘accredited official statistics’. Accredited official statistics are called National Statistics in the Statistics and Registration Service Act 2007. Further explanation of accredited official statistics can be found on the Office for Statistics Regulation website.

Beginning in April 2021, the UK Health Security Agency (UKHSA) was created and is responsible for protecting every member of every community from the effect of infectious diseases, chemical, biological, radiological and nuclear incidents and other health threats. We provide intellectual, scientific and operational leadership at national and local level, as well as on the global stage, to make the nation’s health secure.

The agency replaces Public Health England (PHE) and is an executive agency of the Department of Health and Social Care (DHSC). The transition to UKHSA included the integration of both staff and systems. Accordingly, the systems and processes responsible for the publication of the previous annual epidemiological commentaries were incorporated into UKHSA. The same methods of data capture, analysis and dissemination have been employed in the production of this report.

Corrections to these statistics

Corrected community-onset mortality rates

Since the initial release of these statistics on 28 September 2023, it has been identified that the hospital-onset mortality rates were calculated using national population estimates instead of the appropriate bed-days denominator.

The impact of this error resulted in the incorrect reporting of all hospital-onset mortality rates in the Mortality section of each data collection chapter, and in the following accompanying data table:

• S11: Thirty-day all cause fatality rate by onset of all organisms bacteraemia counts and rates by financial year 2023

All hospital-onset mortality rates have been re-calculated using the correct denominator and all affected tables and report sections have been revised accordingly. Note that these changes led to subtle differences in the values previously reported, some of which resulted in a change in the direction of trend in hospital-onset mortality rates, from comparison periods.

Corrected errors in reporting of figures in data tables

It was identified that some other figures in the data tables had not been reported correctly in the initial release of the report commentary. The full commentary has been reviewed and any reporting errors have been corrected. Corrections have affected the following data collection chapters.

E. coli chapter

The section on prior trust exposure now includes a few updated rates that accurately correspond to the rates referenced in the text. In one instance, this update has led to a change in the direction of the percentage change.

In the age-sex section, adjustments were made to 2 percentages for male and female cases aged 1 to 14 years in the financial year 2022 to 2023, resulting in a slight decrease compared to the previous figures.

Klebsiella spp. chapter

A typo error was corrected from ‘increase’ to ‘decrease’ when referring to the community-onset healthcare associated (COHA) trend during the pandemic.

The percentage change of the rate among females aged 85 years and above has been updated to reflect an increase rather than a decrease between the financial years 2017 to 2018 and 2022 to 2023.

P. aeruginosa chapter

Unknown age-sex cases for the 2017 to 2018 financial year have been removed to align with the data presented in the table.

In the comparison of the rate among females aged 85 years and over, the reported years have been corrected to compare with the 2022 to 2023 financial year instead of the 2021 to 2022 financial year.

MRSA chapter

A small number of population estimates and counts differed slightly in Table 17 and Table 18 from the figures presented in the supplementary data tables and these were updated to correctly reflect the data tables.

MSSA chapter

The reported percentage increase was corrected to reflect the hospital-onset healthcare associated (HOHA) count increase in the 2 most recent financial years, along with the COHA count and rate decrease.

Following review of the revised report, some wider changes were also made to the commentary to improve clarity and readability without altering the original messages.

Mitigating errors

To prevent errors in future reports, UKHSA will implement mitigation measures including:

• introducing an additional cross-check of the numbers, conducted by an independent reviewer, to ensure accuracy
• increasing the use of automation methods during report generation to minimise human error
• reinforcing version control mechanisms to track all changes made to data tables and documents
• reviewing the standardised procedures for reviewing and verifying figures in the report and revising any steps that have potential to introduce errors

Executive summary

This report outlines the latest trends and developments in Gram-negative bacteraemia and C. difficile infections. It aims to provide valuable insights into the incidence, prevalence and future work related to these infections, contributing to the ongoing efforts to enhance patient safety in healthcare settings.

Throughout this report, ‘financial year’ is abbreviated to ‘FY’ and refers to the period between 1 April to 31 March in the years stated.

In the financial year April (FY) 2022 to March 2023 (FY 2022 to 2023), the number of Escherichia coli (E. coli) and meticillin resistant Staphylococcus aureus (MRSA) bacteraemia cases remained lower than those seen prior to the COVID-19 pandemic. However, counts of Klebsiella spp., Pseudomonas aeruginosa and meticillin sensitive Staphylococcus aureus (MSSA) bacteraemia returned to levels observed in the financial year preceding the start of the pandemic. Clostridiodies difficile infections (CDI) case counts reached a 10-year high of 15,583, higher than any other year since the FY 2011 to 2012 (n=18,022).

Counts of hospital-onset in Klebsiella spp. and P. aeruginosa bacteraemia peaked in January 2021; marking the highest monthly case count since the mandatory surveillance of these pathogens started (April 2017), and was the first instance where the number of hospital-onset cases for P. aeruginosa surpassed those originating from the community. Numbers of hospital-onset bacteraemia cases in proportion to community onset continued to be high for Klebsiella spp., P. aeruginosa and MRSA in FY 2022 to 2023, albeit lower than the number of hospital-onset cases reported during FY 2020 to 2021.

During FY 2022 to 2023, there was a substantial increase in reported CDI cases and a slight increase in MRSA bacteraemia. This is in contrast with the consistent decline and low level fluctuations in such cases observed in these infections prior to the COVID-19 pandemic, which can be attributed to the success of various healthcare-associated interventions. This shift to an upward trend, especially for CDI, which was first observed in FY 2021 to 2022, suggests a need for additional efforts in order to return to and maintain previously low prevalence levels.

Following an initial sharp rise in MRSA hospital onset cases in 2020 to 2021, cases dropped back to pre-pandemic levels in 2021 to 2022, with the most recent financial year (FY 2022 to 2023) seeing an increase in cases, consistent with the pre-pandemic trend. During the same period, there was a large increase in the total number of CDI cases. This increase occurred in both hospital-onset and community-onset cases.

The epidemiological trends of the infections covered in this report, prior to the start of the COVID-19 pandemic, were shifting towards increasingly higher proportions of community-onset cases. This highlights the need to concentrate on interventions aimed at reducing community-onset cases. It is currently unclear if the increases in Klebsiella spp., MSSA and CDI hospital-onset infections observed in the last few years mark a long-term shift in the epidemiology; more data and further investigation is required.

Although a large proportion of cases start in the community rather than the hospital, there is still substantial number with recent healthcare interactions. For example, previous sentinel surveillance estimated approximately half of patients with a community onset E. coli bacteraemia had healthcare interactions in the preceding month. Therefore, to reduce infection rates further, it is imperative to uphold or intensify control measures within hospitals, while simultaneously, increasing focus on healthcare interventions within the community and strengthen the collaboration between hospital and community infection control teams.

New analyses, included for the first time in this report, show infection data broken down by deprivation and ethnicity. We also for the first time in this report include data on 30-day all-cause mortality (the annual mortality report will be published this year for the final time).

Incidence rates across all infections were higher in the most deprived areas across the whole time period. Also, differences were observed in ethnicity, with greater age-standardised incidence rates amongst the Asian and black ethnic groups for Gram-negative and MRSA bacteraemia, whilst MSSA bacteraemia and CDI were seen to be more frequent in the white ethnic group.

In FY 2022 to 2023, there were 14,621 deaths within 30 days of a specimen being taken (blood culture for bacteraemia, faecal sample for CDI). The highest number of deaths were associated with E. coli at 6,087, which accounts for 41.6% of deaths covered by this report. The overall mortality rate for all infections was 26.2 deaths per 100,000 population, with a case fatality rate (CFR) of 17.6%.

Knowledge mobilisation

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Highlights

During the financial year (FY) April 2022 to March 2023 (FY 2022 to 2023) NHS Acute Trusts reported the below highlights, with all rates reported as per 100,000 population.

The rate of total E. coli infections increased from 67.3 to 68.5, remaining below pre-pandemic levels. After E. coli, Klebsiella spp. and P. aeruginosa were the most common causative organisms of Gram-negative bacteraemia.

Since surveillance began, the total infection rate of Klebsiella spp. has increased year-on-year from 17.6 in FY 2017 to 2018 to 20.9 in FY 2022 to 2023. Rates of P. aeruginosa bacteraemia have remained stable throughout the 5 years of surveillance for this species, fluctuating between 7.5 and 7.8.

Urinary tract infections remained the most common primary focus among and accounting for 34.0% of Klebsiella cases, 28.3% of P. aeruginosa cases and 45.2% of E. coli bacteraemia cases.

The rate of total MSSA bacteraemia exceeded pre-pandemic levels, reaching 23.2. Prior to a decline at the start of the COVID-19 pandemic, rates of MSSA showed a moderate year on year increases.

Total rates of MRSA, in contrast to those for MSSA, bacteraemia decreased notably during the initial surveillance years (2007 to 2012), before easing into a gentler generally tend decreasing trend over time. However, an increase was observed in the most recent FY but the overall rate remained relatively low at 1.4.

Since March 2021, the total CDI rate has moved to an upward trajectory, rising to 27.6 during FY 2022 to 2023. This is the highest all case rate since FY 2012 to 2013. These recent CDI rate increases are driven by both hospital and community-onset cases, though the trend appears notably steeper in the hospital-onset category.

The CFR for Staphylococcus aureus bacteraemia decreased from last year (FY 2021 to 2022). MSSA bacteraemia decreased from 22.1% to 21.9% and MRSA bacteraemia 26.2% to 22.5%. A slight increase in CFR was observed for all other bacteraemia collections (FY 2022 to 2023).

Since FY 2021 to 2022, the mortality rate (expressed as deaths per 100,000 population per financial year) increased in FY 2022 to 2023 across all bacteraemia, except for MRSA, which maintained a steady rate.

The COVID-19 pandemic has affected the incidence of bacteraemia across all surveillance collections over the past 3 years. This is likely a result of the interventions and changes in healthcare practices during the COVID-19 pandemic. From an analysis of data collected from the voluntary laboratory surveillance scheme during FY 2020 to 2021, there was a reduction, of varying extents, in the number of cases of other bloodstream infections, besides those covered by mandatory surveillance. Thus, reductions unlikely to merely be ascertainment due to lack of resources for surveillance case capture.

For these reasons, caution is advised in interpreting trends in data from the last 3 years. It remains unclear if any comparative changes between FY 2020 to 2021 and FY 2022 to 2023 are a result of lasting effects from the COVID-19 pandemic, or whether these differences constitute the start of a genuine shift in the epidemiology of bacteraemia and CDI in England.

This report constitutes a descriptive analysis of the data from the mandatory surveillance of bacteraemia and CDI programmes in England. Detailed information on each pathogen can be found in the individual sections of the report.

Epidemiological analysis of Gram-negative organisms

Escherichia coli bacteraemia

Total reports

A total of 38,757 cases of E. coli bacteraemia were reported by NHS acute trusts in England during the FY 2022 to 2023, of which, 7,881 (20.3%) were hospital-onset cases. The total number of cases reported in FY 2022 to 2023 increased by 1.9% compared to FY 2021 to 2022 (n=38,037) and represents an overall increase of 20.0% when compared to the first full financial year of E. coli bacteraemia surveillance (FY 2012 to 2013, n=32,309). Within the same period, the rate of E. coli cases per 100,000 population has risen from 60.4 to 68.5 cases per 100,000 population. Figure 1 shows the trends in the rates of E. coli cases between FY 2012 to 2013 and FY 2022 to 2023.

Figure 1. Trends in the rate of E. coli bacteraemia in England, by financial year: April 2012 to March 2023

*Mid-year population estimates for January to December 2021 onwards were not available at time of publication and so population data for January to December 2020 was used as a proxy

Hospital-onset cases

The rate (cases per 100,000 bed-days) of E. coli hospital-onset cases was relatively stable at around 22.0 between FY 2012 to 2013 and FY 2018 to 2019 apart from a temporary decrease to 21.1 during FY 2014 to 2015 (Table 1). Rates continued to increase from FY 2018 to 2019 to FY 2020 to 2021 (22.1 to 23.6), this rate declined the following year to 21.5 but increased to 22.2 in FY 2022 to 2023.

Table 1. E. coli bacteraemia counts and rates by financial year, England, by financial year: April 2012 to March 2023

Financial year	Mid-year population estimate*	All reported cases	Rate (all reported cases per 100,000 population)	Total bed-days	Hospital-onset cases	Rate (hospital-onset per 100,000 bed-days)
2012 to 2013	53,475,357	32,309	60.4	34,633,855	7,552	21.8
2013 to 2014	53,976,973	34,286	63.5	34,514,871	7,558	21.9
2014 to 2015	54,432,437	35,822	65.8	34,972,728	7,381	21.1
2015 to 2016	55,018,883	38,313	69.6	34,752,604	7,744	22.3
2016 to 2017	55,240,934	40,682	73.6	35,148,014	7,888	22.4
2017 to 2018	55,707,642	41,140	73.8	34,903,075	7,683	22.0
2018 to 2019	56,053,563	43,284	77.2	34,538,184	7,630	22.1
2019 to 2020	56,468,265	43,400	76.9	34,637,156	7,839	22.6
2020 to 2021	56,550,138	36,804	65.1	27,628,155	6,525	23.6
2021 to 2022	56,550,138	38,037	67.3	32,905,086	7,081	21.5
2022 to 2023	56,550,138	38,757	68.5	35,478,624	7,881	22.2

*Mid-year population estimates for January to December 2021 onwards were not available at time of publication and so population data for January to December 2020 was used as a proxy

ICU-associated infections

Intensive Care Unit (ICU) surveillance as part of the Infection in Critical Care Quality Improvement Programme (ICCQIP) started in May 2016. Overall, between FY 2016 to 2017 and FY 2022 to 2023, 891 (1.7%, N=52,527) hospital-onset E. coli infections were considered ICU-associated (patient within ICU for at least 2 days prior to positive specimen being collected). Rates of E. coli ICU-associated bloodstream infections have fluctuated over the years, with spikes seen during FY 2017 to 2018 (0.47 cases per 1,000 ICU bed-days greater than 2 days) and FY 2020 to 2021 (0.51 cases per 1,000 ICU bed-days greater than 2 days)*. During FY 2022 to 2023 there was a decrease reported at 0.3 cases per 1,000 ICU bed-days greater than 2 days. (Figure 2). Of note, only a subset of ICUs across England routinely report data to the ICCQIP surveillance system and so ICU associated data must be interpreted with caution.

Figure 2. Rate of ICU-associated E. coli bacteraemia cases, England, by financial year: April 2016 to March 2023**

*Due to technical issues data from one registered unit was unable to be processed. Further information for the methodology is available in the ICU and mandatory surveillance linkage section of the appendix

**Includes 37 infections for E. coli bacteraemia reported to the mandatory surveillance programme and linked to the ICCQIP surveillance programme, but where E. coli was not listed as the causative organism in the ICCQIP data

Prior trust exposure

In April 2019, the mandatory surveillance programme began capturing mandatory information on whether a patient with E. coli bacteraemia had previously been admitted to the same reporting trust in the previous 28 days.

Cases are classified into specific ‘prior trust exposure’ groups, whose definitions can be found in the appendix.

In comparison to FY 2020 to 2021, FY 2022 to 2023 saw an increase in the total count of hospital-onset healthcare associated (HOHA) E. coli infections from 6,525 to 7,881, with a marginal decrease in the rate of HOHA infections from 23.6 to 22.2 cases per 100,000 bed-days. Contributing factors include the large drop in bed-days at the start of COVID-19, which has since slowly increased in subsequent years surpassing that prior to the pandemic. Thus, leading to a comparable rate in FY 2022 to 2023 (22.2) to pre-pandemic years (22.1 in 2018 to 2019) despite the incremental increase in HOHA observed during the pandemic.

Over the same period, the count of community-onset healthcare associated (COHA) E. coli bacteraemia increased by 3.7% (from 5,127 to 5,315) and the rate decreased by 19.5% (from 16.9 to 13.6 cases per 100,000 bed-days and day admissions). Additionally, both the count and rate of community-onset community associated (COCA) E. coli bacteraemia increased by 2.5% over these time periods, from 24,807 to 25,438 cases and from 43.9 to 45.0 cases per 100,000 population, respectively (Table 2). Caution is advised in the interpretation of the data as the number reported with no information fell from 5,152 during FY 2019 to 2020 to 2 during FY 2022 to 2023. This was due to the fields being mandated in late 2019.

Table 2. E. coli bacteraemia counts by prior trust exposure by financial year, England, by financial year: April 2019 to March 2023

Financial Year*	Total bed-days	Total bed-days inc day-admissions	Mid-year population estimate	HOHA cases	HOHA rate (per 100,000 bed-days)	COHA cases	COHA rate (per 100,000 bed-days and day-admissions)	COCA cases	COCA rate (per 100,000 population)	Unknown **	No information ***
2019 to 2020	34,637,156	38,537,997	56,468,265	7,839	22.6	5,552	14.4	24,355	43.1	502	5,152
2020 to 2021	27,628,155	30,292,124	56,550,138	6,525	23.6	5,127	16.9	24,807	43.9	343	2
2021 to 2022	32,905,086	36,333,468	56,550,138	7,081	21.5	5,422	14.9	25,365	44.9	169	0
2022 to 2023	35,478,624	39,082,838	56,550,138	7,881	22.2	5,315	13.6	25,438	45.0	121	2

*Financial year from April to March of the following year

**The record indicates that it is unknown whether the patient was admitted to the reporting organisation in the past 28 days.

***No information was entered in regard to the prior trust exposure

Rationale for denominator methodology can be found in the Prior trust denominator.

Age and sex distribution

During FY 2012 to 2013, either sex or age was unknown for 815 cases (2.5%), compared to 22 cases (<1%) in FY 2022 to 2023. Figure 3 compares the age and sex distribution of E. coli cases in FY 2012 to 2013 versus in FY 2022 to 2023. Broadly, the distribution of cases is similar across both periods. The greatest burden of infection remains in the adults aged 45 years and over; within this demographic, both male and female cases in the 75 to 84 years age group have the highest proportion of infections, comprising 15.5% and 13.5% of the total infections, respectively.

The percentage distribution of cases amongst males and females were similar within each age group, except in the 15 to 44 years age group, where cases were approximately 2.5 times more common in female cases compared to males in both presented financial years. For all age and sex analyses, cases for which the sex or age was missing or reported as “unknown” were excluded.

Figure 3. Age and sex distribution of E. coli bacteraemia by percentage of cases, England, April 2012 to March 2013 and April 2022 to March 2023

In general, the incidence rate of E. coli bacteraemia is greater in male cases compared to females, and particularly so amongst older age groups.

The rate of infection increased in most age groups, for both sexes between FY 2012 to 2013 and FY 2022 to 2023 (Table 3 and Table 4).

During FY 2022 to 2023, both males and females had the highest rates among individuals aged 85 years and over at 819.2 and 507.7 per 100,000 population, respectively. In this period, the rate ratio between men and woman consistently increased with age in all those age groups containing individuals aged 45 years and over. This was also consistent with FY 2012 to 2013.

Table 3. E. coli bacteraemia counts and rates by age group and sex, England: financial year April 2012 to March 2013

Age group (years)	Population: Males	Population: Females	Cases: Males	Cases: Females	Rate per 100,000 population: Males	Rate per 100,000 population: Females	Rate Ratio (Males: Females)
<1	353,705	336,393	306	195	86.5	58.0	1.5 (95% CI: 1.2-1.8)
1-14	4,503,696	4,294,578	81	136	1.8	3.2	0.6 (95% CI: 0.4-0.7)
15-44	10,724,195	10,658,628	712	2,087	6.6	19.6	0.3 (95% CI: 0.3-0.4)
45-64	6,669,559	6,835,423	2,944	2,878	44.1	42.1	1.0 (95% CI: 1.0-1.1)
65-74	2,347,921	2,530,754	3,520	2,889	149.9	114.2	1.3 (95% CI: 1.3-1.4)
75-84	1,320,659	1,677,572	4,577	4,311	346.6	257.0	1.3 (95% CI: 1.3-1.4)
≥85	408,948	813,327	2,937	3,921	718.2	482.1	1.5 (95% CI: 1.4-1.6)

Table 4. E. coli bacteraemia counts and rates by age group and sex, England: financial year April 2022 to March 2023*

Age group (years)	Population: Males	Population: Females	Cases: Males	Cases: Females	Rate per 100,000 population: Males	Rate per 100,000 population: Females	Rate Ratio (Males: Females)
<1	308,815	293,098	423	280	137.0	95.5	1.4 (95% CI: 1.2 – 1.7)
1-14	4,929,714	4,682,857	117	115	2.4	2.5	1.0 (95% CI: 0.7 – 1.3)
15-44	10,817,218	10,581,832	808	2,029	7.5	19.2	0.4 (95% CI: 0.4 – 0.4)
45-64	7,135,608	7,336,977	3,479	3,619	48.8	49.3	1.0 (95% CI: 0.9 – 1.0)
65-74	2,691,641	2,906,787	4,275	3,585	158.8	123.3	1.3 (95% CI: 1.2 – 1.3)
75-84	1,574,092	1,885,089	6,005	5,227	381.5	277.3	1.3 (95% CI: 1.3 – 1.4)
≥85	525,730	880,680	4,307	4,471	819.2	507.7	1.6 (95% CI: 1.5 – 1.7)

*Mid-year population estimates for January to December 2021 onwards were not available at time of publication and so population data for January to December 2020 was used as a proxy

Compared to previous years, FY 2022 to 2023 displayed a slight increase in rate of infection in all age groups, except among males aged 65 to 75 years, whose incidence rate continued to decline.

In the last 2-year period (from April 2021 to March 2023) the rate among males aged 85 years and over increased by 2.1% from 802.5 to 819.2 per 100,000 population, while the rate among those aged 75 to 84 years increased by 0.9%, from 378.2 to 381.5 per 100,000 population. As mentioned above, a decline of 1.6% was observed for the male 65 to 74 years age group with the rate dropping from 161.5 to 158.8.

Between the same 2 financial years, increases were also observed among females, though they were most pronounced in the 75 to 84 years age group, where an increase of 2.5% (270.4 to 277.3 per 100,000) was observed. The rate among females aged 85 years and over, however, only increased by 0.5% from 505.4 to 507.7 per 100,000 population.

All other age groups showed varying degrees of fluctuation but remained relatively low (< 200.0 per 100,000 population) during FY 2022 to 2023.

Figure 4. Trend in age and sex rates per 100,000 population* of E. coli bacteraemia cases, England, by financial year: April 2012 to March 2023

*Mid-year population estimates for January to December 2021 onwards were not available at time of publication and so population data for January to December 2020 was used as a proxy

The rates of E. coli bacteraemia broadly increased between the FY 2012 to 2013 and FY 2018 to 2019. This was then followed by a large decline between the April 2020 and March 2021 period due to the pandemic. In FY 2021 to 2022, there was an increase in rate in most age groups and in both males and females. The highest increase in rate in FY 2022 to 2023, when compared to FY 2021 to 2022, was noted in the 1 to 14 years age groups for both males (41.2%) and females (13.6%) (Figure 4).

Trends by deprivation

During FY 2022 to 2023, 73.9 cases occurred per 100,000 people living in the 20% most deprived areas of England, compared with 61.6 cases per 100,000 people living in the 20% least deprived areas. The observed incidence rate of E. coli bacteraemia increased with increasing deprivation.

Additionally, incidence rates were age-standardised to account for differences in age distribution across the different index of multiple deprivation (IMD) groups. The age-standardised incidence rates showed that if people living in these areas had the same age distribution, the difference in incidence of E. coli would be much greater than what is currently observed (97.5 and 53.5 cases per 100,000 population in the 20% most and least deprived areas, respectively).

This can be attributed to people living in more deprived areas being younger, on average, than those living in less deprived areas, and acquiring E. coli BSI at a disproportionately high rate.

Figure 5 and Supplementary Table 12 show the incidence of E. coli bacteraemia across 5 IMD levels between April 2018 and March 2023. Shaded areas indicate 95% confidence intervals.

Figure 5. Index of Multiple Deprivation distribution of E. coli bacteraemia, England, April 2018 to March 2023

Between FY 2018 to 2019 and FY 2019 to 2020, there was a decline in the 20% most deprived group, unlike in other groups. Otherwise, the differences have mostly been consistently observed between April 2018 and March 2023 where there was a substantial decline across all groups between FY 2019 to 2020 and FY 2020 to 2021, followed by a slow increase in the following years.

Trends by ethnicity

During FY 2022 to 2023, the observed incidence rate of E. coli bacteraemia was highest in the white ethnic group (71.6 cases per 100,000 White population), intermediate in the Asian and black ethnic groups (45.7 and 43.3 cases per 100,000 population, respectively), and lowest in the other and mixed ethnic groups (19.7 and 16.3 cases per 100,000 population, respectively).

Age standardisation was used for the same purposes as the deprivation analyses, as the white ethnic group has a higher average age compared with other ethnic groups. The age-standardised incidence rate was higher in the Asian and black ethnic groups (95.2 and 79.7 cases per 100,000 population, respectively) compared with the white ethnic group (66.2 per 100,000 population).

Figure 6 and Supplementary Table 13 show the incidence of E. coli bacteraemia across ethnic groups between April 2018 and March 2023. Shaded areas indicate 95% confidence intervals.

Figure 6: Ethnic group distribution of E. coli bacteraemia, England, April 2018 to March 2023

Since April 2021, the rate of E. coli bacteraemia has increased across the Asian and black ethnic groups, whilst the rate has remained stationary in the white ethnic group.

Seasonal trends in E. coli bacteraemia

Seasonality is assessed as the number of hospital-onset cases each quarter as a percentage of total hospital-onset cases for the financial year or as the number of community-onset cases each quarter as a percentage of total community-onset cases for the financial year. In general, community-onset E. coli cases peak in the July to September quarter of every year, a trend that has not changed since April 2012 (Table S7 of the supplementary dataset). Historically, there was much less seasonal variation in hospital-onset E. coli cases; however, during FY 2020 to 2021, hospital-onset E. coli cases peaked in October to December 2020 and January 2021 to March 2021 compared to previous trends where it peaked in the July to September quarter. This phenomenon was subsequently observed again for these same 2 quarters within FY 2022 to 2023. Whilst these increases occurred during the second wave of COVID-19, it is unclear if they are associated with the COVID-19 pandemic. The increase observed during these periods is currently under investigation.

Primary focus of E. coli bacteraemia

The provision of data on the most likely primary focus of E. coli infection information is voluntary. The percentage of cases where this information has been provided has declined over time from 85.5% (n=27,610) in FY 2012 to 2013 to 47.1% (n=18,239) in FY 2022 to 2023 (Table 5). This was a 2.1% decrease compared to the previous FY 2021 to 2022 (n=18,631).

Of cases with a reported primary focus of infection, urinary tract has consistently been the most frequent primary focus for E. coli bacteraemia cases. In FY 2012 to 2013 48.9% of cases had a most likely primary focus of urinary tract, and by FY 2022 to 2023 it was 45.2% (n= 8,238). Hepatobiliary as a primary focus has slightly increased between FY 2012 to 2013 and FY 2022 to 2023 from 13.6% to 15.5%. Conversely, the percentage of records for which the primary focus was reported as unknown has decreased from 20.2% in FY 2012 to 2013 to 18.1% in FY 2022 to 2023. All other primary focuses have remained relatively low between FY 2012 to 2013 and FY 2022 to 2023.

Table 5. E. coli bacteraemia counts and rates by primary focus of bacteraemia in England, by financial year: April 2011 to March 2023

Financial year	Total E. coli reported	Primary focus ascertained: n (%)	Gastro-intestinal: n (%**)	Hepatobiliary: n (%***)	Urinary tract: n (%***)	Respiratory tract: n (%***)	Other: n (%*)	Unknown: n (%***)
2012 to 2013	32,309	27,610 (85.5)	1,782 (6.5)	3,756 (13.6)	13,501 (48.9)	1,050 (3.8)	1,936 (7.0)	5,585 (20.2)
2013 to 2014	34,286	28,300 (82.5)	1,711 (6.0)	3,855 (13.6)	13,393 (47.3)	1,016 (3.6)	1,873 (6.6)	6,452 (22.8)
2014 to 2015	35,820	28,729 (80.2)	1,640 (5.7)	3,818 (13.3)	13,088 (45.6)	1,099 (3.8)	1,851 (6.4)	7,233 (25.2)
2015 to 2016	38,312	26,446 (69.0)	1,492 (5.6)	3,556 (13.4)	12,220 (46.2)	1,068 (4.0)	1,703 (6.4)	6,407 (24.2)
2016 to 2017	40,678	22,727 (55.9)	1,237 (5.4)	3,277 (14.4)	10,724 (47.2)	1,028 (4.5)	1,553 (6.8)	4,908 (21.6)
2017 to 2018	41,128	25,615 (62.3)	1,717 (6.7)	4,035 (15.8)	12,566 (49.1)	1,575 (6.1)	1,757 (6.9)	3,965 (15.5)
2018 to 2019	43,265	28,468 (65.8)	1,969 (6.9)	4,686 (16.5)	13,914 (48.9)	1,728 (6.1)	1,989 (7.0)	4,182 (14.7)
2019 to 2020	43,393	27,513 (63.4)	1,774 (6.4)	4,351 (15.8)	13,241 (48.1)	1,673 (6.1)	1,898 (6.9)	4,576 (16.6)
2020 to 2021	36,801	20,386 (55.4)	1,351 (6.6)	3,720 (18.2)	8,968 (44.0)	1,250 (6.1)	1,523 (7.5)	3,574 (17.5)
2021 to 2022	38,037	18,631 (49.0)	1,328 (7.1)	3,105 (16.7)	8,393 (45.0)	1,091 (5.9)	1,489 (8.0)	3,225 (17.3)
2022 to 2023	38,757	18,239 (47.1)	1,258 (6.9)	2,824 (15.5)	8,238 (45.2)	1.044 (5.7)	1,580 (8.7)	3,295 (18.1)

*Gastrointestinal (not hepatobiliary)

**‘Other’ includes the following options HCAI DCS: bone and joint, central nervous system, genital tract (including prostate), indwelling intravascular device, other, respiratory tract, skin or soft tissue, no clinical signs of bacteraemia

***percentage within those records ascertained

During FY 2022 to 2023, the primary focus of E. coli bacteraemia also varies according to time to onset (Figure 7). Urinary tract as the primary focus was the most commonly reported among those with less than 2 days, 2 to 6 days and ≥7 days between admission and positive specimen. Furthermore, urinary tract as a primary focus was more common in cases where the time to onset was less than 2 days (46.8%) compared to cases where it was between 2 and 6 days (33.0%) and ≥ 7 days (37.0%).

Figure 7. Distribution of primary focus E. coli bacteraemia, by time to onset, England: financial year April 2022 to March 2023

Geographic distribution of E. coli bacteraemia

The regional distribution of cases is presented across Integrated Care Boards (ICBs) for FY 2022 to 2023. Some geographical variation in rates of E. coli bacteraemia is noted (Figure 8). There is some evidence of similar and relatively high infection rates among neighbouring ICBs in the North East, parts of the North West and South West. Accordingly, the highest rates (cases per 100,000 population) were found among ICBs within these areas; Somerset ICB (89.6), North East and North Cumbria ICB (85.7) and Nottingham and Nottinghamshire ICB (85.3 cases per 100,000 population). The lowest incidence rates were observed in Gloucestershire ICB (32.2), Bristol, North Somerset and South Gloucestershire ICB (50.7) and Leicester, Leicestershire and Rutland ICB (50.7).

Figure 8. Geographic distribution of E. coli bacteraemia rates per 100,000 population, England: financial year April 2022 to March 2023*

*Mid-year population estimates for January to December 2021 onwards were not available at time of publication and so population data for January to December 2020 was used as a proxy

Mortality

During FY 2022 to 2023, 38,757 E. coli bacteraemia cases were reported in England. Information on mortality was available for 98.6% (38,197) of these cases (Table S2 of the supplementary dataset). There were 6,087 deaths within 30 days of an E. coli bacteraemia diagnosis, indicating a mortality rate of 10.9 deaths per 100,000 population and a CFR of 15.9%.

As reported in previous reports there was a declining trend in CFR starting from the beginning of surveillance (FY 2012 to 2013), until FY 2018 to 2019, after which the CFR then increased to 16.0% by FY 2020 to 2021. Since, the trend in CFR has fluctuated in the last 2 years, with a CFR of 15.9% being reported for FY 2022 to 2023. The mortality rate increased between FY 2021 to 2022 and FY 2022 to 2023 from 10.1 to 10.9 deaths per 100,000 population.

Figure 9. Case fatality rate and mortality rate of E. coli bacteraemia per 100,000 population, England: financial year April 2012 to March 2023*

Variation by onset of bacteraemia

In the last 2 years of surveillance, between 2021 to 2023, the community-onset (CO) mortality rate has increased from 7.1 to 7.7 deaths per 100,000 population. This increase in mortality rate was also reflected in hospital-onset (HO) cases where the rate increased from 4.7 to 4.9 cases per 100,000 bed-days, respectively. In comparison the CFR of HO cases decreased from 22.6% to 22.2% between FY 2021 to 2022 and FY 2022 to 2023. CO cases however saw an increase in CFR from 13.3% to 14.3% over the same period.

Variation by age and sex

During FY 2022 to 2023, among male cases, the highest mortality rates were in those aged 85 years and over (200.5 deaths  per 100,000 population) and those aged 75 to 84 years (70.4  per 100,000 population). This equated to CFRs of 24.7% and 18.6%, respectively, indicating a slight increase from the previous FY 2021 to 2022 which recorded CFRs of 22.3% and 17.3% for the same age groups. Likewise, the mortality rate for both groups increased by 24.3 (85 years and over age group) and 5.8 deaths (75 to 84 years age group) per 100,000 population, from the previous year. Mortality rate in males increased between FY 2021 to 2022 and FY 2022 to 2023 in all age groups except for those aged 45 to 64 years. However, numbers of deaths are very small in this age group so differences should be interpreted with caution.

Similar to male cases, the highest mortality rates among female cases during FY 2022 to 2023 were seen in those aged over 85 years at 105.4 per 100,000 population, followed by those aged 75 to 84 years at 40.3. This equated to CFRs of 20.9% and 14.7% of cases, respectively. The mortality rate in female cases aged under 1 year increased from 5.8 to 7.8 per 100,000 population between FY 2021 to 2022 and FY 2022 to 2023. The CFR also increased from 8.9% to 9.5% in the same period. Mortality rates and CFRs also increased in females in all age groups except in those aged 15 to 44 years which remained the same, although numbers of deaths are small in these groups so differences should be interpreted with caution.

Klebsiella spp. bacteraemia

Total reports

A total of 11,823 cases of Klebsiella spp. bacteraemia were reported by NHS acute trusts in England during FY 2022 to 2023. Of these, 3,935 (33.3%) were hospital-onset cases (Figure 10). The total count of Klebsiella spp. BSI has increased year-on-year since the start of surveillance (FY 2017 to 2018) from 9,806 to 11,823. This increase was also reflected in the incidence rate of total Klebsiella spp. BSI cases, which increased from 17.6 per 100,000 population in FY 2017 to 2018 to 20.9 in FY 2022 to 2023.

Figure 10. Trends in the rate of Klebsiella spp. bacteraemia in England, by financial year: April 2017 to March 2023

*Mid-year population estimates for January to December 2021 onwards were not available at time of publication and so population data for January to December 2020 was used as a proxy

Hospital-onset cases

The rate (cases per 100,000 bed-days) of hospital-onset Klebsiella spp. BSI cases was 8.4 during FY 2017-2018 and 9.3 in both financial years from April 2018 to March 2020 (Table 6). However, this increased sharply to 13.7 in FY 2020 to 2021. This observed increase has 2 components. Firstly, the number of hospital-onset cases increased from 3,225 to 3,795 between April 2019 and March 2021. Secondly, there was a drop off in hospital activity (occupied bed-days) between the April 2019 and March 2021 period, increasing rates. However, hospital activity has since been increasing, thus, the rate of hospital-onset infections has decreased in comparison to FY 2020 to 2021 to 11.3 cases per 100,000 bed-days the following year and is consistent in its gradual decrease to 11.1 in FY 2022 to 2023. Rates are still higher to pre-pandemic levels.

Table 6. Klebsiella spp. bacteraemia counts and rates by financial year, England, by financial year: April 2017 to March 2023

Financial year	Mid-year population estimate*	All reported cases	Rate (all reported cases per 100,000 population)	Total bed-days	Hospital-onset cases	Rate (Hospital-onset cases per 100,000 bed-days)
2017 to 2018	55,707,642	9,806	17.6	34,903,075	2,928	8.4
2018 to 2019	56,053,563	10,724	19.1	34,538,184	3,212	9.3
2019 to 2020	56,468,265	11,074	19.6	34,637,156	3,225	9.3
2020 to 2021	56,550,138	11,180	19.8	27,628,155	3,795	13.7
2021 to 2022	56,550,138	11,438	20.2	32,905,086	3,704	11.3
2022 to 2023	56,550,138	11,823	20.9	35,478,624	3,935	11.1

*Mid-year population estimates for January to December 2021 onwards were not available at time of publication and so population data for January to December 2020 was used as a proxy

During FY 2020 to 2021, the overall count and the count of hospital-onset cases peaked in January 2021 at 1,078 and 486 cases, respectively (Figure 11). These were the highest numbers of hospital-onset cases observed since the start of mandatory surveillance for Klebsiella spp. bacteraemia. These increases coincided with the COVID-19 pandemic and are likely a result of the change to the usual hospital population and practices during this period. Following the trend from the previous year, the count and percentage of hospital-onset Klebsiella spp. BSI declined in February 2022 before rising and peaking in September 2022. All counts and percentages of hospital-onset infections were higher during July 2022 to December 2022 in comparison to the pre-pandemic periods of surveillance (Figure 11).

Figure 11. Monthly counts of Klebsiella spp. bacteraemia by onset of infection, April 2018 to March 2023

ICU-associated infections

ICU surveillance as part of ICCQIP started in May 2016. Between FY 2017 to 2018 and FY 2022 to 2023, 1,156 (5.6%, N=20,799) hospital-onset Klebsiella spp. Infections were reported to the mandatory surveillance programme and linked to the ICCQIP surveillance programme were considered ICU-associated.

Rates of Klebsiella spp. infections in ICUs have remained below 0.5 cases per 1,000 ICU bed-days greater than 2 days between FY 2017 to 2018 and FY 2019 to 2020 (Figure 12). A steep rise was seen between FY 2019 to 2020 and FY 2020 to 2021, with the rate rising from 0.42 to 1.05 cases per 1,000 ICU bed-days greater than 2 days, respectively. The rate subsequently declined to 0.70 cases per 1,000 ICU bed-days greater than 2 days in FY 2021 to 2022; however, this remains higher than the rates observed in pre-pandemic years. During FY 2022 to 2023 the rate has continued to decrease to 0.5 cases per 1,000 ICU bed-days greater than 2 days. The trend in the rate of ICU-associated Klebsiella spp. infections follows a similar trajectory to the rate of all hospital-onset Klebsiella spp. infections (Figure 12), with both ICU-associated and hospital-onset Klebsiella spp. infections peaking in FY 2020 to 2021. Of note, only a subset of ICUs across England routinely report data to the ICCQIP surveillance system and so ICU associated data must be interpreted with caution.

Figure 12. Rate of ICU-associated hospital-onset Klebsiella spp. bacteraemia cases, England, by financial year: April 2016 to March 2023**

*Due to technical issues data from one registered unit was unable to be processed.

**Includes 30 infections for Klebsiella spp. bacteraemia reported to the mandatory surveillance programme and linked to the ICCQIP surveillance programme, but where Klebsiella spp. was not listed as the causative organism in the ICCQIP data.

Distribution of Klebsiella species

Between April 2022 and March 2023, Klebsiella pneumoniae (72.7%) was the most frequently reported species, followed by Klebsiella oxytoca (16.7%). This distribution was similar regardless of onset of infection (Table 7).

Table 7. Counts and percentages of Klebsiella species bacteraemia, England, April 2022 to March 2023

Species	All cases	Hospital-onset cases	Community-onset cases
K. pneumoniae	8,599 (72.7%)	2,818 (32.8%)	5,781 (67.2%)
K. oxytoca	1,974 (16.7%)	641 (32.5%)	1,333 (67.5%)
K. aerogenes	457 (3.9%)	220 (48.1%)	237 (51.9%)
Other named species	442 (3.7%)	146 (33.0%)	296 (67.0%)
Not speciated	351 (3.0%)	110 (31.3%)	241 (68.7%)

Prior trust exposure

Information on prior trust exposure is available from April 2019, when the mandatory surveillance programme began capturing information on whether a patient had previously been admitted to the same reporting trust within the previous 28 days. These cases are further split into specific groups, a guide to these groups and a full definition can be found in the appendix.

In comparison to FY 2019 to 2020, FY 2022 to 2023 observed an increase in both the count and rate of HOHA Klebsiella spp. infections from 3,225 to 3,935, with a corresponding increase in the rate of HOHA infections from 9.3 cases per 100,000 bed-days to 11.1 (Table 8). Comparing FY 2019 to 2020 and FY 2022 to 2023, the number of COHA Klebsiella spp. cases remained broadly similar with a slight decrease observed from 1,671 and 1,642, respectively. Over the same periods, the rate of COHA cases decreased by 3.1% from 4.3 to 4.2 cases per 100,000 bed-days and day-admissions, a decrease that was affected by the concomitant reduced hospital activity. Between FY 2019 to 2020 and FY 2022 to 2023, the number of cases and rate of COCA Klebsiella spp. infections respectively increased by 22.1%, (from 5,093 to 6,219) and by 21.9% (from 9.0 to 11.0 cases per 100,000 population). It should be noted, however, that FY 2019 to 2020 had 1,009 cases with no information, compared to one case in FY 2020 to 2021 and none for the last 2 years, so these comparisons should be interpreted with caution.

Table 8. Klebsiella spp. bacteraemia counts by prior trust exposure by financial year, England, by financial year: April 2019 to March 2023

Financial Year*	Total bed-days	Total bed-days inc day-admissions	Mid-year population estimate	HOHA cases	HOHA rate (per 100,000 bed-days)	COHA cases	COHA rate (per 100,000 bed-days and day-admissions)	COCA cases	COCA rate (per 100,000 population)	Unknown **	No information***
2019 to 2020	34,637,156	38,537,997	56,468,265	3,225	9.3	1,671	4.3	5,093	9.0	76	1,009
2020 to 2021	27,628,155	30,292,124	56,550,138	3,795	13.7	1,506	5.0	5,827	10.3	51	1
2021 to 2022	32,905,086	36,333,468	56,550,138	3,704	11.3	1,656	4.6	6,036	10.7	42	-
2022 to 2023	35,478,624	39,082,838	56,550,138	3,935	11.1	1,642	4.2	6,219	11.0	27	-

*Financial year from April to March

**The record indicates that it is unknown whether the patient was admitted to the reporting organisation in the past 28 days.

***No information was entered in regard to the prior trust exposure

Rationale for denominator methodology can be found in the Prior trust denominator.

Age and sex distribution

For all age and sex analyses, cases in which the age and or sex was missing or given as unknown were excluded. In FY 2017 to 2018, 8 cases were reported with an ‘unknown’ age or sex compared to 11 cases of ‘unknown’ age or sex in FY 2022 to 2023.

Figure 13 compares the age and sex distribution of Klebsiella spp. bacteraemia cases as a percentage of all reported cases in FY 2017 to 2018 and FY 2022 to 2023. There has been little change to the distribution of cases with the majority of the burden of infection being among males. During FY 2022 to 2023, most cases occurred in adults aged 45 years and over, with the highest percentage being among males aged 75 to 84 years (18.4% of cases). Among females, those aged 45 to 64 years had the greatest burden, representing 9.5% of cases. This is consistent with what was observed during FY 2021 to 2022. Unlike E. coli bacteraemia cases, the percentage of male Klebsiella spp. cases among the 15 to 44 years age group during FY 2022 to 2023 was similar to their female counterparts at 3.8% and 4.1%, respectively.

Figure 13. Age and sex distribution of Klebsiella spp. bacteraemia by percentage of cases, England, April 2017 to March 2018 and April 2022 to March 2023

*Mid-year population estimates for January to December 2021 onwards were not available at time of publication and so population data for January to December 2020 was used as a proxy

In FY 2022 to 2023, the incidence rates of Klebsiella spp. bacteraemia were greater in males than females, particularly so amongst older age groups aged 45 years and over. The rate of infection increased in all age groups, for both sexes, between FY 2017 to 2018 and FY 2022 to 2023. In FY 2022 to 2023, both males and females had their highest rates among individuals aged 85 years and over at 257.5 and 76.8 per 100,000 population, respectively.

For age groups with individuals aged 15 years and over, the rate ratio between males and females increased with each advancing age group in FY 2022 to 2023, remaining consistent with the first reported financial year.

Trends in the age- and sex-specific rates of Klebsiella spp. bacteraemia are shown (Figure 14). Rates of Klebsiella spp. bacteraemia have increased across most age and sex groups between the start of the surveillance up until 2019 to 2020. However, many age and sex groups observed a sharp decline during the 2020 to 2021 financial period. As observed during FY 2022 to 2023, many age and sex groups, particularly among younger ages, have returned to similar levels reported during FY 2019 to 2020.

Table 9. Klebsiella spp. bacteraemia counts and rates by age group and sex, England: financial year April 2017 to March 2018

Age group (years)	Population: Males	Population: Females	Cases: Males	Cases: Females	Rate per 100,000 population: Males	Rate per 100,000 population: Females	Rate Ratio (Males: Females)
<1	333,130	316,482	105	76	31.5	24.0	1.3 (1.0 to 1.8)
1-14	4,827,472	4,595,038	68	53	1.4	1.2	1.2 (0.9 to 1.8)
15-44	10,750,236	10,570,942	305	401	2.8	3.8	1.5 (1.4 to 1.6)
45-64	7,027,291	7,219,864	1,266	874	18.0	12.1	1.5 (1.4 to 1.6)
65-74	2,652,198	2,855,857	1,455	827	54.6	29.0	1.9 (1.7 to 2.1)
75-84	1,446,314	1,757,576	1,724	926	119.0	52.7	2.3 (2.1 to 2.5)
≥85	490,494	864,748	1,094	619	223.0	71.6	3.1 (2.8 to 3.4)

Table 10. Klebsiella spp. bacteraemia counts and rates by age group and sex, England: financial year April 2022 to March 2023*

Age group (years)	Population: Males	Population: Females	Cases: Males	Cases: Females	Rate per 100,000 population: Males	Rate per 100,000 population: Females	Rate Ratio (Males: Females)
<1	308,815	293,098	150	109	48.6	37.2	1.3 (1.0 to 1.7)
1-14	4,929,714	4,682,857	117	74	2.4	1.6	1.5 (1.0 to 1.7)
15-44	10,817,218	10,581,832	443	490	4.1	4.6	0.9 (0.8 to 1.0)
45-64	7,135,608	7,336,977	1,492	1,122	20.9	15.3	1.4 (1.3 to 1.5)
65-74	2,691,641	2,906,787	1,655	926	61.5	31.9	1.9 (1.8 to 2.1)
75-84	1,574,092	1,885,089	2,173	1,031	138.0	54.7	2.5 (2.3 to 2.7)
≥85	525,730	880,680	1,354	676	257.5	76.8	3.4 (3.1 to 3.7)

*Mid-year population estimates for January to December 2021 onwards were not available at time of publication and so population data for January to December 2020 was used as a proxy

Between FY 2017 to 2018 and FY 2022 to 2023, the highest rates of infection among males were observed in those aged 75 to 84 years and 85 years and over age group, increasing by 15.8% from 119.2 to 138.0 per 100,000 population and by 15.5% from 223.0 to 257.5 per 100,000 population, respectively. Similarly, between those financial years the highest rates of infection among females were observed in those aged 75 to 84 years and aged 85 years and over. The rate among females aged 85 years and above increased by 6.7% from 71.9 to 76.8 per 100,000 population respectively, and similarly to the male group, the rate in females aged 75 to 84 years also increased by 3.8% from 52.7 to 54.7 per 100,000 population in this duration.

Between the 2 compared financial years the rate of infection increased for both males and females in the less than one year category, by 54.1% from 31.5 to 48.6 per 100,000 population and by 54.9% from 24.0 to 37.2 per 100,000 population, respectively. It should be noted, the numbers remain low across all years, and the rate and number of cases remain lower in this group compared to some of the previous years. It should be noted, numbers in these groups are small, and thus should be interpreted with caution.

Figure 14. Trend in age and sex rates per 100,000 population* of Klebsiella spp. bacteraemia cases, England, by financial year: April 2017 to March 2023

*Mid-year population estimates for January to December 2021 onwards were not available at time of publication and so population data for January to December 2020 was used as a proxy

Trends by deprivation

During FY 2022 to 2023, 23.8 cases occurred per 100,000 people living in the 20% most deprived areas of England, compared with 17.8 cases per 100,000 people living in the 20% least deprived areas. The observed incidence rate of Klebsiella spp. bacteraemia increased with increasing deprivation.

The age-standardised incidence rates show that if people living in these areas had the same age distribution, the difference in incidence of Klebsiella spp. bacteraemia would be greater than what is currently observed (30.7 and 15.6 cases per 100,000 population in the 20% most and least deprived areas, respectively).

Figure 15 and Supplementary Table 12 show the incidence of Klebsiella spp. bacteraemia across 5 IMD levels between April 2018 and March 2023. Shaded areas indicate 95% confidence intervals.

Figure 15: Index of Multiple Deprivation distribution of Klebsiella spp. bacteraemia, England, April 2018 to March 2023

These differences have been consistent between April 2018 and March 2023.

Trends by ethnicity

During FY 2022 to 2023, the observed incidence rate of Klebsiella spp. bacteraemia was highest in the black and white ethnic group (21.1 and 21.0 cases per 100,000 population, respectively), intermediate in the Asian ethnic group (16.7 per 100,000 population), and lowest in the other and mixed ethnic groups (6.8 and 6.3, respectively).

However, after adjusting for age, the age-standardised incidence rates were substantially higher and increasing in the black, Asian and mixed ethnic groups compared with the white ethnic group.

Figure 16 and Supplementary Table 13 show the incidence of Klebsiella spp. bacteraemia across ethnic groups between April 2018 and March 2023. Shaded areas indicate 95% confidence intervals.

Figure 16: Ethnic group distribution of Klebsiella spp. bacteraemia, England, April 2018 to March 2023

During FY 2020 to 2021, there were a peak in incidence in the Asian ethnic group and an increasing trend among the black, Asian, and other ethnic groups.

Seasonal trends in Klebsiella spp. bacteraemia

Seasonality is assessed as the number of cases each quarter as a percentage of total cases for the financial year. This analysis is performed separately for hospital-onset and community-onset cases. In the first 3 financial years of Klebsiella spp. bacteraemia mandatory surveillance (between April 2017 to March 2020), the percentage of hospital-onset cases was greatest in the July to September quarter (Table S7 of the supplementary dataset). FY 2020 to 2021 saw a considerable change in the percentage distribution by quarter for hospital-onset Klebsiella spp. cases. For the first time the percentage of cases increased in the October to December 2020 financial quarter with a further increase during January and March 2021. This is the first time since the start of Klebsiella spp. mandatory surveillance (FY 2017 to 2018) that this trend was observed. During FY 2022 to 2023, the percentage of hospital-onset cases increased during the tail end of the July to September quarter, peaking in the October to December quarter before decreasing in the last quarter.

Between FY 2017 to 2018 and FY 2020 to 2021 the percentage of community-onset cases was greatest in the July to September quarter. In FY 2022 to 2023, there was a greater fluctuation in the seasonal distribution, with the percentage of community-onset cases being greater in the October to December financial quarter.

Primary focus of Klebsiella spp. bacteraemia

Reporting of primary focus for Klebsiella spp. bacteraemia remains low; during FY 2022 to 2023 the primary focus was reported for 44.4% of all cases. The most frequently reported primary focus of Klebsiella spp. bacteraemia was the urinary tract, constituting 34.0% of cases with a reported primary focus of infection in FY 2022 to 2023. This percentage has remained broadly consistent since the inception of Klebsiella spp. surveillance (Table 11 and Figure 17). Hepatobiliary and respiratory tract as primary focuses have decreased between FY 2017 to 2018 and FY 2022 to 2023, from 20.2% to 16.5% and from 9.2% to 8.8% respectively.

Table 11. Klebsiella spp. bacteraemia counts and rates by primary focus of bacteraemia, England, by financial year: April 2017 to March 2023

Financial year	Total Klebsiella spp. reported	Primary focus ascertained: n (%)	Urinary tract: n (%***)	Gastrointestinal: n (%***)	Hepatobiliary: n (%***)	Respiratory tract: n (%***)	Other: n (%*)	Unknown: n (%***)
2017 to 2018	9,789	5,110 (52.2)	1,681 (32.9)	374 (7.3)	1,032 (20.2)	470 (9.2)	603 (11.8)	950 (18.6)
2018 to 2019	10,698	5,816 (54.4)	1,943 (33.4)	493 (8.5)	1,105 (19.0)	585 (10.1)	768 (13.2)	922 (15.9)
2019 to 2020	11,061	6,108 (55.2)	2,056 (33.7)	484 (7.9)	1,221 (20.0)	581 (9.5)	745 (12.2)	1,021(16.7)
2020 to 2021	11,157	5,491 (49.2)	1,724 (31.4)	409 (7.4)	984 (17.9)	709 (12.9)	693 (12.6)	972 (17.7)
2021 to 2022	11,823	5,255 (44.4)	1,729 (32.9)	367 (7.0)	938 (17.9)	550 (10.5)	745 (14.2)	926 (17.6)
2022 to 2023	11,823	5,239 (44.3)	1,782 (34.0)	429 (8.2)	864 (16.5)	461 (8.8)	748 (14.3)	955 (18.2)

*Gastrointestinal (not hepatobiliary)

**‘Other’ includes the following options HCAI DCS: bone and joint, central nervous system, genital tract (including prostate), indwelling intravascular device, other, respiratory tract, skin or soft tissue, no clinical signs of bacteraemia

***percentage within those records ascertained

During FY 2022 to 2023, for inpatients with a time to onset (time between hospital admission and specimen date) of less than 2 days the most common primary focus of bacteraemia was the urinary tract (34.0%, Figure 17). However, as time between admission and positive specimen increased, the percentage of inpatients reporting primary focus as the urinary tract decreased from 37.9% among those with a time to onset of less than 2 days to 23.0% among those with a time to onset of ≥ 7 days. Conversely, as the number of days between admission and positive specimen increased, so did the percentage of individuals with a reported respiratory tract primary focus, from 5.1% among those with less than 2 days to 11.8% among those with ≥ 7 days.

Figure 17. Distribution of primary focus of Klebsiella spp. bacteraemia, by time to onset, England: financial year April 2022 to March 2023

**Data restricted to cases for which date of admission available

Geographic distribution of Klebsiella spp. bacteraemia

Regional distribution of cases is presented across ICBs for FY 2022 to 2023. There was more geographical variation in rates than was observed for E. coli. The North tended to have higher rates than the South (Figure 18). The highest incidence rates were observed in the North East and North Cumbria ICB (27.6 per 100,000 population), South Yorkshire ICB (27.5 per 100,000 population) and Somerset ICB (25.5 per 100,000 population), while the lowest incidence rates were observed in Gloucestershire ICB (9.4 per 100,000 population), Bath and North East Somerset, Swindon and Wiltshire ICB (14.5 per 100,000 population) and Bristol, North Somerset and South Gloucestershire (15.5 per 100,000 population).

Figure 18. Geographic distribution of Klebsiella spp. bacteraemia rates per 100,000 population, England: financial year April 2022 to March 2023

*Mid-year population estimates for January to December 2021 onwards were not available at time of publication and so population data for January to December 2020 was used as a proxy

Mortality

During FY 2022 to 2023, 11,823 Klebsiella spp. bacteraemia cases were reported in England. Information on mortality was available for 98.3% (11,617) of these cases. There were 2,281 deaths within 30 days of a Klebsiella spp. bacteraemia diagnosis, giving a mortality rate of 4.0 deaths per 100,000 population. The CFR was 19.6% (Figure 19).

Mandatory surveillance of Klebsiella spp. bacteraemia started during FY 2017 to 2018, as such, trends are not as established as those in data collections such as MRSA or E. coli bacteraemia. The mortality rate increased from 3.4 to 4.0 deaths per 100,000 population between FY 2017 to 2018 and FY 2022 to 2023. Conversely, the CFR decreased marginally from 20.2% (1,896 deaths) to 19.6% (2,821 deaths) between FY 2017 to 2018 and FY 2022 to 2023.

Figure 19. Case fatality rate and mortality rate of Klebsiella spp. bacteraemia per 100,000 population, England: financial year April 2017 to March 2023*

*Mid-year population estimates for January to December 2021 onwards were not available at time of publication and so population data for January to December 2020 was used as a proxy

Variation by onset of bacteraemia

The mortality rate in CO cases increased from 2.2 (1,259 deaths) during FY 2021 to 2022 to 2.4 deaths per 100,000 population (1,375 deaths) during FY 2022 to 2023. The corresponding CFRs also increased from 16.7% to 17.7%. Over the same period, the mortality rate of HO cases decreased from 2.8 (916 deaths) to 2.6 deaths per 100,000 bed-days (906 deaths). In line with this, CFR for HO cases also decreased from 25.6% to 23.6%.

Variation by age and sex

During FY 2022 to 2023, the mortality rate and CFR increased with age, except among children aged under one year. The mortality rate was greater in male cases while the CFR was greater in female cases.

Among male cases, the highest mortality rate was observed in those aged 85 years and over (70.4 deaths per 100,000 population) and those aged 75 to 84 years (28.5 deaths per 100,000 population). This equates to CFRs of 27.5% and 20.8% of cases, respectively. Both mortality rates and CFRs increased in these age groups compared with FY 2021 to 2022, when the mortality rate was 60.3 deaths per 100,000 population in those aged 85 years and over and 24.2 deaths per 100,000 population in those aged 75 to 84 years. During the same period, CFR was 26.0% in cases aged 85 years and over and 19.6% of cases in those aged 75 to 84 years.

Among female cases, the mortality rate was also highest among those aged 85 years and over (24.3 deaths per 100,000 population) followed by 75 to 84 years (12.3 deaths per 100,000 population). CFRs for these groups were 31.9% and 22.7% respectively. This is an increase in females aged 75 to 84 years from the previous year, where the mortality rate was 11.2 deaths per 100,000 population and the CFR was 22.5% of cases. Similar to trends seen in males, the mortality rate and CFR were higher than in the FY 2021 to 2022, when mortality rate was 21.3 deaths per 100,000 population and CFR was 27.6% for females over 85 years.

Pseudomonas aeruginosa bacteraemia

Total reports

A total of 4,409 cases of P. aeruginosa bacteraemia were reported by NHS acute trusts in England in the financial year April 2022 to March 2023 (FY 2022 to 2023). Of the 4,409 P. aeruginosa cases, 1,700 (38.6%) were hospital-onset cases. There was little variation in the counts and rates of total P. aeruginosa cases since the start of enhanced surveillance in 2017, except for a spike in the rate of hospital-onset infections during FY 2020 to 2021 (Figure 20).

Figure 20. Trends in the rate of P. aeruginosa bacteraemia in England, by financial year: April 2017 to March 2023

*Mid-year population estimates for January to December 2021 onwards were not available at time of publication and so population data for January to December 2020 was used as a proxy

Hospital-onset cases

The rate (cases per 100,000 bed-days) of P. aeruginosa hospital-onset cases was relatively stable between FY 2017 to 2018 and FY 2019 to 2020, ranging from 4.4 to 4.7 cases per 100,000 bed-days (Table 12). However, this increased to 6.0 in FY 2020 to 2021. This observed increase was similar to that observed for hospital-onset Klebsiella spp. bacteraemia and has occurred due to an increase in the reported number of hospital-onset cases (from 1,583 in 2019 to 2020 to 1,630 in 2021 to 2022) and was compounded by a drop off in the number of bed-days being reported in FY 2020 to 2021, due to the COVID-19 pandemic. During FY 2022 to 2023, the rate of hospital-onset infections has decreased in comparison to FY 2020 to 2021 but has returned to pre-pandemic levels. Additionally, in this year, the number of hospital-onset infections remained similar to FY 2020 to 2021, at 1,670 and 1,700 respectively. However, the rate declined to 4.8 from 6.0 cases per 100,000 bed-days due to an increase in bed-day hospital activity.

Table 12. P. aeruginosa bacteraemia counts and rates by financial year, England, by financial year: April 2017 to March 2023

Financial year	Mid-year population estimate*	All reported cases	Rate (all reported cases per 100,000 population)	Total bed-days	Hospital-onset cases	Rate (Hospital-onset cases per 100,000 bed-days)
2017 to 2018	55,707,642	4,309	7.7	34,903,075	1,626	4.7
2018 to 2019	56,053,563	4,191	7.5	34,538,184	1,521	4.4
2019 to 2020	56,468,265	4,348	7.7	34,637,156	1,583	4.6
2020 to 2021	56,550,138	4,291	7.6	27,628,155	1,670	6.0
2021 to 2022	56,550,138	4,341	7.7	32,905,086	1,630	5.0
2022 to 2023	56,550,138	4,409	7.8	35,478,624	1,700	4.8

*Mid-year population estimates for January to December 2021 onwards were not available at time of publication and so population data for January to December 2020 was used as a proxy

During FY 2020 to 2021, the hospital-onset counts of P. aeruginosa bacteraemia peaked in January 2021 at 213 cases, which was 54.5% of all reported cases (n=391). This was the first time since the start of mandatory surveillance (April 2017 to March 2018) when there were more hospital-onset cases than community-onset cases (Figure 21). These increases coincided with the COVID-19 pandemic and are likely a result of the change to the usual hospital population and practice during this period. There was some evidence to suggest many cases were COVID-19 co-infections (2). After the January 2021 peak, the count and percentage of hospital-onset P. aeruginosa declined, although numbers and the percentage of hospital-onset infections increased again during the July 2021 to October 2021 period, compared to the same period in the pre-COVID-19 pandemic periods of surveillance.

Figure 21. Monthly counts of P. aeruginosa bacteraemia by onset of infection, April 2018 to March 2023

ICU-associated infections

ICU surveillance as part of ICCQIP started in May 2016. Overall, 439 (4.5%, N=9,730) hospital-onset cases P. aeruginosa were identified as ICU-associated between FY 2017 to 2018 and FY 2022 to 2023.

Rates of ICU-associated P. aeruginosa infections have also gradually increased year-on-year between the FY 2017 to 2018 and FY 2020 to 2021 (Figure 22). A steep increase was observed in FY 2020 to 2021 (0.31 ICU-associated cases per 1,000 CU bed-days greater than 2 days), compared to the previous financial year (0.20 ICU-associated cases per 1,000 ICU bed-days greater than 2 days). This increase mirrors the sharp increase that was observed among hospital-onset P. aeruginosa infections during FY 2020 to 2021. Despite the hospital-onset infections reducing slightly for FY 2021 to 2022, the rate of ICU-associated P. aeruginosa infections remained relatively high at 0.30 cases per 1,000 ICU bed-days greater than 2 days*. The rate of ICU-associated P. aeruginosa infections has decreased for FY 2022 to 2023 to 0.2 cases per 1,000 ICU bed-days greater than 2 days.

Figure 22. Rate of ICU-associated hospital-onset P. aeruginosa bacteraemia cases, England, by financial year: April 2017 to March 2023**

*Due to technical issues, data from one registered unit was unable to be processed.

**Includes 14 infections for P. aeruginosa bacteraemia reported to the mandatory surveillance programme and linked to the ICCQIP surveillance programme, but where P. aeruginosa was not listed as the causative organism in the ICCQIP data

Prior trust exposure

From April 2019, as with the other Gram-negative bacteraemia, the mandatory surveillance programme began capturing information on whether a patient had previously been admitted to the same reporting trust from in the previous 28 days.

With the prior trust exposure, cases are split into specific groups, a full guide and definition of these groups can be found in the appendix.

In comparison to FY 2019 to 2020, FY 2022 to 2023 observed an increase in the number of HOHA P. aeruginosa cases from 1,583 to 1,700 with a corresponding increase in the rate of HOHA infections from 4.6 to 4.8 cases per 100,000 bed-days. Over the same period, the number of COHA cases increased 2.2% from 727 to 743 cases, with the rate staying consistent at 1.9 cases per 100,000 bed-days and day admissions. Over the same period COCA cases increased 22.7% from 1,583 to 1,942 and 22.5% from 2.8 to 3.4 cases per 100,000 population, respectively (Table 13).

Table 13. P. aeruginosa bacteraemia counts by prior trust exposure by financial year, England, by financial year: April 2019 to March 2023

Financial Year*	Total bed-days	Total bed-days inc day-admissions	Mid-year population estimate	HOHA cases	HOHA rate (per 100,000 bed-days)	COHA cases	COHA rate (per 100,000 bed-days and day-admissions)	COCA cases	COCA rate (per 100,000 population)	Unknown **	No information ***
2019 to 2020	34,637,156	38,537,997	56,468,265	1,583	4.6	727	1.89	1,583	2.80	25	430
2020 to 2021	27,628,155	30,292,124	56,550,138	1,670	6.0	752	2.48	1,838	3.25	31	-
2021 to 2022	32,905,086	36,333,468	56,550,138	1,630	5.0	818	2.25	1,880	3.32	13	-
2022 to 2023	35,478,624	39,082,838	56,550,138	1,700	4.8	743	1.90	1,942	3.43	24	-

*Financial year from April to March

**The record indicates that it is unknown whether the patient was admitted to the reporting organisation in the past 28 days.

***No information was entered in regard to the prior trust exposure

Rationale for denominator methodology can be found in the Prior trust denominator.

Age and sex distribution

Cases in which the age and or sex was missing or reported as ‘unknown’ were excluded. In FY 2022 to 2023, 3 cases were reported as an ‘unknown’ age or sex.

Figure 23 compares the age and sex distribution of P. aeruginosa cases as a percentage of all reported cases in the FY 2017 to 2018 and FY 2022 to 2023. There has been little change to the distribution of cases. Most infections occur in males and in adults aged 45 years and over. In males, the highest percentage of total cases during FY 2022 to 2023 was among those aged 75 to 84 years (18.2%). During the same period, the highest proportion of total cases among females was among those aged 45 to 64 years (8.2%). This is consistent with observations made in the first financial year of P. aeruginosa surveillance. Like Klebsiella spp., in FY 2022 to 2023, the percentage of male cases among those aged 15 to 44 years was the similar to their female counterparts. This is unlike what was observed for E. coli bacteraemia.

Figure 23. Age and sex distribution of P. aeruginosa bacteraemia by percentage of cases, England, April 2017 to March 2018 and April 2022 to March 2023

*Mid-year population estimates for January to December 2021 onwards were not available at time of publication and so population data for January to December 2020 was used as a proxy

During FY 2022 to 2023, the incidence rate of P. aeruginosa bacteraemia increased with age for both sexes, with the highest rate observed for both sexes among those aged 85 years and over. Despite this, the rate in males aged 85 years and over declined by 7.8% between FY 2017 to 2018 to FY 2022 to 2023 from 108.7 to 100.2 per 100,000 population. Similarly, over the same period, the rate among females aged 85 years and over decreased 13.1% from 34.2 to 29.7 per 100,000 population.

Those aged less than one year are a relatively high-risk group. Of note, between FY 2017 to 2018 and FY 2022 to 2023, the rate among females in this age group increased 12.7% from 7.3 to 8.2 per 100,000 population. However, rates in all other age groups remained comparable between the 2 financial years, including males aged under one year.

Compared to E. coli cases, the difference in incidence rates between sexes were greater. For example, the rate ratio for male and female cases aged 85 years and over was 3.4 (95% CI: 2.9 to 3.9) for P. aeruginosa cases during FY 2022 to 2023, while E. coli was 1.6 over the same period. For comparison, the rate ratio for male and female cases aged 85 years and over for Klebsiella spp. during the same financial year was also 3.4 cases. During the financial year, with the exception of the age groups involving children younger than 15 years, the rate ratio between men and woman increased with each subsequent age group. This was consistent with FY 2017 to 2018.

Table 14. P. aeruginosa bacteraemia counts and rates by age group and sex, England: financial year April 2017 to March 2018

Age group (years)	Population: Males	Population: Females	Cases: Males	Cases: Females	Rate per 100,000 population: Males	Rate per 100,000 population: Females	Rate Ratio (Males: Females)
<1	333,130	316,482	36	23	10.8	7.3	1.5 (95% CI: 0.9 to 2.6)
1-14	4,827,472	4,595,038	60	49	1.2	1.1	1.2 (95% CI: 0.8 to 1.7)
15-44	10,750,236	10,570,942	179	155	1.7	1.5	1.1 (95% CI: 0.9 to 1.4)
45-64	7,027,291	7,219,864	540	352	7.7	4.9	1.6 (95% CI: 1.4 to 1.8)
65-74	2,652,198	2,855,857	650	366	24.5	12.8	1.9 (95% CI: 1.7 to 2.2)
75-84	1,446,314	1,757,576	750	320	51.9	18.2	2.8 (95% CI: 2.5 to 3.3)
≥85	490,494	864,748	533	296	108.7	34.2	3.2 (95% CI: 2.8 to 3.7)

Table 15. P. aeruginosa bacteraemia counts and rates by age group and sex, England: financial year April 2022 to March 2023*

Age group (years)	Population: Males	Population: Females	Cases: Males	Cases: Females	Rate per 100,000 population: Males	Rate per 100,000 population: Females	Rate Ratio (Males: Females)
<1	308,815	293,098	39	24	12.6	8.2	1.5 (95% CI: 0.9 to 1.7)
1-14	4,929,714	4,682,857	82	61	1.7	1.3	1.3 (95% CI: 0.9 to 1.8)
15-44	10,817,218	10,581,832	204	177	1.9	1.7	1.1 (95% CI: 0.9 to 1.4)
45-64	7,135,608	7,336,977	564	361	7.9	4.9	1.7 (95% CI: 1.4 to 1.9)
65-74	2,691,641	2,906,787	641	327	23.8	11.2	2.1 (95% CI: 1.8 to 2.4)
75-84	1,574,092	1,885,089	800	337	50.8	17.9	2.9 (95% CI: 2.5 to 3.2)
≥85	525,730	880,680	527	262	100.2	29.7	3.4 (95% CI: 2.9 to 3.9)

*Mid-year population estimates for January to December 2021 onwards were not available at time of publication and so population data for January to December 2020 was used as a proxy

Trends in the age- and sex-specific rates of P. aeruginosa bacteraemia are shown in Figure 24. The rates are higher in males compared to females, especially in the older age groups (65 years and over). The rate of infection among males aged 85 years and over has fluctuated between 108.7 and 100.2 per 100,000 population between FY 2017 to 2018 and FY 2022 to 2023. In FY 2022 to 2023, the rate in males aged 85 years and over is 100.2 per 100,000 population, a 7.8% decline from 108.7 per 100,000 population during FY 2017 to 2018. Subsequently, the rate increased from 99.7 to 100.2 per 100,000 population between the last 2 financial years (FY 2021 to 2022 and FY 2022 to 2023). The rate of infection among females aged 85 years and over declined by 13.1% between FY 2017 to 2018 and FY 2022 to 2023 from 34.2 to 29.7 per 100,000 population.

Generally, all other age groups, for both sexes, have remained broadly stable over the 5 years since the start of P. aeruginosa bacteraemia mandatory surveillance. The exception to this was in females aged less than one year where the rate of infection increased from 7.5 in FY 2020 to 2021 to 11.6 cases per 100,000 population in FY 2021 to 2022, a 54.5% increase. Since, the rate has declined from 11.6 to 8.2 per 100,000 population, thus being more in line with previous years. Caution should be applied when interpreting the under one age group due to smaller case numbers.

Figure 24. Trend in age and sex rates per 100,000 population* of P. aeruginosa bacteraemia cases, England, by financial year: April 2017 to March 2023

*Mid-year population estimates for January to December 2021 onwards were not available at time of publication and so population data for January to December 2020 was used as a proxy

Trends by deprivation

During FY 2022 to 2023, unlike other collections, the observed incidence rate for P. aeruginosa BSI did not substantially differ across deprivation areas, with slightly higher rates in the middle deprivation group.

However, the age-standardised incidence rates show that if people living in these areas had the same age distribution, the incidence of P. aeruginosa BSI would vary according to deprivation group, with 9.6 and 6.3 cases per 100,000 population in the 20% most and least deprived areas, respectively.

Figure 25 and Supplementary Table 12 show the incidence of P. aeruginosa bacteraemia across 5 IMD levels between April 2018 and March 2023. Shaded areas indicate 95% confidence intervals.

Figure 25: Index of Multiple Deprivation distribution of P. aeruginosa bacteraemia, England, April 2018 to March 2023

These differences were consistent between April 2018 and March 2023.

Trends by ethnicity

During FY 2022 to 2023, the observed incidence rate of P. aeruginosa bacteraemia was highest in the white and black ethnic groups (8.0 and 7.8 cases per 100,000 population, respectively), intermediate in the Asian ethnic group (5.1 cases per 100,000 population), and lowest in the mixed and other ethnic groups (2.6 and 1.7, respectively).

However, after adjusting for age, the age-standardised incidence rate was far greater in the black ethnic group (13.4 cases per 100,000), followed with some temporal fluctuation by the Asian, white and mixed ethnic groups, and lowest in the other ethnic group.

Figure 26 and Supplementary Table 13 show the incidence of P. aeruginosa bacteraemia across ethnic groups between April 2018 and March 2023. Shaded areas indicate 95% confidence intervals.

Figure 26: Ethnic group distribution of P. aeruginosa bacteraemia, England, April 2018 to March 2023

There was a peak in incidence in FY 2020 to 2021 among the Asian and other ethnic groups, and a steady increase since FY 2019 to 2020 among the mixed ethnic group.

Seasonal trends in P. aeruginosa bacteraemia

Seasonality is assessed as the number of hospital-onset cases each quarter as a percentage of total hospital-onset cases for the financial year or as the number of community-onset cases each quarter as a percentage of total community-onset cases for the financial year. When assessing the distribution of community-onset P. aeruginosa cases per financial year, the greatest percentage occurs in the July to September quarter of the financial year. This has occurred for each year since the beginning of P. aeruginosa surveillance in FY 2017 to 2018. However, there has been no consistent trend in the distribution of hospital-onset cases by quarter in the first 5 years of P. aeruginosa bacteraemia mandatory surveillance.

Primary focus of P. aeruginosa bacteraemia

Reporting of primary focus for P. aeruginosa remains low, during FY 2022 to 2023, the primary focus was reported for 42.8% of cases, a decrease from 49.4% from the start of surveillance in FY 2017 to 2018. The most frequent primary focus of P. aeruginosa bacteraemia, was the urinary tract, constituting 28.3% of cases during FY 2022 to 2023. Over the 6 financial years from April 2017 to March 2023, the percentage of primary focus reported as the urinary tract was similar (range: 31.1% to 28.3%) and was the most commonly reported primary focus of bacteraemia Table 16. As observed with Klebsiella spp., in the 3 financial years between April 2017 and March 2020 the percentage of cases reporting respiratory tract as a primary focus remained stable (range:13.0% to 16.1%). However, in FY 2020 to 2021, there was an increase in the percentage of primary focus reported as the respiratory tract to 16.1% which coincided with the first 2 waves of the COVID-19 pandemic. This reduced to 15.1% during FY 2021 to 2022 and then further declined to 14.0% in FY 2022 to 2023, back to pre-pandemic levels.

Table 16. P. aeruginosa bacteraemia counts and rates by primary focus of bacteraemia, England, by financial year: April 2017 to March 2023

Financial year	Total P. aeruginosa reported	Primary focus ascertained: n (%)	Urinary tract: n (%***)	Gastro-intestinal: n (%***)	Hepatobiliary: n (%***)	Respiratory tract: n (%***)	Other: n (%*)	Unknown: n (%***)
2017 to 2018	4,309	2,129 (49.4)	638 (30)	126 (5.9)	102 (4.8)	300 (14.1)	491 (23.1)	472 (22.2)
2018 to 2019	4,191	2,161 (51.6)	644 (29.8)	156 (7.2)	100 (4.6)	281 (13.0)	557 (25.8)	423 (19.6)
2019 to 2020	4,348	2,320 (53.4)	716 (30.9)	160 (6.9)	123 (5.3)	313 (13.5)	549 (23.7)	459 (19.8)
2020 to 2021	4,291	2,117 (49.3)	611 (28.9)	129 (6.1)	116 (5.5)	341 (16.1)	503 (23.8)	417 (19.7)
2021 to 2022	4,341	1,931 (44.5)	600 (31.1)	117 (6.1)	93 (4.8)	291 (15.1)	461 (23.9)	369 (19.1)
2022 to 2023	4,409	1,889 (42.8)	535 (28.3)	146 (7.7)	95 (5.0)	265 (14.0)	452 (23.9)	396 (21.0)

*Gastrointestinal (not hepatobiliary)

**‘Other’ includes the following options HCAI DCS: bone and joint, central nervous system, genital tract (including prostate), indwelling intravascular device, other, respiratory tract, skin or soft tissue, no clinical signs of bacteraemia

***percentage within those records ascertained

In FY 2022 to 2023, for inpatients with a time to onset (days between hospital admission and specimen date) of less than 2 days, the most important primary focus of bacteraemia was the urinary tract (32.0%, Figure 27). However, as time between admission and positive specimen increased, the percentage of inpatients reporting primary focus as the urinary tract remained stable, with a small observed increase from 20.7% among those with a time to onset of 2 to 6 days, to 21.4% among those with a time to onset of ≥ 7 days. Conversely, as the number of days between admission and positive specimen increased, so did the percentage of individuals with a reported respiratory tract primary focus from 10.6% among those with less than 2 days, peaking at 22.0% in the 2 to 6 days group before declining to 14.8% among those with ≥ 7 days. Similarly, gastrointestinal (not hepatobiliary) increased from 7.2% among those with less than 2 days to 11.0% in the 2 to 6 days group followed by a decline to 9.8% among those with ≥ 7 days.

Figure 27. Distribution of primary focus of P. aeruginosa bacteraemia, by time to onset, England: financial year April 2022 to March 2023

Geographic distribution of P. aeruginosa bacteraemia

Regional distribution of cases is presented across ICBs for FY 2022 to 2023. There is some evidence of similar and relatively high infection rates of P. aeruginosa among neighbouring ICBs in the North and the isolated regions centrally and in the South. Areas across the Midlands, North West and South West typically had lower rates of infections per 100,000 population (Figure 28). The highest incidence rate was observed in Lincolnshire ICB (11.5 per 100,000 population), Norfolk and Waveney ICB (10.2 per 100,000 population) and Kent and Medway ICB (10.0 per 100,000 population), while the lowest incidence rate were observed in Gloucestershire ICB (4.4 per 100,000 population), Birmingham and Solihull ICB (5.2 per 100,000 population) and Bath and North East ICB (5.7 per 100,000 population).

Figure 28. Geographic distribution of P. aeruginosa bacteraemia rates per 100,000 population, England: financial year April 2022 to March 2023*

*Mid-year population estimates for January to December 2021 onwards were not available at time of publication and so population data for January to December 2020 was used as a proxy

Mortality

During FY 2022 to 2023, 4,409 P. aeruginosa bacteraemia cases were reported in England. Information on mortality was available for 98.4% (4,339) of these cases. There were 1,111 deaths within 30 days of a P. aeruginosa bacteraemia, giving a mortality rate of 2.0 deaths per 100,000 population and a CFR of 25.6%.

Mandatory surveillance of P. aeruginosa bacteraemia started during FY 2017 to 2018, as such, trends are not as established as those in data collections such as MRSA or E. coli bacteraemia. The CFR had decreased from 26.9% (1,121 deaths) during FY 2017 to 2018 to 25.6% (1,111 deaths) during FY 2022 to 2023. The mortality rate also remained consistent at around 2.0 per 100,000 population between FY 2021 to 2022 and FY 2022 to 2023. However, a slight increase was observed in the CFR from 25.0% to 25.6%, respectively.

Figure 29. Case fatality rate and mortality rate of P. aeruginosa bacteraemia per 100,000 population, England: financial year April 2017 to March 2023*

Variation by onset of bacteraemia

During FY 2022 to 2023, the mortality rate of HO cases increased to 1.4 deaths per 100,000 bed-days (482 deaths) compared with 1.3 (430 deaths) in FY 2021 to 2022. Over the same period, the mortality rate in CO remained at 1.1 per 100,000 population (631 deaths and 629 deaths, respectively).

Between the FY 2019 to 2020 and FY 2022 to 2023, the CFR of HO cases increased from 28.5% to 29.0%, while for CO cases there was also an increase from 22.3% to 23.5% over the same period. Compared with FY 2021 to 2022, CFR in HO cases in FY 2022 to 2023 increased from 27.0% to 29.0%, while CO cases decreased from 23.8% to 23.5%.

Variation by age and sex

During FY 2022 to 2023, mortality rate and CFR increased with age, except among children aged under one year. The mortality rate was greater in male cases while the CFR was greater in female cases.

During FY 2022 to 2023, among male cases, the highest mortality rates were in those aged 85 years and over (27.2 deaths per 100,000 population) and those aged 75 to 84 years (13.2 deaths per 100,000 population), which corresponded to CFRs of 27.4% and 26.0% of cases, respectively.

In female cases of the same age groups, the mortality rates were far lower than their male counterparts; 13.6 deaths per 100,000 population (aged 85 years and over) and 6.7 deaths per 100,000 population (aged 75 to 84 years). These equated to CFRs of 45.8% and 37.8% of all cases in these age groups.

Among children aged under one year, the mortality rate in male cases was 2.9 deaths per 100,000 population (25.0% of cases) compared with 1.4 (19.0% of cases) in female cases. Although, caution is required in interpreting these data, as the number of deaths was relatively small in both groups.

Discussion

Data on bacteraemia caused by E. coli, Klebsiella spp. and P. aeruginosa shows both similarities and differences in their epidemiology.

E. coli continues to have the highest rate of all the Gram-negative organisms, with 68.5 cases of bacteraemia per 100,000 population in the FY 2022 to 2023. This is followed by Klebsiella spp. (20.9 per 100,000 population) and then P. aeruginosa cases (7.8 per 100,000 population). In FY 2022 to 2023, a greater proportion of P. aeruginosa and Klebsiella spp. cases were categorised as hospital-onset, accounting for 38.5% and 33.3% of all reported cases, respectively, compared to E. coli bacteraemia, which constituted 20.3% of hospital-onset cases. The hospital-onset rate for Klebsiella spp. bacteraemia have returned to pre-pandemic levels and continue to climb year on year.

Among the Gram-negative bacteraemia cases with a known primary focus of infection, the urinary tract remains a major source of bacteraemia for all Gram-negatives, accounting for 45.2% of E. coli, 34.0% of Klebsiella spp. and 28.3% of P. aeruginosa. In FY 2020 to 2021, P. aeruginosa and Klebsiella spp. had their highest percentage of infections reported with a respiratory tract as primary focus with 16.1% and 12.9%, respectively. This was predominantly driven by peaks observed in January 2021 and February 2021 at 25.1% and 22.7% for P. aeruginosa and Klebsiella spp., respectively. This peak in the respiratory tract as focus of infection coincided with the months of highest counts and rates of SARS-CoV-2 infection during the second COVID-19 wave in England, with further investigations revealing a proportion as being co-infected with COVID-19 (2). This declined for both P. aeruginosa (14.0%) and Klebsiella spp. (8.8%) consecutively for the second financial year for FY 2022 to 2023, returning back to reported pre-pandemic levels.

The age distribution of cases is similar in all 3 Gram-negative bacteraemia included in the mandatory surveillance programme. However, there are differences in the distribution of cases by sex. E. coli cases are more evenly distributed between male and female cases, while Klebsiella spp. and P. aeruginosa cases are more common in male cases, particularly in the older age groups.

Rates of each of the Gram-negative bacteraemia show unique geographic distributions in England with evidence of similar and relatively high infection rates among neighbouring ICBs for both E. coli and P. aeruginosa is suggested by the observed data, although differences exist within the organism-specific patterns. More broadly, higher rates were generally found in each organism in the Northern and some South West regions of England.

Long term trends of each bacteraemia show increasing incidence rates since the start of enhanced surveillance until at least the start of the COVID-19 pandemic. E. coli cases increased annually between FY 2012 to 2013 and FY 2019 to 2020, except in the FY 2016 to 2017, where a small fluctuation was noted. During FY 2020 to 2021, the incidence rate of E. coli bacteraemia declined considerably. While the rate of incidence has now increased from 65.1 (FY 2020 to 2021) to 68.5 (FY 2022 to 2023), the rate is still appreciably lower than the pre-pandemic period, although follow the same upward trajectory seen pre-pandemic. Further investigative work and more data is required to fully determine if the recent changes indicate a shift in the long-term trends of E. coli bacteraemia.

The observed decline of E. coli was not observed for Klebsiella spp. and P. aeruginosa bacteraemia with the number of hospital-onset Klebsiella spp. and P. aeruginosa cases increasing during the pandemic period. Further analysis conducted by UKHSA identified the increases corresponded with increases in hospital-onset cases that are a) secondary to COVID-19 cases, b) reported with respiratory tract as the primary focus of infection and c) reported in intensive care units (2). Suggesting that the increases in hospital-onset cases are likely related to the COVID-19 pandemic. E. coli bacteraemia is less commonly associated with these factors compared to Klebsiella spp. and P. aeruginosa potentially explaining the difference in trend.

Following this period, the rate of hospital-onset infection declined for Klebsiella spp. and P. aeruginosa bacteraemia in the reporting financial year. It is important to note the rate for Klebsiella spp. remained high compared to pre-pandemic levels, however, the rate for P. aeruginosa returned to similar levels in line with earlier years of surveillance.

For the first time this report publishes data on incidence by deprivation and ethnicity. Data across all 3 Gram-negative bloodstream infections (GNBSIs) suggests those living in more deprived areas are disproportionately affected. Age standardised ethnicity data also demonstrates ethnic minorities, particularly black and Asian, having greater incidence of GNBSIs. It should be noted that ethnicity and deprivation are linked, with those in some ethnic minorities known to reside in more deprived areas. These findings highlight potential health inequalities and require further investigation to identify appropriate interventions.

Even with the observed changes during the COVID-19 pandemic period, the number of Gram-negative bacteraemia’s far exceeds those caused by S. aureus. As such, the Secretary of State for Health and Social Care introduced an ambition to reduce healthcare-associated Gram-negative bacteraemia by March 2024 (3). Currently, further work is being carried out by UKHSA to review current data and set future goals to support the reduction of Gram-negative bacteraemia. The historical reduction in rates of CDI and MRSA bacteraemia show what is possible with targeted interventions. However, there are key difference in the epidemiology of Gram-negative bacteraemia and CDI and MRSA bacteraemia. Thus, highlighting the need of different interventions required to target and effectively control GNBSIs. Emphasis also needs to be focused on community-onset cases, which account for a much larger proportion of the overall incidence, however, interventions can be more difficult in this setting.

Epidemiological analysis of Staphylococcus aureus bacteraemia

A total of 13,912 Staphylococcus aureus bacteraemia cases were reported in the financial year April 2022 to March 2023 (FY 2022 to 2023) through both the MRSA bacteraemia and MSSA bacteraemia surveillance schemes. This represents a 7.2% increase in the numbers of bacteraemias caused by S. aureus from the previous financial year (April 2021 to March 2022, n = 12,981) and a 40.8% increase from the FY 2011 to 2012 (n = 9,883) when MSSA reporting was made mandatory.

In FY 2022 to 2023, 5.7% (n = 787) of S. aureus bacteraemia reports were caused by MRSA. This is a decrease from FY 2011 to 2012, in which 11.3% (n = 1,116) of reports were caused by MRSA. In addition, there is a small increase from FY 2021 to 2022, in which 5.2% (n = 675) of reports were caused by MRSA. At its peak MRSA bacteraemias accounted for approximately 40% of all S. aureus bacteraemia cases in England (4).

The following sections will describe the epidemiology of MRSA and MSSA in England separately.

Meticillin resistant Staphylococcus aureus bacteraemia

Total reports

A total of 787 cases of MRSA bacteraemia were reported by acute NHS acute trusts in England in FY 2022 to 2023. This is an increase of 16.6% from FY 2021 to 2022 period (n = 673), and a decrease of 82.3% from FY 2007 to 2008 (n = 4,451) when MRSA surveillance began. Figure 30 shows the trends in rates of MRSA cases for all cases and hospital-onset cases for financial years between April 2007 and March 2022. The rate of all MRSA cases per 100,000 population, per year has fallen from 8.6 in FY 2007 to 2008 to 1.4 in FY 2022 to 2023.

Figure 30. Trends in the all case and hospital-onset rate of MRSA bacteraemia in England, by financial year: April 2007 to March 2023

*Mid-year population estimates for January to December 2021 onwards were not available at time of publication and so population data for January to December 2020 was used as a proxy

The rate of all MRSA has plateaued since FY 2014 to 2015 ranging between 1.4 to 1.5 cases per 100,000 population. A slight decline in rates was observed between FY 2019 to 2020 (1.4) and April 2020 to March 2022 (1.2) financial years of 14.3% during this period of relative stability. In FY 2022 to 2023, the rate of all MRSA returned to the previously observed level of 1.4 per 100,000 population.

Hospital-onset reports

Of the 787 cases reported during FY 2022 to 2023, 295 (37.5%) were hospital-onset (0.8 per 100,000 bed-days). Overall, there has been a declining trend in the rate of hospital-onset MRSA cases from 4.2 per 100,000 bed-days in FY 2008 to 2009 (when this metric was first collected) to 0.7 per 100,000 bed-days in FY 2021 to 2022 (Table 17). In the reporting financial period (April 2022 to March 2023), a 26.6% increase was observed in case numbers (from 233 to 295), with an incremental increase in rate from 0.7 to 0.8 per 100,000 bed-days. Of note, an increase in the hospital setting was observed during FY 2020 to 2021, which was the first full financial year after the World Health Organization (WHO) declared COVID-19 a pandemic. During this year, the hospital-onset counts increased by 7.7% from 260 to 280 cases and the rate compared to FY 2019 to 2020 also increased by 25% to 1.0 per 100,000 bed-days. This was the largest annual increase in the hospital-onset rate of infection since enhanced mandatory surveillance began collecting the hospital-onset metric in fin FY 2008 to 2009. This increase was also observed in the surveillance of Klebsiella spp. and P. aeruginosa bacteraemia. The rate subsequently dropped sharply in FY 2021 to 2022, but has risen slightly in the most recent financial year to 0.8 per 100,000 bed days, however, it should be noted the case numbers have risen more sharply for this period (26.6%), and are its highest since FY 2017 to 2018, but rates appear lower due to the increased hospital activity.

Since the inception of MRSA surveillance, the percentage of cases that were hospital-onset has declined from 54.7% in FY 2008 to 2009 to 37.5% in FY 2022 to 2023.

Table 17. MRSA counts and rates by financial year, England, by financial year: April 2007 to March 2023

Financial year	Mid-year population estimate*	All reported cases	Rate (all reported cases per 100,000 population)	Total bed-days	Hospital-onset cases	Rate (Hospital-onset cases per 100,000 bed-days)
2007 to 2008	51,594,959	4,451	8.6	37,451,698	-	-
2008 to 2009	51,803,017	2,935	5.7	37,823,000	1,606	4.2
2009 to 2010	52,306,371	1,898	3.6	37,441,591	1,004	2.7
2010 to 2011	52,757,039	1,481	2.8	35,206,316	688	2.0
2011 to 2012	53,312,604	1,116	2.1	34,669,499	473	1.4
2012 to 2013	53,475,357	924	1.7	34,633,855	398	1.1
2013 to 2014	53,976,973	862	1.6	34,514,871	364	1.1
2014 to 2015	54,432,437	800	1.5	34,972,728	285	0.8
2015 to 2016	55,018,883	823	1.5	34,752,604	298	0.9
2016 to 2017	55,240,933	825	1.5	35,148,014	315	0.9
2017 to 2018	55,707,642	850	1.5	34,903,075	276	0.8
2018 to 2019	56,053,563	807	1.4	34,538,184	271	0.8
2019 to 2020	56,468,265	815	1.4	34,637,156	260	0.8
2020 to 2021	56,550,138	696	1.2	27,628,155	280	1.0
2021 to 2022	56,550,138	675	1.2	32,905,086	233	0.7
2022 to 2023	56,550,138	787	1.4	35,478,624	295	0.8

In FY 2020 to 2021, the counts of all reported cases peaked in January 2021 at 84 cases and of these, 50% of cases were hospital-onset (Figure 31). In FY 2021 to 2022, the counts of hospital-onset cases were comparable to pre-pandemic periods, although a reduction in community-onset cases also meant that percentage of cases that were hospital-onset were still relatively high compared to the 2-year period between April 2018 to March 2020. This trend remained evident and stable in FY 2022 to 2023. The gap between community-onset and hospital-onset cases also appears to be closing in the recent financial years, with hospital-onset cases overtaking community-onset on 2 separate occasions during the COVID-19 pandemic.

Figure 31. Monthly counts of MRSA bacteraemia by onset of infection, April 2018 to March 2023

ICU-associated infections

ICU surveillance as part of ICCQIP started in May 2016. Overall, between FY 2016 to 2017 and FY 2022 to 2023, 53 (2.75%, N=1,930) hospital-onset MRSA infections were also recorded as ICU-associated infections.

Rates of MRSA infections in ICUs have fluctuated year-on-year between FY 2016 to 2017 and FY 2018 to 2019 (Figure 32). Between FY 2018 to 2019 and FY 2021 to 2022, the rates increased in each consecutive financial year from 0.012 to 0.034 cases per 1,000 ICU bed-days greater than 2 days. During FY 2022 to 2023 the rate decreased to 0.016 cases per 1,000 ICU bed-days greater than 2 days*. Caution is required in the interpretation of this due to the relatively small numbers of MRSA infections.

Figure 32. Rate of ICU-associated hospital-onset MRSA bacteraemia cases, England, by financial year: April 2017 to March 2023**

*Due to technical issues data from one registered unit was unable to be processed

**Includes 6 infections for MRSA bacteraemia reported to the mandatory surveillance programme and linked to the ICCQIP surveillance programme, but where S. aureus was not listed as the causative organism in the ICCQIP data

Prior trust exposure

During the FY 2019 to 2020, the mandatory surveillance programme began capturing information on whether a patient had previously been admitted to the same reporting trust in the previous 28 days. With the prior trust exposure, cases are split into specific groups, a full guide and definition of these groups can be found in the appendix.

In comparison to FY 2019 to 2020, FY 2022 to 2023 saw an increase in the count and rate of HOHA MRSA bacteraemia from 260 to 295, correspondingly, the rate of HOHA infections also increased from 0.75 to 0.83 cases per 100,000 bed-days (Table 18). Over the same period, the number of COHA cases reduced by 34.5% from 142 to 93 cases, corresponding to a rate decrease of 50% from 0.4 to 0.2 cases per 100,000 bed-days and day-admissions. Over the same period, COCA MRSA bacteraemia decreased from 404 to 394 (2.5%). This decline was also reflected in the COCA rate which fell from 0.72 to 0.70 cases per 100,000 population. Caution should be used in the interpretation of these as the underlying numbers are relatively small.

Table 18. MRSA counts by prior trust exposure by financial year, England, by financial year: April 2019 to March 2023

Financial Year*	Total bed-days	Total bed-days inc day-admissions	Mid-year population estimate	HOHA cases	HOHA rate (per 100,000 bed-days)	COHA cases	COHA rate (per 100,000 bed-days and day-admissions)	COCA cases	COCA rate (per 100,000 population)	Unknown **	No information ***
2019 to 2020	34,637,156	38,537,997	56,468,265	260	0.8	142	0.4	404	0.7	9	0
2020 to 2021	27,628,155	30,292,124	56,550,138	280	1.0	86	0.3	326	0.6	4	0
2021 to 2022	32,905,086	36,333,468	56,550,138	233	0.7	100	0.3	339	0.6	3	0
2022 to 2023	35,478,624	39,082,838	56,550,138	295	0.8	93	0.2	394	0.7	5	0

*Financial year from April to March

**The record indicates that it is unknown whether the patient was admitted to the reporting organisation in the past 28 days

***No information was entered in regard to the prior trust exposure

Rationale for denominator methodology can be found in the Prior trust denominator.

Age and sex distribution

For all age and sex analyses, cases in which the age and or sex was missing or given as unknown were excluded. In FY 2007 to 2008, 53 cases (1.2%) were reported with the age or sex as unknown while in FY 2022 to 2023, no cases were reported with the age or sex as unknown.

Figure 33 compares the age and sex distribution of MRSA cases as a percentage of all reported cases in financial years April 2007 to March 2008 and April 2022 to March 2023. Since the start of surveillance there have been some changes observed in the distribution of cases between these periods. In general, the percentage of cases in most age groups is lower in the April 2022 to March 2023 period compared to FY 2007 to 2008, with greater reduction observed in the older age groups (65 years and over). This is particularly evident for males and females aged 75 to 84 years. During FY 2007 to 2008 males aged 75 to 84 years accounted for 20.1% of all infections and females of the same age group made up 9.9%. During FY 2022 to 2023, the percentage of male cases within the 75 to 84 years age group reduced to 12.2%, while in females it reduced to 6.7%. There was also a reduction in the burden of infection among individuals aged 65 to 74 years. Conversely, younger age groups have seen a relative increase, particularly in children aged 14 years and younger. Furthermore, there was an increase in the percentage of all cases that occurred in male cases in under 65 years of age during FY 2022 to 2023 compared to FY 2007 to 2008.

Figure 33. Age and sex distribution of MRSA bacteraemia by percentage of cases, England, April 2007 to March 2008 and April 2022 to March 2023

*Mid-year population estimates for January to December 2021 onwards were not available at time of publication. Therefore, population data for January to December 2020 was used as a proxy for 2021 to 2022 and 2022 to 2023 financial years

At the start of MRSA surveillance (FY 2007 to 2008), higher MRSA rates were found across all age groups within males compared to females. This remained consistent for FY 2022 to 2023 (Table 19 and Table 20), with the greatest difference in rates observed among males aged 85 years and over, with a rate of 14.6 cases per 100,000 population, compared to 4.9 cases per 100,000 in females. Both sexes observed increasing rates of infection with age, with the exception among those aged less than one year. Caution is advisable when interpreting data for MRSA among those aged 14 years and under as the number of cases is small.

Table 19. MRSA counts and rates by age group and sex, England: financial year April 2007 to March 2008

Age group (years)	Population: Males	Population: Females	Cases: Males	Cases: Females	Rate per 100,000 population: Males	Rate per 100,000 population: Females	Rate Ratio (Males: Females)
<1	333,165	317,036	31	28	9.3	8.8	1.1 (95% CI: 0.7 to 1.5)
1-14	4,369,017	4,166,420	22	14	0.5	0.3	1.5 (95% CI: 0.9 to 2.7)
15-44	10,728,560	10,721,325	252	149	2.3	1.4	1.7 (95% CI: 1.5 to 1.9)
45-64	6,322,973	6,463,537	594	306	9.4	4.7	2.0 (95% CI: 1.8 to 2.2)
65-74	2,011,279	2,205,682	583	269	29.0	12.2	2.4 (95% CI: 2.1 to 2.7)
75-84	1,208,494	1,655,253	884	436	73.1	26.3	2.8 (95% CI: 2.5 to 3.0)
≥85	337,200	755,019	497	334	147.4	44.2	3.3 (95% CI: 3.0 to 3.6)

Table 20. MRSA counts and rates by age group and sex, England: financial year April 2022 to March 2023*

Age group (years)	Population: Males	Population: Females	Cases: Males	Cases: Females	Rate per 100,000 population: Males	Rate per 100,000 population: Females	Rate Ratio (Males: Females)
<1	308,815	293,098	15	10	4.9	3.4	1.4 (95% CI: 1.3 to 1.5)
1-14	4,929,714	4,682,857	18	17	0.4	0.4	1.0 (95% CI: 0.5 to 2.1)
15-44	10,817,218	10,581,832	82	55	0.8	0.5	1.5 (95% CI: 1.2 to 1.8)
45-64	7,135,608	7,336,977	155	51	2.2	0.7	3.1 (95% CI: 2.3 to 4.3)
65-74	2,691,641	2,906,787	76	39	2.8	1.3	2.1 (95% CI: 1.4 to 3.2)
75-84	1,574,092	1,885,089	96	53	6.1	2.8	2.2 (95% CI: 1.5 to 3.1)
≥85	525,730	880,680	77	43	14.6	4.9	3.0 (95% CI: 2.0 to 4.4)

*Mid-year population estimates for January to December 2021 onwards were not available at time of publication and so population data for January to December 2020 was used as a proxy

Since the start of MRSA surveillance, there have been huge reductions in the number of bacteraemia reported, this decrease is reflected in the age- and sex-specific rates and trends of MRSA bacteraemia (Figure 34). The sharpest decline is observed among males and females aged 75 to 84 years from 73.1 cases per 100,000 population (91.7% decline) to 6.1 in males and from 26.3 cases per 100,000 population to 2.8 (89.4% decline) in females. Those aged 85 years and over also observed similar sharp declines for both males and females from 147.4 cases per 100,000 population to 14.6 (90.1% decline) and from 44.2 cases per 100,000 population to 4.9 (89.0% decline), respectively. Over the same period, the decline in younger age groups has been relatively moderate, with rates in males aged 1 to 14 years declining from 0.5 cases per 100,000 population to 0.4 (20.0% decline). Females in the same group observed the same rate of 0.4 cases per 100,000 population.

Figure 34. Trends in age and sex rates per 100,000 population* of MRSA cases, England, by financial year: April 2007 to March 2023

*Mid-year population estimates for January to December 2021 onwards were not available at time of publication and so population data for January to December 2020 was used as a proxy

Trends by deprivation

During FY 2022 to 2023, 2.0 cases occurred per 100,000 people living in the 20% most deprived areas of England, compared with 1.0 cases per 100,000 people living in the 20% least deprived areas. The observed incidence rate of MRSA BSI increased with increasing deprivation.

The age-standardised incidence rates show that if people living in these areas had the same age distribution, the difference in incidence of MRSA BSI would be much greater than observed, with 2.4 and 0.9 cases per 100,000 population in the 20% most and least deprived areas, respectively. In the last 2 financial years there has been a steeper increase in incidence in the 20% most deprived areas of the country.

Figure 35 and Supplementary Table 12 show the incidence of MRSA bacteraemia across 5 IMD levels between April 2018 and March 2023. Shaded areas indicate 95% confidence intervals.

Figure 35: Index of Multiple Deprivation distribution of MRSA bacteraemia, England, April 2018 to March 2023

Trends by ethnicity

During FY 2022 to 2023, the observed incidence rate of MRSA bacteraemia was comparable in the Asian, black, and white ethnic groups (1.7, 1.6 and 1.3 cases per 100,000 population, respectively) and lowest in the mixed and other ethnic groups (0.8 and 0.7, respectively).

However, after adjusting for age, there is some evidence that the age-standardised incidence rate was higher in the Asian and black ethnic groups, compared with the white ethnic group.

Figure 36 and Supplementary Table 13 show the incidence of MRSA bacteraemia across ethnic groups between April 2018 and March 2023. Shaded areas indicate 95% confidence intervals.

Figure 36: Ethnic group distribution of MRSA bacteraemia, England, April 2018 to March 2023

Note: age-standardised rates for the mixed or multiple and for the other ethnic groups could not be calculated when yearly counts in these groups were less than 10 for any given ethnic-age subgroup.

Source of MRSA bacteraemia

The Healthcare Associated Infections (HCAI) Data Capture System (DCS) provides Trust users the opportunity to add information regarding the likely source of bacteraemia. Source of bacteraemia refers to the likely cause of the bacteraemia, such as an intravenous catheter, rather than an organ where the infection first arose as in primary focus for Gram-negative bacteraemia. The provision of this information is also voluntary and has declined over time for MRSA. In FY 2007 to 2008, a total of 54.2% (n=2,414) of MRSA records had entries (including ‘Unknown’) for the source of bacteraemia. By FY 2022 to 2023, only 28.0% (n=221) MRSA records had entries for the source of bacteraemia.

Amongst MRSA cases with a reported source of bacteraemia, there were large declines in the percentage of MRSA cases in which the source of bacteraemia was a catheter and line between FY 2007 to 2008 and FY 2014 to 2015 period from 25.6% to 11.9%. However, between the April 2014 to March 2015 period and FY 2022 to 2023; this had increased to 16.7%

In contrast, the percentage of cases caused by skin and soft tissue infections there has seen continual increase from 16.4% in FY 2007 to 2008 to 37.1% in FY 2022 to 2023. Between FY 2007 to 2008 and FY 2014 to 2015, the percentage of cases for which the source of bacteraemia was pneumonia increased from 6.6% to 15.4%. However, this then plateaued until FY 2020 to 2021 (range: 13.1% to 15.4%). The percentage ascribed to pneumonia has subsequently seen a sharp decline during FY 2021 to 2022 to 6.3%. However, for FY 2022 to 2023, the percentage of cases have seen a small increase to 7.2%. It is important to interpret these numbers with caution, as the underlying number (n) that the percentage is derived from can be small. Trends in sources of bacteraemia are shown in Table 21.

Table 21. MRSA counts and rates by source of bacteraemia, England, by financial year: April 2007 to March 2023

Financial year	Total MRSA	Source of bacteraemia reported: n (%)	Catheters and lines*: n (%†)	Skin and soft tissue: n (%†)	Pneumonia: n (%†)	Other**: n (%†)	Unknown: n (%†)
2007 to 2008	4,451	2,414 (54.2)	617 (25.6)	395 (16.4)	160 (6.6)	705 (29.2)	537 (22.2)
2008 to 2009	2,935	1,541 (52.5)	346 (22.5)	276 (17.9)	113 (7.3)	552 (35.8)	254 (16.5)
2009 to 2010	1,898	915 (48.2)	178 (19.5)	191 (20.9)	63 (6.9)	328 (35.8)	155 (16.9)
2010 to 2011	1,481	676 (45.6)	118 (17.5)	146 (21.6)	47 (7.0)	251 (37.1)	114 (16.9)
2011 to 2012	1,116	482 (43.2)	71 (14.7)	98 (20.3)	41 (8.5)	177 (36.7)	95 (19.7)
2012 to 2013	924	394 (42.6)	72 (18.3)	74 (18.8)	34 (8.6)	128 (32.5)	86 (21.8)
2013 to 2014	862	294 (34.1)	39 (13.3)	57 (19.4)	33 (11.2)	100 (34.0)	65 (22.1)
2014 to 2015	800	253 (31.6)	30 (11.9)	53 (20.9)	39 (15.4)	64 (25.3)	67 (26.5)
2015 to 2016	823	245 (29.8)	38 (15.5)	56 (22.9)	25 (10.2)	89 (36.3)	37 (15.1)
2016 to 2017	825	255 (30.9)	51 (20.0)	80 (31.4)	21 (8.2)	88 (34.5)	15 (5.9)
2017 to 2018	850	325 (38.2)	50 (15.4)	101 (31.1)	40 (12.3)	118 (36.3)	16 (4.9)
2018 to 2019	807	289 (35.8)	37 (12.8)	97 (33.6)	30 (10.4)	115 (39.8)	10 (3.5)
2019 to 2020	815	258 (31.7)	33 (12.8)	81 (31.4)	24 (9.3)	96 (37.2)	24 (9.3)
2020 to 2021	696	214 (30.8)	34 (15.9)	55 (25.7)	28 (13.1)	91 (42.5)	6 (2.8)
2021 to 2022	675	176 (25.1)	31 (17.6)	60 (34.1)	11 (6.3)	65 (36.9)	9 (5.1)
2022 to 2023	787	221 (28.0)	37 (16.7)	82 (37.1)	16 (7.2)	76 (34.4)	10 (4.5)

*‘Catheters and lines’ include the following options from the HCAI DCS question: dialysis lines, central venous catheter (CVC) associated, peripheral venous catheter (PVC) associated and intravenous (IV) lines

**‘Other’ includes the following options HCAI DCS: endocarditis, osteomyelitis, other, prosthetic joint, surgical site infection (SSI), septic arthritis, urinary tract and ventilator-associated pneumonia

† Percentage within those records ascertained

Geographic distribution of MRSA bacteraemia

Regional distribution of cases is presented across ICB for FY 2022 to 2023. While the overall rate of MRSA is comparatively low, there is some evidence of similar and relatively high infection rates among some neighbouring ICBs (Figure 37). Broadly speaking the majority of ICBs have an average rate of less than 1.4 MRSA bacteraemia cases per 100,000 population. The highest incidence rates were observed in; Bristol, North Somerset and South Gloucestershire ICB (3.0 per 100,000 population), Mid and South Essex ICB (2.5 per 100,000 population) and Bedfordshire, Luton and Milton Keynes ICB (2.4 per 100,000 population), while the lowest incidence rate were observed in Gloucestershire ICB (0.5 per 100,000 population), Norfolk and Waveney ICB (0.6 per 100,000 population) and Hertfordshire and West Essex ICB as well as Coventry and Warwickshire ICB (both 0.7 per 100,000 population).

Figure 37. Geographic distribution of MRSA rates per 100,000 population, England: financial year April 2021 to March 2023*

*Mid-year population estimates for January to December 2021 onwards were not available at time of publication and so population data for January to December 2020 was used as a proxy

Mortality

During FY 2022 to 2023, 787 MRSA bacteraemia cases were reported in England. Information on mortality was available for 98.5% (775 reports) of these cases. There were 174 deaths within 30 days of an MRSA bacteraemia diagnosis, a mortality rate of 0.3 deaths per 100,000 population. The CFR was 22.5% of cases.

The CFR in FY 2022 to 2023 has been declining since the start of the surveillance (FY 2007 to 2008) and is at its lowest, 22.5% versus 38.9%, respectively. The overall trend of mortality rate decreased from 2.6 to 0.3 deaths per 100,000 population between FY 2007 to 2008 and FY 2022 to 2023.

Figure 38. Case fatality rate and mortality rate of MRSA bacteraemia per 100,000 population, England: financial year April 2007 to March 2023*

Variation by onset of bacteraemia

The mortality rate of CO cases has remained consistent at 0.2 per 100,000 population since FY 2019 to 2020 (Table S11 of the supplementary dataset). The mortality rate of HO cases also remained at 0.2 deaths per 100,000 bed-days (68 deaths) during FY 2021 to 2022 and (85 deaths) during FY 2022 to 2023.

In FY 2022 to 2023, the CFR decreased in HO cases from 29.7% to 29.6% and decreased in CO cases from 24.3% to 18.2%, when compared to last financial year (FY 2021 to 2022).

Variation by age and sex

During FY 2022 to 2023, mortality rate and CFR increased with age. Over the same period, the mortality rate was greater in males, while CFR was near equal in both sexes.

Among male cases, the highest mortality rate was in those aged 85 years and over (7.6 deaths per 100,000 population) and those aged 75 to 84 years (2.5 deaths per 100,000 population), with CFRs being 51.9% and 41.1%, respectively.

In female cases, the mortality rate was also higher in the oldest age groups, 2.3 deaths per 100,000 population among those aged 85 years and over and 1.0 deaths per 100,000 population among those in the group aged 75 to 84 years. CFRs of these 2 groups were 46.5% and 35.8%, respectively.

Compared with other infections covered in this report, there were relatively fewer deaths in cases aged under one year compared with other age groups. During FY 2022 to 2023, there were 0 deaths within 30-days in <1 age groups in both sex groups.

Discussion

MRSA bacteraemia rates declined consistently each year between FY 2007 to 2008 and FY 2014 to 2015. In FY 2015 to 2016, the rate increased slightly but remained relatively stable until FY 2020 to 2021, when there was a considerable decline in the rate of all reported cases compared to the previous years. The rate remained at a similar level for FY 2021 to 2022 and returned to previous pre pandemic levels by FY 2022 to 2023. An increase in hospital-onset cases was observed during FY 2022 to 2023, with the rate increasing from 0.7 cases per 100,000 bed-days to 0.8. Indicating the return of MRSA rates to pre-pandemic levels. It should be noted that while the rate of hospital-onset increased during FY2020 to 2021; this was largely due to the significant decreases in hospital activity and increase in number of infections.

Numerous interventions aimed at reducing the incidence of MRSA bacteraemia and other infections have been introduced from the beginning of the surveillance programme for MRSA, contributing to the large reductions seen. These include the Department of Health (DoH) policy document ‘Winning Ways,’ published in 2003 (5). The ‘Clean Your Hands’ campaign launched by the National Patient Safety Agency in 2014 (6), the ‘Saving Lives’ programme launched by the DoH in 2005 which included the ambition to halve MRSA rates by 2008 (7), the 2006 Health Act which introduced a code of practice to provide guidance on reducing HCAI including MRSA (8), and the Health and Social Care Act, 2008 which requires the code of practice to be regularly updated (9).

The epidemiology of MRSA has changed since its peak in FY 2007 to 2008, and since FY 2010 to 2011 community-onset cases have been the most common. This switch in setting is most likely due to most MRSA bacteraemia interventions being concentrated in the acute care setting, and thus the largest reductions in MRSA bacteraemias were seen in hospital-onset cases. The duration of stay of hospital patients is also on the decline. In 2001 the average length of stay for a hospitalised patient was 7.4 days in the UK, this decreased to 6.2 days in 2019 (10). The reduced hospital stay lessens the risk of acquiring a hospital-acquired infection (HAI).

The percentage of MRSA bacteraemia where the likely source of infection was a catheter or a line has shown steady decrease since the early years of surveillance, however between FY 2019 to 2020 and FY 2021 to 2022, the percentage increased from 12.8% to 17.2%. In FY 2022 to 2023, the percentage decreased to 16.7% in line with earlier years of surveillance. Despite some fluctuations in the recent years, an overall decline is observed from the start of the surveillance period.

The initial declines in the percentage of MRSA bacteraemia where the most likely source of infection is a catheter or line have been noted previously and may have been due to greater clinical awareness of the importance of this route of infection, the introduction of care bundles aimed at reducing infections in intravascular lines and selective decolonisation of patients with MRSA carriage. After the increase observed in the last financial year, the decline in the current year may be an indicator of recovery from the pandemic but will require further investigation to determine exact cause. Please note that the numbers that make up this period are relatively small compared to those observed at the MRSA peak.

The percentage where the source of bacteraemia is something other than catheters or intravenous lines have fluctuated considerably over time. The percentage of infections whose likely source is skin or soft tissue infection does appear to have increased over time, from 16.4% in FY 2007 to 2008 to 37.1% in FY 2022 to 2023. However, during FY 2022 to 2023 only a quarter (29.2%) of records had information on the likely source of bacteraemia and therefore, interpretation of this data should be approached cautiously.

It is also noted that cases are more prevalent in the most deprived areas of England, and also those of an ethnic minority background, mostly black and Asian, highlighting potential health inequalities.

The marginal increase in the rate of hospital-onset cases but decrease in the percentage of infections due to catheters and lines since FY 2014 to 2015 may point to a need to continue concentration on Trust-based infection prevention initiatives to reduce hospital-onset cases further. In addition to maintaining good practice in an acute trust setting, interventions are required in the community setting, considering most MRSA cases are community-onset.

Meticillin susceptible Staphylococcus aureus bacteraemia

Total reports

A total of 13,125 cases of MSSA bacteraemia were reported by NHS acute Trusts in England during the financial year April 2022 to March 2023 (FY 2022 to 2023). This is an increase of 6.7% compared to FY 2021 to 2022 (n = 12,306), and an increase of 49.7% compared to FY 2011 to 2012 (n = 8,767). Figure 39 shows the trends in rates of MSSA cases for all cases and hospital-onset cases between the start of MSSA mandatory surveillance (in FY 2011 to 2012) and FY 2022 to 2023. The rate of all MSSA cases per 100,000 population, per year has risen from 16.4 during FY 2011 to 2012 to 23.2 in FY 2022 to 2023. This represents a 6.9% increase when compared with the rate of 21.7 cases per 100,000 observed during FY 2019 to 2020. This is comparable to the pre-COVID-19 period and marks an increase compared to FY 2020 to 2021, when the overall incidence rate fell to 20.7 infections per 100,000 population. This decline in the rate of all reported MSSA bacteraemia was the first decline in the incidence rate of MSSA since the inception of surveillance, and it coincided with similar declines observed in other organisms under surveillance, around the beginning of the COVID-19 pandemic.

Figure 39. Trends in the rate of MSSA bacteraemia in England, by financial year: April 2011 to March 2023

*Mid-year population estimates for January to December 2021 onwards were not available at time of publication and so population data for January to December 2020 was used as a proxy

Hospital-onset reports

Between FY 2011 to 2012 and FY 2019 to 2020, the incidence rate of hospital-onset infections has been steadily increasing from 8.2 cases per 100,000 population to 9.6, respectively, an increase of 17.1% (Table 22). In the following financial year (FY 2020 to 2021), the incidence rate of hospital-onset infections increased sharply to 12.1 cases per 100,000 population (n=3,341 of 11,705), a one-year increase of 26.6%, however, it must be noted, hospital activity was reduced compared to other years. The hospital-onset incidence rate has subsequently declined in FY 2022 to 2023, to 11.0 cases per 100,000 population (n=3,903 of 13,125), although the rate has declined it is still higher than the pre-pandemic period. The exact reasons for this increase in hospital-onset cases since FY 2020 to 2021 are unclear and under investigation but are likely related to the effect of the COVID-19 pandemic on hospital activity and population during that period. It is worth noting that overall, between FY 2011 to 2012 and FY 2022 to 2023, there has been a decrease in the percentage of hospital-onset cases of all reported cases of MSSA from 32.6% to 29.7%. This was preceded by the observed increase in the percentage of hospital-onset cases between FY 2019 to 2020 until FY 2021 to 2022, from the lowest point of 27.0% to 30.2%.

Table 22. MSSA counts and rates by financial year, England, by financial year: April 2011 to March 2023

Financial year	Mid-year population estimate*	All reported cases	Rate (all reported cases per 100,000 population)	Total bed-days	Hospital-onset cases	Rate (Hospital-onset cases per 100,000 bed-days)
2011 to 2012	53,312,604	8,767	16.4	34,669,499	2,854	8.2
2012 to 2013	53,475,357	8,812	16.5	34,633,855	2,700	7.8
2013 to 2014	53,976,973	9,290	17.2	34,514,871	2,696	7.8
2014 to 2015	54,432,437	9,863	18.1	34,972,728	2,807	8.0
2015 to 2016	55,018,883	10,608	19.3	34,752,604	2,921	8.4
2016 to 2017	55,240,934	11,499	20.8	35,148,014	3,098	8.8
2017 to 2018	55,707,642	11,955	21.5	34,903,075	3,154	9.0
2018 to 2019	56,053,563	12,103	21.6	34,538,184	3,330	9.6
2019 to 2020	56,468,265	12,242	21.7	34,637,156	3,311	9.5
2020 to 2021	56,550,138	11,705	20.7	27,628,155	3,339	12.1
2021 to 2022	56,550,138	12,306	21.8	32,905,086	3,714	11.3
2022 to 2023	56,550,138	13,125	23.2	35,478,624	3,903	11.0

*Mid-year population estimates for January to December 2021 onwards were not available at time of publication and so population data for January to December 2020 was used as a proxy

ICU-associated infections

ICU surveillance as part of ICCQIP started in May 2016. Between FY 2016 to 2017 and FY 2022 to 2023, 750 (3.14%, N=23,849) cases of hospital-onset MSSA infection were identified as ICU-associated (Figure 40).

After an initial rise in rates of ICU-associated MSSA bacteraemia between FY 2016 to 2017 and FY 2017 to 2018 from 0.31 to 0.34 cases per 1,000 ICU bed-days greater than 2 days, there was a subsequent period of decline with rates dropping to their lowest levels during FY 2019 to 2020 at 0.27 cases per 1,000 ICU bed-days greater than 2 days. However, between FY 2019 to 2020 and FY 2020 to 2021 the ICU-associated MSSA bacteraemia rate increased by 72.5% from 0.27 to 0.47 cases per 1,000 ICU bed-days greater than 2 days. This is the highest level recorded since bacteraemia surveillance in ICUs began and coincides with the peak in overall hospital-onset infections. This rate was maintained during FY 2021 to 2022 (0.47 cases per 1,000 ICU bed-days greater than 2 days). The rate then decreased back to 0.28 cases per 1,000 ICU bed-days greater than 2 days in FY 2022 to 2023, similar to the rate before the increase*.

Figure 40. Rate of ICU-associated hospital-onset MSSA bacteraemia cases, England, by financial year: April 2017 to March 2023**

*Due to technical issues data from one registered unit was unable to be processed.

**Includes 32 infections for MSSA bacteraemia reported to the mandatory surveillance programme and linked to the ICCQIP surveillance programme, but where S. aureus was not listed as the causative organism in the ICCQIP data

Prior trust exposure

In April 2019, the mandatory surveillance programme began capturing information on whether a patient had previously been admitted to the same reporting trust in the previous 28 days. With the prior trust exposure, cases split into specific groups, a full definition of these groups can be found in the appendix.

In comparison to FY 2021 to 2022, FY 2022 to 2023 observed a 5.1% increase in the count of HOHA MSSA bacteraemia from 3,714 to 3,903, however, a decrease in the rate of HOHA infections from 11.3 cases per 100,000 bed-days to 11.0 (Table 23). Following trend, the number of COHA increased 1.9% from 1,559 to 1,589 while the COHA rate decreased 4.7% from 4.3 to 4.1 cases per 100,000 bed-days and day admissions. Between the previous and reporting financial year, COCA increased from 6,961 to 7,592 (9.1%) which corresponded to an increase in the COCA rate (8.9%) from 12.3 to 13.4 cases per 100,000 population.

Table 23. MSSA counts by prior trust exposure by financial year, England, by financial year: April 2019 to March 2023

Financial Year*	Total bed-days	Total bed-days inc day-admissions	Mid-year population estimate	HOHA cases	HOHA rate (per 100,000 bed-days)	COHA cases	COHA rate (per 100,000 bed-days and day-admissions)	COCA cases	COCA rate (per 100,000 population)	Unknown **	No information ***
2019 to 2020	34,637,156	38,537,997	56,468,265	3,311	9.6	1,692	4.4	7,086	12.5	153	-
2020 to 2021	27,628,155	30,292,124	56,550,138	3,341	12.1	1,430	4.7	6,855	12.1	79	-
2021 to 2022	32,905,086	36,333,468	56,550,138	3,714	11.3	1,559	4.3	6,961	12.3	72	-
2022 to 2023	35,478,624	39,082,838	56,550,138	3,903	11.0	1,589	4.1	7,592	13.4	39	-

*Financial year from April to March

**The record indicates that it is unknown whether the patient was admitted to the reporting organisation in the past 28 days

***No information was entered in regard to the prior trust exposure

Rationale for denominator methodology can be found in the Prior trust denominator.

Age and sex distribution

For all age and sex analyses, cases in which the age or sex was missing or given as unknown were excluded. In the first financial year of MSSA mandatory surveillance (FY 2011 to 2012), 265 cases (3.0%) gave the age or sex as ‘unknown’ compared to 9 cases (≤ 1%) during FY 2022 to 2023.

Figure 41 compared the age and sex distribution of MSSA cases as a percentage of all reported cases in FY 2011 to 2012 and FY 2022 to 2023. Unlike MRSA, there has been no change to the distribution of cases by age and sex despite general increasing numbers of MSSA infection and increasing incidence rate. During FY 2022 to 2023, the bulk of the burden of disease was in adults aged 45 years and over with more infections among males in comparison to females. Furthermore, both males and females had the greatest percentage of cases among individuals aged 45 to 64 years at 17.6% and 8.3%, respectively. Compared to FY 2011 to 2012, this was an increase among males aged 45 to 64 years from 16.6% and a decrease for females from 8.6%, respectively. Conversely, the percentage of both males and females in the aged less than one year group declined during FY 2022 to 2023 in comparison to FY 2011 to 2012 to 1.6% and 1.1% respectively, a reduction from 3.0% and 2.2%.

Figure 41. Age and sex distribution of MSSA bacteraemia by percentage, England, financial years April 2011 to March 2012 and April 2022 to March 2023*

*Mid-year population estimates for January to December 2021 onwards were not available at time of publication and so population data for January to December 2020 was used as a proxy.

During FY 2022 to 2023, the incidence rates of MSSA bacteraemia are greater in male cases compared to females (Table 24 and Table 25). With the exception of those aged less than one year, the rate of infection increased for both sexes with each subsequent age group. This is consistent with what was observed during FY 2011 to 2012. During FY 2022 to 2023 both males and females had their highest rate of infection among individuals aged 85 years and over at 205.8 per 100,000 population and 94.9 per 100,000 population, respectively. This was also consistent with FY 2011 to 2012, although the rates of infection were higher during FY 2022 to 2023. Those aged less than one year are still a relatively high-risk group. Between FY 2011 to 2012 and FY 2022 to 2023, the rate in males aged less than one year declined by 4.9% from 71.5 to 68.0 per 100,000 population, while the rate in females fell between the 2 periods by 13.2% from 56.6 to 49.1 per 100,000 population.

Despite the general shift in the incidence of infection that has seen a reduction amongst the very young (less than one year) and an increase in the elderly (85 years and over), the rate ratio between the sexes remains broadly similar for all age groups (range: 1.2 to 2.2) between FY 2011 to 2012 and FY 2022 to 2023.

Table 24. MSSA counts and rates by age group and sex, England: financial year April 2011 to March 2012

Age group (years)	Population: Males	Population: Females	Cases: Males	Cases: Females	Rate per 100,000 population: Males	Rate per 100,000 population: Females	Rate Ratio (Males: Females)
<1	350,818	333,993	251	189	71.5	56.6	1.2 (95% CI: 1.1 to 1.4)
1-14	4,476,176	4,269,122	283	174	6.3	4.1	1.6 (95% CI: 1.4 to 1.7)
15-44	10,799,092	10,738,537	849	482	7.9	4.5	1.8 (95% CI: 1.6 to 1.9)
45-64	6,678,566	6,837,399	1,409	731	21.1	10.7	2.0 (95% CI: 1.8 to 2.1)
65-74	2,241,804	2,422,556	947	516	42.2	21.3	2.0 (95% CI: 1.9 to 2.1)
75-84	1,295,415	1,666,592	964	646	74.4	38.8	1.9 (95% CI: 1.8 to 2.0)
≥85	394,885	807,649	531	530	134.5	65.6	2.0 (95% CI: 1.9 to 2.2)

Table 25. MSSA counts and rates by age group and sex, England: financial year April 2022 to March 2023*

Age group (years)	Population: Males	Population: Females	Cases: Males	Cases: Females	Rate per 100,000 population: Males	Rate per 100,000 population: Females	Rate Ratio (Males: Females)
<1	308,815	293,098	210	144	68.0	49.1	1.4 (95% CI: 1.2 to 1.6)
1-14	4,929,714	4,682,857	288	209	5.8	4.5	1.3 (95% CI: 1.2 to 1.5)
15-44	10,817,218	10,581,832	1,179	758	10.9	7.2	1.5 (95% CI: 1.4 to 1.6)
45-64	7,135,608	7,336,977	2,311	1,090	32.4	14.9	2.2 (95% CI: 2.1 to 2.3)
65-74	2,691,641	2,906,787	1,531	742	56.9	25.5	2.2 (95% CI: 2.1 to 2.3)
75-84	1,574,092	1,885,089	1,728	1,008	109.8	53.5	2.0 (95% CI: 2.0 to 2.2)
≥85	525,730	880,680	1,082	836	205.8	94.9	2.2 (95% CI: 2.0 to 2.3)

*Mid-year population estimates for January to December 2021 onwards were not available at time of publication and so population data for January to December 2020 was used as a proxy

Between FY 2011 to 2012 and FY 2022 to 2023 the highest rates of MSSA infection amongst males were observed among individuals aged 75 to 84 years and 85 years and over when compared to the other age groups, increasing 47.5% from 74.4 to 109.8 per 100,000 population and 53.0% from 134.5 to 205.8 per 100,000 population, respectively (Figure 42). Similarly, to males, the highest rates of infection among females between FY 2011 to 2012 and FY 2022 to 2023 were observed among individuals aged 75 to 84 years and 85 years and over when compared to the other age groups, increasing 37.9% from 38.8 to 53.5 per 100,000 population and 44.7% from 65.6 to 94.9 per 100,000 population, respectively. It should be noted that those aged less than one year also have relatively high rates in comparison to other age groups. While the rates for individuals aged less than one year have fluctuated between FY 2011 to 2012 and FY 2022 to 2023, the rate of infection for both males and females was lower during FY 2022 to 2023 in comparison to FY 2011 to 2012.

Figure 42. Trend in age and sex rates per 100,000 population* of MSSA cases, England, by financial year: April 2011 to March 2023

*Mid-year population estimates for January to December 2021 onwards were not available at time of publication and so population data for January to December 2020 was used as a proxy

Trends by deprivation

The observed incidence rate of MSSA BSI increased with increasing deprivation. During FY 2022 to 2023, 29.3 cases occurred per 100,000 people living in the 20% most deprived areas of England, compared with 18.3 cases per 100,000 people living in the 20% least deprived areas.

Moreover, the age-standardised incidence rates show that if people living in these areas had the same age profiles, the difference in incidence of MSSA BSI would be much greater than observed, 35.4 and 16.4 cases per 100,000 population in the 20% most and least deprived areas, respectively.

Figure 43 and Supplementary Table 12 show the incidence of MSSA bacteraemia across 5 IMD levels between April 2018 and March 2023. Shaded areas indicate 95% confidence intervals.

Figure 43: Index of Multiple Deprivation distribution of MSSA bacteraemia, England, April 2018 to March 2023

These differences have been consistent between April 2018 and March 2023.

Trends by ethnicity

During FY 2022 to 2023, the observed incidence rate of MSSA bacteraemia was highest in the white ethnic group (24.5 cases per 100,000 White population), followed by the black and Asian ethnic groups (15.2 and 12.7 cases per 100,000 population, respectively) and finally by the mixed and other ethnic groups (7.1 and 6.5, respectively).

In contrast with other organisms, even after adjusting for age, the age-standardised incidence rate was still higher in the White ethnic group, although the difference was less pronounced than what the observed rates show.

Figure 44 and Supplementary Table 13 show the incidence of MSSA bacteraemia across ethnic groups between April 2018 and March 2023. Shaded areas indicate 95% confidence intervals.

Figure 44: Ethnic group distribution of MSSA bacteraemia, England, April 2018 to March 2023

During FY 2020 to 2021 there was a peak in incidence in the Asian ethnic group, mirrored by a dip in incidence in the white ethnic group.

Source of MSSA bacteraemia

The number of records reporting a source of bacteraemia has been steadily declining since the start of MSSA mandatory surveillance. In FY 2011 to 2012, a total of 3,305 (37.7%) records had entries for the source of bacteraemia. However, by FY 2022 to 2023, a total of 3,267 (24.9%) had entries for the source of bacteraemia.

The percentage of cases caused by skin and soft tissue infections has increased from 20.3% in FY 2011 to 2012 to 28.8% in FY 2022 to 2023. Similarly, the percentage of infections caused by pneumonia has also risen substantially from 6.0% in FY 2011 to 2012 to 12.2% in FY 2022 to 2023. The percentage of MSSA cases with catheter or line as the reported source of infection fluctuated between 13.1% and 15.7% between FY 2012 to 2013 and FY 2019 to 2020. However, since FY 2019 to 2020 the percentage of MSSA cases with catheter or line as the reported source of infection increased to 16.2% during FY 2020 to 2021 and then increased again to 17.6% during FY 2021 to 2022.The percentage of these cases has fluctuated again for FY 2022 to 2023, declining to 16.3, remaining in line with historic observations of this case burden.

Although the percentage of records for which the source of infection was not reported has increased, the percentage of cases for which the source of infection was reported as ‘unknown’ has decreased from 23.6% during FY 2011 to 2012 to 3.5% during FY 2022 to 2023. Trends in sources of MSSA bacteraemia are shown in Table 26.

Table 26. MSSA counts and rates by source of bacteraemia, England, by financial year: April 2022 to March 2023

Financial year	Total MSSA	Source of bacteraemia reported: n (%)	Catheters and lines*: n (%†)	Skin and soft tissue: n (%†)	Pneumonia: n (%†)	Other**: n (%†)	Unknown: n (%†)
2011 to 2012	8,767	3,305 (37.7)	565 (17.1)	670 (20.3)	197 (6.0)	1,093 (33.1)	780 (23.6)
2012 to 2013	8,812	3,266 (37.1)	492 (15.1)	699 (21.4)	232 (7.1)	1,088 (33.3)	755 (23.1)
2013 to 2014	9,290	3,236 (34.8)	435 (13.4)	684 (21.1)	218 (6.7)	1,124 (34.7)	775 (23.9)
2014 to 2015	9,863	3,398 (34.5)	445 (13.1)	706 (20.8)	305 (9.0)	1,087 (32.0)	855 (25.2)
2015 to 2016	10,609	3,224 (30.4)	493 (15.3)	769 (23.9)	306 (9.5)	1,170 (36.3)	487 (15.1)
2016 to 2017	11,499	3,185 (27.7)	500 (15.7)	871 (27.3)	365 (11.5)	1,280 (40.2)	169 (5.3)
2017 to 2018	11,955	3,529 (29.5)	516 (14.6)	1,003 (28.4)	445 (12.6)	1,440 (40.8)	125 (3.5)
2018 to 2019	12,103	3,732 (30.8)	579 (15.5)	1,030 (27.6)	450 (12.1)	1,595 (42.7)	78 (2.1)
2019 to 2020	12,242	4,152 (33.9)	633 (15.2)	1,175 (28.3)	503 (12.1)	1,729 (41.6)	112 (2.7)
2020 to 2021	11,705	3,565 (30.5)	579 (16.2)	968 (27.2)	447 (12.5)	1,497 (42.0)	74 (2.1)
2021 to 2022	12,306	3,401 (27.6)	600 (17.6)	968 (28.5)	352 (10.3)	1,391 (40.9)	90 (2.7)
2022 to 2023	13,125	3,267 (24.9)	533 (16.3)	941 (28.8)	398 (12.2)	1,282 (39.2)	113 (3.5)

*‘Catheters and lines’ include the following options from the HCAI DCS question: dialysis lines, central venous catheter (CVC) associated, peripheral venous catheter (PVC) associated and intravenous (IV) lines

**‘Other’ includes the following options HCAI DCS: endocarditis, osteomyelitis, other, prosthetic joint, surgical site infection (SSI), septic arthritis, urinary tract and ventilator-associated pneumonia

† Percentage within those records ascertained

Geographic distribution of MSSA bacteraemia

Regional distribution of MSSA bacteraemia cases for FY 2022 to 2023 are presented across ICB in Figure 45. It appears incidence rates of MSSA bacteraemia are similar in neighbouring ICBs, this is observed from the lowest rates to the highest. The ICBs with the highest rates were Cornwall and the Isles of Scilly (32.0 per 100,000 population), North East and North Cumbria ICB (31.8 per 100,000 population) and Somerset ICB (29.1 per 100,000 population) reporting the highest incidence rates. In contrast a large band of ICBs running from the West Midlands, through the Heartlands, Bedfordshire and into London have the lowest reported incidence rates. As such the lowest incidence rate were observed in the Gloucestershire ICB (16.2 per 100,000 population), South West London ICB (16.8 per 100,000 population) and Leicester, Leicestershire and Rutland ICB (17.7 per 100,000 population).

Figure 45. Geographic distribution of MSSA rates per 100,000 population, England: financial year April 2022 to March 2023

*Mid-year population estimates for January to December 2021 onwards were not available at time of publication and so population data for January to December 2020 was used as a proxy

Mortality

During FY 2022 to 2023, 13,125 MSSA bacteraemia cases were reported in England. Information on mortality was available for 97.6% (12,950) of these cases (Table S18 of the supplementary dataset). There were 2,837 deaths within 30 days of an MSSA bacteraemia diagnosis which gave a mortality rate of 5.1 deaths per 100,000 population. The CFR was 21.9%. Between FY 2021 to 2022 and FY 2022 to 2023, the trend in CFR declined from 22.1% to 21.9%. In the same period, the overall trend of mortality rate increased from 4.8 to 5.1 deaths per 100,000 population.

Figure 46. Case fatality rate and mortality rate of MSSA bacteraemia per 100,000 population, England: financial year April 2011 to March 2023*

• Mid-year population estimates for January to December 2021 onwards were not available at time of publication and so population data for January to December 2020 was used as a proxy

Variation by onset of bacteraemia

The CO of mortality rate cases increased from 3.0 deaths per 100,000 population (1,706 deaths) during FY 2021 to 2022 to 3.3 deaths (1,859 deaths) during FY 2022 to 2023. The mortality rate of HO cases decreased from 2.9 deaths per 100,000 bed-days (956 deaths), during FY 2021 to 2022 to 2.8 deaths per 100,000 bed-days (978 deaths) during FY 2022 to 2023.

The CFR of HO cases decreased from 26.3% during FY 2021 to 2022 to 25.4% during FY 2022 to 2023, while for CO cases, there was a marginal increase from 20.3% to 20.4% during the same period.

Variation by age and sex

During FY 2022 to 2023, the mortality rate and CFR increased with age, except among children aged under one year. Mortality rate was greater in male cases while CFR was close to equal in both sexes.

Among male cases, the highest mortality rates were in those aged 85 years and over (92.3 deaths per 100,000 population) and those aged 75 to 84 years (33.6 deaths per 100,000 population) with CFRs of these age groups at 45.2% and 30.9%, respectively. The mortality rate in males aged over 85 years has been increasing over time and was the highest since the start of surveillance during FY 2022 to 2023. In female cases, the mortality rates were also higher in older age groups, 45.8 deaths per 100,000 population (aged 85 years and over) and 17.5 deaths per 100,000 population (aged 75 to 84 years). These equate to a CFR of 48.3% and 32.8% of all cases in those respective age groups.

Among children aged under one year, the mortality rate in male cases was 3.6 deaths per 100,000 population compared with 2.4 per 100,000 population in female cases. Please note, the numbers of deaths in this group are small and data should be analysed with caution.

Discussion

The mandatory surveillance of MSSA bacteraemia was introduced in January 2011. The total number of MSSA bacteraemia has been steadily increasing throughout the 11-year surveillance period, although this trend has 2 components. Firstly, between the financial years encompassing April 2011 and March 2017 there were observed increases in community-onset bacteraemia from 5,913 to 8,401 (a 42.1% increase). The second component relates to hospital-onset infections during the time between the financial years April 2017 and March 2022. During this time the number of community-onset infections remained relatively stable while hospital-onset infections increased from 3,098 to 3,903 (26.0% increase).

In the first 2 financial years of surveillance, the MSSA bacteraemia all case rates were stable (between FY 2011 to 2012 and FY 2012 to 2013), before increasing each subsequent financial year until FY 2020 to 2021, when the rate fell from 21.7 cases per 100,000 population to 20.7. For FY 2022 to 2023, the rate has increased from 20.7 to 23.2, signifying a return to the trend seen in earlier years of surveillance. In contrast, hospital-onset rates have seen a small reduction from 11.3 to 11.0 in FY 2022 to 2023, compared to the previous year. Beyond this, hospital-onset rates have mostly been increasing annually and a sharp rise has been observed between FY 2019 to 2020 and FY 2020 to 2021 from 9.6 to 12.1 cases per 100,000 bed-days, respectively, a period which coincided with the total case incidence rate reduction. Subsequently, the incidence rate of hospital-onset infections declined to 11.0 cases per 100,000 population, although this figure in isolation remains relatively high compared to the pre-pandemic period. Concerningly, with the rise in incidence, all-cause mortality rates are also seen to be rising.

Age and sex structure of MSSA bacteraemia has changed little over the time since surveillance began. The trend shows that the number of infections and rate have increased more in the older age groups, particularly amongst males, with the highest rate seen in male aged 85 years and over. The highest all-cause mortality rates are also seen in the older aged population, particularly those aged 85 and over, and has been rising annually. Furthermore, males also generally show higher rates of MSSA bacteraemia compared to females. However, rates of MSSA bacteraemia are also seen to be very high in the youngest age group (under one year of age), compared to young cases (one to 14 years old) and adults (15 to 74 years old).

Research using data gathered by the mandatory surveillance found that cases arising in very young cases were most likely attributable to healthcare-associated infections, rather than community-associated infections and were related to intravascular devices (11). Despite the increase in MSSA bacteraemia rates over the years, little change has been observed in the rate ratios between males and females.

Similar to the Gram-negative and MRSA bactaeramias, those living in the most deprived areas have a disproportionately higher incidence of infection. However, age standardised ethnicity data shows the white ethnic group to have the higher incidence, unlike the other infections described earlier.

In FY 2022 to 2023, there appears to be no similarities between the geographical distribution of MRSA and MSSA. MSSA appears to be most prevalent in the northern and south-west regions of England, while rates of MRSA bacteraemia are more evenly distributed.

As rates of MRSA continue to fall, we see the MSSA bacteraemia rates increasing to concerning rates, associated with rises in the 30-day all-cause mortality rates, sparking concerns. Focus needs to be shifted to better understanding the driving factors and potential interventions.

Epidemiological analysis of Clostridioides difficile infection

Total reports

A total of 15,583^{[footnote ]} cases of Clostridioides difficile infection (CDI) were reported by NHS acute trusts in England during financial year 2022 to 2023 (FY 2022 to 2023). This is an increase of 9.2% from the previous financial year (FY 2021 to 2022, n = 14,276). The epidemiology of CDI for all reported cases can be broken down into 3 distinct periods. The first period, between FY 2007 to 2008 and FY 2013 to 2014, is characterised by a rapid decline in the incidence rate from 107.6 infections per 100,000 population to 24.8 per 100,000 population, representing a 77.0% decline. The second period is one of continued gradual decline and relative stability in the incidence rate, where there was little fluctuation between FY 2013 to 2014 (24.8 infections per 100,000 population) until FY 2020 to 2021. The third period, starting in FY 2021 to 2022 sees a shift in trend for the first time since the inception of the surveillance programme, with a significant increase in cases annually, with a rate of 27.6 infections per 100,000 population, an 11.3% increase in the most recent financial year. Overall, since the inception of mandatory CDI surveillance during FY 2007 to 2008, the total rate of CDI has decreased by 74.4%. Figure 47 shows the trends in rates of CDI cases for all cases and hospital-onset cases from FY 2007 to 2008 to FY 2022 to 2023.

Figure 47. Trends in the rate of CDI in England, by financial year: April 2007 to March 2023

*Mid-year population estimates for January to December 2021 onwards were not available at time of publication and so population data for January to December 2020 was used as a proxy

Hospital-onset reports

Of the 15,583 total cases reported in FY 2022 to 2023, 6,499 were hospital-onset (18.3 per 100,000 bed-days). It should be noted that CDI cases are considered hospital-onset if they occur 4 or more days after admission to an acute trust, where day of admission is day one. This data is provided for historical context only as this definition contrasts with 3 or more days for bacteraemia cases and with the definitions now imposed with the information on prior healthcare exposure for CDI. The incidence rate for hospital-onset CDI cases mirrors the trends in incidence for all cases in the early stages of the surveillance, with a decline of 83.7% in infection rates from FY 2007 to 2008 (89.3 infections per 100,000 bed-days) to FY 2013 to 2014 (14.6 per 100,000 bed-days). This declining trend with some fluctuations continued between FY 2013 to 2014 and FY 2018 to 2019 with the rate dropping from 14.6 cases per 100,000 bed-days to 12.2 cases per 100,000 bed-days (16.6% decrease). Following this, the rate of hospital-onset CDI cases has begun an upward trajectory, increasing each year consecutively from 12.2 cases per 100,000 bed-days during FY 2018 to 2019 to 18.3 cases per 100,000 bed-days, a 50.6 % increase (Table 17).

Table 27. CDI counts and rates by financial year, England, by financial year: April 2007 to March 2023

Financial year	Mid-year population	All reported cases	Rate (all reported cases per 100,000 population)	Total bed-days	Hospital-onset cases	Rate (Hospital-onset cases per 100,000 bed-days)
2007 to 2008	51,594,959	55,498	107.6	37,451,698	33,434	89.3
2008 to 2009	51,803,017	36,095	69.7	37,823,000	19,927	52.7
2009 to 2010	52,306,371	25,604	49	37,441,591	13,220	35.3
2010 to 2011	52,757,039	21,707	41.1	35,206,316	10,417	29.6
2011 to 2012	53,312,604	18,022	33.8	34,669,499	7,689	22.2
2012 to 2013	53,475,357	14,694	27.5	34,633,855	5,980	17.3
2013 to 2014	53,976,973	13,362	24.8	34,514,871	5,034	14.6
2014 to 2015	54,432,437	14,193	26.1	34,972,728	5,233	15
2015 to 2016	55,018,883	14,143	25.7	34,752,604	5,162	14.9
2016 to 2017	55,240,934	12,849	23.3	35,148,014	4,621	13.1
2017 to 2018	55,707,642	13,296	23.9	34,903,075	4,739	13.6
2018 to 2019	56,053,563	12,274	21.9	34,538,184	4,201	12.2
2019 to 2020	56,468,265	13,213	23.4	34,637,156	4,712	13.6
2020 to 2021	56,550,138	12,503	22.1	27,628,155	4,251	15.4
2021 to 2022	56,550,138	14,276	25.2	32,905,086	5,355	16.3
2022 to 2023	56,550,138	15,583	27.6	35,478,624	6,499	18.3

Prior trust exposure

In April 2017, the mandatory surveillance programme began capturing information on whether a patient with CDI had previously been admitted to the same reporting trust within the past 84 days (12 weeks). With the prior trust exposure, cases are split into specific groups, a full definition of these groups can be found in the appendix. Of note, unlike the bacteraemia where hospital-onset infections are the equivalent of HOHA infections, for CDI, the HOHA is based on an infection being detected within a hospital stay that is at least 3 days long, with the day of admission being counted as day one, compared to the historical definition used in the hospital-onset section where the hospital stay is required to be at least 4 days long to be defined as a hospital-onset infection.

The count of HOHA cases increased from 5,463 in FY 2017 to 2018 to 7,198 in FY 2022 to 2023 (Table 28). In contrast, the number of COHA cases increased 34.3% between FY 2017 to 2018 and FY 2022 to 2023 from 1,891 to 2,539. This increase was reflected in the COHA rate, which increased 33.2% over the same period, from 4.9 to 6.5 cases per 100,000 bed-days and day admissions. COCA cases increased 49.7% in the same period from 2,858 to 4,277 cases, this corresponded to an increase of 47.4% in the rate of COCA infection from 5.1 to 7.6 cases per 100,000 population. Counts of community-onset indeterminate association (COIA) also increased by 42.7% with an observed increase in the rate of infection from 1.9 to 2.7 cases per 100,000 population. Caution is advised in interpretation as the number of cases reported with no information declined from 1,758 (13%) records during the FY 2017 to 2018 period to one (<0.01%) during FY 2022 to 2023, as a result of making the questions around prior trust exposure mandatory.

Table 28. Prior healthcare exposure of CDI cases, England, by financial year: April 2017 to March 2023*

Financial Year*	Total bed-days	Total bed-days inc day-admissions	Mid-year population estimate	HOHA cases	HOHA rate (per 100,000 bed-days)	COHA cases	COHA rate (per 100,000 bed-days and day-admissions)	COCA cases	COCA rate (per 100,000 population)	COIA cases	Unknown **	No Information ***
2017 to 2018	34,903,075	38,781,512	55,707,642	5,463	15.7	1,891	4.9	2,858	5.1	1.9	1,064	262
2018 to 2019	34,538,184	38,421,535	56,053,563	4,869	14.1	2,318	6.0	3,442	6.1	2.4	1,335	305
2019 to 2020	34,637,156	38,537,997	56,468,265	5,358	15.5	2,472	6.4	3,803	6.7	2.7	1,525	53
2020 to 2021	27,628,155	30,292,124	56,550,138	4,897	17.7	2,210	7.3	3,934	7.0	2.5	1,436	26
2021 to 2022	32,905,086	36,333,468	56,550,138	6,034	18.3	2,445	6.7	4,182	7.4	2.8	1,580	35
2022 to 2023	35,478,624	39,082,838	56,550,138	7,198	20.3	2,539	6.5	4,277	7.6	2.7	1,541	27

*Financial year from April to March

**The record indicates that it is unknown whether the patient was admitted to the reporting organisation in the past 84 days (12 weeks)

***No information was entered in regard to the prior trust exposure

Rationale for denominator methodology can be found in the Prior trust denominator.

Seasonal trends in CDI

Seasonality is assessed as the number of hospital-onset cases each quarter as a percentage of total hospital-onset cases for the financial year or as the number of community-onset cases each quarter as a percentage of total community-onset cases for the financial year (Table S7 of the supplementary dataset). No clear trend in hospital-onset CDI (occurring in hospital admissions at NHS acute Trusts on day 4 or more, where day of admissions is considered day one) has been exhibited during the FY 2010 to 2011 and FY 2022 to 2023. Potential patterns of seasonal distribution have fluctuated. For example, during FY 2010 to 2011, 28.8% of hospital-onset cases were reported in the first quarter of the financial year, with declining percentages reported in the subsequent quarters of the financial year. Hospital-onset cases during the FY 2011 to 2012 period showed a similar distribution. However, an initial change was detected in FY 2012 to 2013 with a further shift in the distribution of CDI by financial quarter in the FY 2014 to 2015 period, when hospital-onset cases were more evenly distributed throughout the year. Between FY 2015 to 2016 and FY 2018 to 2019, the second financial quarter of the year (July to September) saw the greatest percentage of cases reported (between 26% and 29% of cases each year), with further shifts in the distribution for FY 2019 to 2020 and FY 2022 to 2023.

In contrast, community-onset cases have always shown a peak in the second quarter of the financial year, with this quarter forming 28% to 29% of cases for the financial year.

Age and sex distribution

For all age and sex analyses, cases in which the age and or sex was missing or given as unknown were excluded. In FY 2007 to 2008, the age or sex was reported as unknown for 667 cases (1.2%), while during FY 2022 to 2023, the age or sex was reported as unknown for 4 cases (<1.0%).

While number of infections have decreased dramatically over the course of CDI surveillance, there has been a subtle shift in the age and sex distribution of CDI infections (Figure 48). During FY 2007 to 2008 each increasing age group contributed a higher percentage of CDI infections towards the overall burden of infection; ranging from 0.3% in the 2 to 14 years age group to 9.3% in the 85 years and over group. During reporting financial year, the distribution among females had a marked increase in burden among the 2 to 14 years age group in comparison to the FY 2007 to 2008 period. During FY 2022 to 2023, 1.3% of female CDI infections were among the 2 to 14 years age group compared to 0.3% during the FY 2007 to 2008 period. Furthermore, during this financial year, 3.6% of female CDI infections were among the 15 to 44 years age group compared to 2.3% during the FY 2007 to 2008 financial period. In contrast, between the 2 years there has been a reduction within females in the oldest age group (85 years and over) from 9.3% during the FY 2007 to 2008 period, to 8.9% during the FY 2022 to 2023 period. The distribution among males has remained more consistent between the compared financial years, although the most recent data shows an increase from 0.3% to 1.3% years in the 2 to 14 years age group, and a slight decline among the 75 to 84 years age group from 14.5% to 13.5% between the 2 periods.

It is important to note that this only reflects percentages of cases and that, because of the age and sex structure of the population of England, the incidence rate by age and sex can be very different.

Figure 48. Age and sex distribution of CDI by percentage, England, by financial year: April 2007 to March 2023

Table 29 and Table 30 show counts and rates of CDI in FY 2007 to 2008 and FY 2022 to 2023. The rates have reduced across most age groups and for both sexes between FY 2007 to 2008 and FY 2022 to 2023, although the largest reductions have been observed among older age groups (45 years and over). The exception to this being the 2 to 14 years age group, where an incremental increase in rate has been observed in both for males and females from 4.4 to 4.6 and 4.1 to 4.2, respectively. During FY 2007 to 2008, the highest rate amongst males was 1,504.7 per 100,000 population and amongst females was 1,490.2 per 100,000 population. In the current reporting financial year, the highest rate remained among males and females aged 85 years and over but was substantially lower than the 2007 to 2008 financial period, at 262.3 per 100,000 population (82.6% decline) and 260.8 per 100,000 population (82.5% decline), respectively.

During the FY 2022 to 2023, the rate ratio between men and woman was similar across age groups, with slightly higher rates amongst females than males. While incidence rates have declined across most age and sex groups; except for the 2 to 14 years age group, between the financial years between April 2007 and March 2023, the rate ratios between the sexes remain similar in FY 2022 to 2023 to those observed during FY 2007 to 2008.

Table 29. CDI counts and rates by age group and sex, England: financial year April 2007 to March 2008

Age group (years)	Population: Males	Population: Females	Cases: Males	Cases: Females	Rate per 100,000 population: Males	Rate per 100,000 population: Females	Rate Ratio (Males: Females)
2-14	4,046,150	3,858,142	178	157	4.4	4.1	1.1 (95% CI: 0.9 to 1.3)
15-44	10,728,560	10,721,325	1,251	1,539	11.7	14.4	0.8 (95% CI: 0.8 to 0.9)
45-64	6,322,973	6,463,537	3,479	3,333	55.0	51.6	1.1 (95% CI: 1.0 to 1.1)
65-74	2,011,279	2,205,682	4,970	4,904	247.1	222.3	1.1 (95% CI: 1.1 to 1.2)
75-84	1,208,494	1,655,253	7,974	10,721	659.8	647.7	1.0 (95% CI: 1.0 to 1.0)
≥85	337,200	755,019	5,074	11,251	1504.7	1,490.2	1.0 (95% CI: 1.0 to 1.0)

Table 30. CDI counts and rates by age group and sex, England: financial year April 2022 to March 2023*

Age group (years)	Population: Males	Population: Females	Cases: Males	Cases: Females	Rate per 100,000 population: Males	Rate per 100,000 population: Females	Rate Ratio (Males: Females)
2-14	4,608,336	4,378,759	210	183	4.6	4.2	1.1 (95% CI: 0.9 to 1.3)
15-44	10,817,218	10,581,832	557	845	5.1	8.0	0.6 (95% CI: 0.6 to 0.7)
45-64	7,135,608	7,336,977	1,159	1,405	16.2	19.1	0.8 (95% CI: 0.8 to 0.9)
65-74	2,691,641	2,906,787	1,394	1,563	51.8	53.8	1.0 (95% CI: 0.9 to 1.0)
75-84	1,574,092	1,885,089	2,097	2,490	133.2	132.1	1.0 (95% CI: 1.0 to 1.1)
≥85	525,730	880,680	1,379	2,297	262.3	260.8	1.0 (95% CI: 0.9 to 1.1)

*Mid-year population estimates for January to December 2021 onwards were not available at time of publication and so population data for January to December 2020 was used as a proxy

The sharp decline in the overall all case rate of CDI between FY 2007 to 2008 and FY 2013 to 2014, followed by a period of relative stability, is reflected across most of the age sex categories. The sharpest declines, for both sexes occurred in the age groups of adults aged 45 years and over. Some exceptions to this were present. For both sexes in the aged 85 years and over age group, males and females had observed declines leading into FY 2020 to 2021 before increasing consecutively for the last 2 financial years. With the overall rate for the reporting period increasing, from 212.3 to 262.3 CDI per 100,000 population (males) and from 208.9 to 260.8 CDI per 100,000 population (females), respectively (Figure 49).

Figure 49. Trend in age and sex rates per 100,000 population* of CDI cases, England, by financial year: April 2007 to March 2023

*Mid-year population estimates for January to December 2021 onwards were not available at time of publication and so population data for January to December 2020 was used as a proxy

Trends by deprivation

The observed incidence rate of CDI increased slightly with increasing deprivation. During FY 2022 to 2023, 29.0 cases occurred per 100,000 people living in the 20% most deprived areas of England, compared with 26.3 cases per 100,000 people living in the 20% least deprived areas.

Moreover, the age-standardised incidence rates show that if people living in these areas had the same age distribution, the difference in incidence of CDI would be much greater than observed (38.2 and 23.1 cases per 100,000 population in the 20% most and least deprived areas, respectively).

Figure 50 and Supplementary Table 12 show the incidence of C. difficile infection across 5 IMD levels between April 2018 and March 2023. Shaded areas indicate 95% confidence intervals.

Figure 50: Index of Multiple Deprivation distribution of C. difficile infection, England, April 2018 to March 2023

These differences were consistent between April 2018 and March 2023.

Trends by ethnicity

During FY 2022 to 2023, the observed incidence rate of CDI was highest in the white ethnic group (30.9 cases per 100,000 white population), intermediate in the black and Asian ethnic groups (12.2 and 10.5 cases per 100,000 population, respectively), and lowest in the mixed and other ethnic groups (5.2 and 4.3, respectively).

Even after adjusting for age, the age-standardised incidence rate remained highest in the white ethnic group.

Figure 51 and Supplementary Table 13 show the incidence of C. difficile infection across ethnic groups between April 2018 and March 2023. Shaded areas indicate 95% confidence intervals.

Figure 51: Ethnic group distribution of C. difficile infection, England, April 2018 to March 2023

These differences were mostly consistent between April 2018 and March 2023.

Geographic distribution of CDI

Regional distribution of cases is presented across ICB for FY 2022 to 2023. There was no clear pattern to the distribution of CDI, although there were parts of England where rates were particularly high in the North West, followed by the East of England and parts of the South West (Figure 52). It was within these areas that the highest rates of CDI were observed. The 3 highest areas were Lancashire and South Cumbria ICB (45.9 per 100,000 population), Cornwall and the Isles of Scilly ICB (39.3 per 100,000 population) and Mid and South Essex ICB (37.9 per 100,000 population). Conversely, parts of the Midlands leading down into London and the South East generally had the lowest rates of CDI, many of the lowest incidence rates were reported from ICBs within these areas, with the 3 lowest rates being North East London ICB (12.9 per 100,000 population), North West London ICB (15.9 per 100,000 population) and Frimley ICB (16.9 per 100,000 population).

Figure 52. Geographic distribution of CDI rates per 100,000 population, England: financial year April 2022 to March 2023

*Mid-year population estimates for January to December 2021 onwards were not available at time of publication and so population data for January to December 2020 was used as a proxy

Mortality

During FY 2022 to 2023, 15,583 CDI cases were reported in England. Information on mortality was available for 99.0% (15,421) of these cases. There were 2,131 deaths within 30 days of a CDI, resulting in a CFR of 13.8%, which was a marginal increase from the 13.7% of the previous year (FY 2021 to 2022). In the same period, the mortality rate increased to 3.8 deaths per 100,000 population in FY 2022 to 2023 from 3.5 deaths per 100,000 population (FY 2021 to 2022).

Figure 53. Case fatality rate and mortality rate of C. difficile infection per 100,000 population, England: financial year April 2007 to March 2023

Variation by onset of bacteraemia

The mortality rate of CO cases remained at 1.5 deaths per 100,000 population (867 deaths) in the previous FY 2021 to 2022 and (833 deaths) during FY 2022 to 2023. The mortality rate of HO cases, however, increased from 3.2 deaths per 100,000 bed-days (1,053 deaths) in 2021 to 2022 to 3.5 deaths per 100,000 bed-days (1,298 deaths) during FY 2022 to 2023.

The CFR of HO cases increased from 20.0% to 20.2% between FY 2021 to 2022 and FY 2022 to 2023. CO cases however decreased from 9.9% to 9.3% between FY 2021 to 2022 and FY 2022 to 2023.

Variation by prior healthcare exposure

The mortality rate of HOHA cases was at 1.9 deaths per 100,000 bed days during FY 2019 to 2020. It then rose to 2.0 deaths per 100,000 bed days (1,089 deaths) during FY 2020 to 2021. It is now at the highest seen since the start of using these prior healthcare exposure definitions at 2.5 per 100,000 bed days (1,420 deaths).

The mortality rate of COCA cases has been relatively stable with fluctuations between FY 2018 to 2019 and FY 2022 to 2023, staying in the 0.4 to 0.6 deaths per 100,000 population range. The observed mortality rate for COCA cases in FY 2022 to 2023 was observed at 0.5. The COHA mortality rate remained consistent with retrospective rates at 0.5 deaths per 100,000 (FY 2022 to 2023) since the start of using these prior healthcare exposure definitions.

Variation by age and sex

CDI surveillance only covers cases aged over 2 years. The CFR following CDI infections increased with age. Mortality rate and CFR increased with age and was greater in male cases compared with female cases.

Among male cases, the highest mortality rates were in those aged 85 years and over (70.2 deaths per 100,000 population) and in those aged 75 to 84 years (23.5 deaths per 100,000 population), with a corresponding CFR of 27.0% and 17.8%, respectively. This was an increase compared to last FY 2021 to 2022’s mortality rates (62.6 in those aged 85 years and over and 20.1 in the 75 to 84 years age groups) and CFR (26.1% in those aged 85 years and over and 17.7% in the 75 to 84 years age groups)

In female cases, the mortality rates were also higher in older age groups, 50.9 deaths per 100,000 population (aged 85 years and over) and 17.0 deaths per 100,000 population (aged 75 to 84 years). These equated to CFRs of 19.7% and 13.0% of all cases in those respective age groups.

Discussion

Between FY 2007 to 2008 and FY 2013 to 2014, rates of CDI fell rapidly and since then there have been relatively stable rates of all reported cases. These sharp declines in CDI counts and rates is likely to be due to the care bundle introduced by the Department of Health and Social Care (DHSC) in 2007 which recommended the use of personal protective equipment, cohort nursing and environmental decontamination which were geared towards preventing HCAI in acute care settings and not community settings (8).

Despite the historic declines in the counts and rates of CDI, FY 2022 to 2023 continued to see a sharp increase in comparison to previous financial years and was the third consecutive financial year increase in the hospital-onset incidence rate that has been observed for CDI. A trend that has seen a 50.6% increase in hospital-onset incidence rate from 12.2 infections per 100,000 bed-days during FY 2018 to 2019 to 18.3 per 100,000 bed-days in FY 2022 to 2023 financial year. The exact reasons for the increases to both hospital-onset and community-onset cases are currently unclear and are under investigation, although colleagues within UKHSA have explored the distribution and rates of individual C. difficile ribotypes for the last 4 years using the Clostridioides difficile ribotyping network (CDRN) database. The data shows that there has not been an expansion of particular ribotypes to explain the increased incidence. The current hypothesis is that possible challenges in infection prevention and control during the pandemic, potential changes in antibiotic prescribing and/or increased CDI case ascertainment are more likely to explain the recent increased incidence.

Historically, CDI was primarily an issue within the hospital setting with around 60.2% of infections defined as hospital-onset (occurring on or after day 4 of an acute NHS Trust admission, where day of admission is counted as day 1). Many of the interventions aimed at the reduction of CDI rates were targeted at the hospital setting, and as a result the sharp decline that has been observed has disproportionately affected hospital-onset infections compared to community-onset infections. Consequently, community-onset cases now constitute the greater burden of all CDI for the reporting period (58.3%). However, the division of cases into hospital-onset and community-onset cases ignores the effect of any prior admissions to hospital which could increase the risk of CDI. For this reason, and to better align surveillance in England with that performed by the European Centre for Disease Prevention and Control (ECDC) and the Centres for Disease Control and Prevention (CDC), information on prior trust exposure was introduced in April 2017.

The prior trust exposure classification groups cases according to whether the patient has previously been admitted to the reporting organisation in the past 12 weeks (84 days). In addition, to further align with the ECDC and CDC definitions, a change was implemented within this categorisation so that HOHA infections were defined as those occurring on or after day 3 of an acute NHS Trust admission, which differs to the historical hospital-onset categorisation for CDI. The prior trust exposure classification has now been running for 5 years and from the first to the second year, data completion improved markedly due to changes in the way questions were asked in the DCS. HOHA cases formed the largest group by the new categorisation method. However, the counts of cases for community-onset groups have each increased since FY 2017 to 2018. During FY 2022 to 2023 the number of HOHA increased to its highest reported since the prior trust exposure surveillance began. COCA CDI cases have consistently increased year-on-year FY 2022 to 2023, by 49.7% across this time period. The underlying cause of this increase is unclear and currently under investigation.

Although the prior trust exposure only records admission to the reporting organisation, work by the mandatory surveillance team using data from the Hospital Episodes Statistics estimates that this captures over 80% of all hospital interactions.

Rates of CDI are highest among older age groups, with those aged 85 years and over being the highest rates for both sexes. Research has shown that over time, elderly individuals are getting frailer and experiencing polypharmacy (15). A frailer population, receiving greater levels of medication would suggest that greater levels of healthcare interaction were being experienced by this age group. Despite this, the greatest reductions between the FY 2007 to 2008 and FY 2022 to 2023 in the rates of CDI have been observed in both males and females in the oldest age groups (65 years and over), although reductions have been observed across most age groups due to the large reductions observed in CDI all case counts over the course of the mandatory surveillance programme. The exception to this being the 2 to 14 years age group where an incremental increase in rate was observed, reflective of the increase in cases in this group. However, it should be noted, the recent rise in incidence appears to be more prominent in those aged 85 years and over, and to a lesser extent those aged 75 to 84 years.

Cases of those aged 85 years and over remain the most frequently affected by CDI and there is little difference in the rates between the sexes, although the highest incidence rates tend to be found among females. This marks a difference in epidemiology between CDI and bacteraemia in which rates are higher among male cases for most age groups.

The multiple shifts in seasonality are intriguing and currently lack an explanation. One observation is that while counts and rates were in steep decline, most hospital-onset cases occurred in the April to June quarter of the financial year. When counts and rates began to stabilise there was a shift. More investigation is required.

Previous work by this group has indicated an association between higher levels of deprivation and higher rates of CDI and historically, there has been a North-South divide in rates of CDI. However, during FY 2022 to 2023 this was not obvious. This change in the geographical distribution requires further detailed investigation.

The most deprived regions again show the highest incidence. However, unlike the other bacteraemia’s (with the exception of MSSA), the white ethnic group had the highest incidence. It should be noted that mortality rates are also rising with the rise in incidence.

Several interventions aimed at reducing MRSA rates were also aimed at reducing CDI (the ‘Clean Your Hands’ and Saving Lives campaigns) (6). It is perhaps not surprising, then, that the trends in rates of CDI mirror those of MRSA. In addition, targets to reduce CDI cases were introduced in 2008. These aimed to reduce the number of cases reported annually to 30% of the FY 2007 to 2008 baseline count by FY 2010 to 2011, a target that was achieved by FY 2008 to 2009 (16). In addition to the objectives and the ‘Clean Your Hands’ and Saving Lives campaigns, guidance was issued aiming to reduce clindamycin, cephalosporin or fluoroquinolone prescribing, which had been shown to promote the spread of epidemic strains of C. difficile (16, 17). The resulting reduction in prescribing of fluoroquinolones and cephalosporins was associated with a significant decline in the incidence of CDI (18).

Future work

During the last 3 years of mandatory surveillance, with particular respect to changes observed during the COVID-19 period, there have been several areas of potential interest that have been identified as requiring further investigation. Some of these are summarised below.

The exact reasons for the increases in both hospital- and community-onset CDI counts are currently unknown, an initial descriptive investigation of ribotype distribution has not led to an explanation. It is unknown if changes in CDI during mandatory surveillance may have been impacted by sampling rates, whereby the more you test for C. difficile the more you might identify. As mentioned in previous reports, a joint DPhil project between the National Institute for Health Research (NIHR)-funded University of Oxford Health Protection Research Unit (HPRU) and the UKHSA, is underway assessing how sampling rates affect the rate of reported infections, alongside the importance of case-mix in acute NHS Trust settings and to model the ‘ideal’ testing rate based on case-mix assumptions.

In addition to the increases in CDI, it has been observed that there have been multiple shifts in seasonality. While counts and rates were in steep decline, most hospital-onset cases occurred in the April to June quarter of the financial year. In recent years, as hospital-onset infections have increased, there has also been an increase in the October to December quarter of each year. It has been hypothesised that this may be related to changes in prescribing and an investigation into the impact of broad-spectrum antibacterial prescribing, particularly quinolones and cephalosporins, on CDI over time is still under investigation.

In the last few financial years, there were substantial increases in MRSA and MSSA rates. While some of this may have been driven by a reduction in hospital activity (occupied overnight bed-days) in FY 2020 to 2021, increases in the count of hospital-onset infections were observed in both surveillance programmes. For MRSA the rate has increased again in FY 2022 to 2023 remaining consistent with pre-COVID-19 pandemic levels. MSSA has also increased in FY 2022 to 2023 compared to the FY 2020 to 2021 period, with the hospital-onset rate of infections remaining substantially higher in the FY 2022 to 2023 period compared to FY 2019 to 2020. As such, further investigation into the impact of both ICU admissions and COVID-19 co- or secondary infections has begun. While some of the work on ICU-associated cases has been presented in this report, the work on COVID-19 co- and secondary S. aureus infections is ongoing.

E. coli cases peaked in the October to December 2020 and January 2021 to March 2021 quarters, which is the opposite to what has been observed previously. This phenomenon was subsequently observed again for these same 2 quarters within FY 2022 to 2023. While these increases occurred during the second wave of COVID-19, it is unclear if these increases are related to the effect of the COVID-19 pandemic. Future data on seasonality will help us determine if this was a temporary shift or whether this marks a change in the long-term seasonality.

In addition, the observed large and sustained decreases in E. coli bacteraemia, particularly amongst community-onset infections, led to a University of Oxford HPRU and UKHSA collaborative project focussing on what may have caused these “missing” E. coli bacteraemia. This investigation is still currently underway.

In pursuit of a deeper understanding of health disparities and to contribute to the field of public health, ongoing research is being conducted to illuminate health inequities by IMD data in conjunction with ethnicity information. This epidemiological investigation aims to shed light on the socio-economic factors, geographic location, ethnic backgrounds and the prevalence of Gram-negative bacteraemia and C. difficile infections.

Appendix

Background to the mandatory surveillance of MRSA and MSSA bacteraemia

During the 1990s, voluntary surveillance of antibiotic resistance by microbiology laboratories across England, Wales and Scotland saw increasing reports of MRSA (4, 19). In response to the concerns about the rising number of reports of MRSA, DHSC made surveillance of MRSA bacteraemia mandatory from April 2001 (20).

UKHSA has managed the mandatory surveillance on behalf of DHSC since the inception of the surveillance. The surveillance initially captured aggregate counts of bacteraemia due to S. aureus and the number of those that were MRSA. The results were reported every 6 months.

The frequency of data collection was increased to every quarter in 2004 and then monthly in 2005. Enhanced surveillance of MRSA bacteraemia was introduced in October 2005, allowing reporting of individual cases, rather than aggregated counts for a time period. This had the advantage of providing information about the date of onset of bacteraemia, relative to the date of admission and the department or specialty in which the patient was being treated (21).

In 2011, the surveillance was further expanded to include enhanced surveillance of MSSA bacteraemia which had not shown the same decline in incidence that MRSA had after the initiation of mandatory MRSA surveillance during FY 2007 to 2008.

Between 2013 and 2014, a Post Infection Review (PIR) process was instigated and the outcome of these assigned cases to either a clinical commissioning group (sub-ICB) or an NHS acute trust. On 1 April 2014, a ‘third-party’ category for cases such as patients not resident in England or intractable cases was added to process (22). PIR data was collected on the HCAI DCS and in previous Annual Epidemiological Commentaries, the PIR outcomes data was published. The PIR process was halted in April 2018 when it became a local-only process (23).

Current surveillance of MRSA and MSSA bacteraemia allows reporting of cases through a web based DCS which provides the means to capture clinical information on the patient and the infection, the likely source of infection and previous healthcare interactions. The DCS also provides dynamic, on-demand reporting to acute trusts, ICBs and other organisations allowing those involved in the care of patients to investigate trends at a local level.

Mandatory surveillance of C. difficile infection

Voluntary surveillance of CDI by microbiology laboratories identified a steady increase in the number of reports between 1990 and 2001. From 2002, this increase accelerated rapidly (8). In 2004, quarterly surveillance of CDI in patients aged 65 years or over was made mandatory. This involved reporting aggregate numbers of cases to UKHSA on the behalf of DHSC. From 2007, the surveillance was expanded to include all patients 2 or more years old and was extended to enhanced surveillance to capture patient-level information (12). Between the FY 2007 to 2008 and FY 2018 to 2019, rates of CDI declined dramatically, however, between FY 2018 to 2019 and FY 2021 to 2022, the rates have begun to increase again.

In April 2017, prior healthcare exposure questions were introduced to the CDI mandatory surveillance programme, to determine onset status of cases. This involved further classification of cases into the following onset statuses, HOHA, COHA, COIA and COCA. Of note, the HOHA definition was also amended from the historical trust apportioning algorithm, so that CDI cases were categorised as being HOHA if a CDI was detected on or after day 3 of an acute NHS Trust admission, where the day admission is considered day one, compared to on or after day 4 in the historical trust apportioning algorithm. These changes and additions have allowed the CDI surveillance to be more comparable with international definitions.

Mandatory surveillance of Gram-negative bacteraemia

E. coli bacteraemia

Since the mid-2000s onwards, E. coli has been the most common pathogen causing bacteraemia in England and has seen year-on-year increases. Given this sustained increasing trend in the number of E. coli bacteraemia reports made through the voluntary service, DHSC made reporting of E. coli bacteraemia mandatory in June 2011.

Klebsiella and P. aeruginosa bacteraemia

From April 2017, it became mandatory for acute trusts to report bacteraemias caused by any species of Klebsiella bacteria and P. aeruginosa. This was introduced to support the UK Government’s ambition to reduce Healthcare-associated Gram-negative bacteraemias by 50% by the end of FY 2023-2024.

In 2017, Enterobacter aerogenes was reclassified to Klebsiella aerogenes. K. aerogenes was eligible for surveillance from April 2017 and trusts which identify isolates on or after 1 April 2017 are expected to report them as K. aerogenes.

A note on terminology

In FY 2018 to 2019, mandatory surveillance stopped using the term Trust-apportioned and started using the terms hospital-onset and community-onset instead. This change was introduced due to the need to increase awareness of cases that occur in the community. The algorithm for the separation of cases into hospital-onset or trust-apportioned and those that are not, is unchanged. Clostridium difficile taxonomy classification has been changed to Clostridioides difficile.

Please note the data included K. aerogenes (previously Enterobacter aerogenes) in surveillance under the Klebsiella genus data collection (24).

Use of mandatory surveillance statistics

The data presented in this commentary, and in the accompanying data files serve several purposes:

• they provide information to clinicians in trusts about rates of bacteraemia and CDI in their organisation, helping to improve care and infection control at their trust
• they provide information on the epidemiology of these infections to clinicians, healthcare researchers and other interested parties, identifying the likely sources of infection
• the information at sub-ICB level allows commissioners of care to understand healthcare-associated infection rates in their catchment area
• the national picture provides information to DHSC, NHS England and Improvement regarding the infection rates across the country, and how these are changing over time, while also providing information about where new interventions could be targeted
• they are used by NHS Choices to assist in providing information to the general public about healthcare-associated infection rates in their area and in facilities where they might receive care

Further information on data purpose, relevance and associated user-need can be found in the Mandatory Health Care Associated Infection Surveillance Data Quality Statement and data included in the April 2020 to March 2021 Annual Epidemiological Commentary (AEC).

Counts and rates of MRSA, MSSA, Gram-negative bacteraemias and CDI included in this report are those with dates of specimens found to be positive within the period 1 April 2007 to 31 March 2023. This report includes data, extracted from the HCAI DCS on 25 April 2022, from 137 NHS acute trusts, 106 sub locations ICBs, each mapped to 42 ICBs. Data is published in line with organisational arrangements on the HCAI DCS as of 31 March 2023. Data by national level trend, acute trust and sub-ICB is presented on an annual (Annual Epidemiology Commentary) basis. National level trend data is also produced on a quarterly (Quarterly Epidemiology Commentary) basis. Data tables included in the Annual Epidemiological Commentary can also be found in OpenDocument Spreadsheets (.ods) format on the Annual Epidemiological Commentary webpage.

This publication forms part of a range of National Statistics outputs routinely published by UKHSA. Epidemiological analyses included in this report are on an annual (financial year) basis. Further epidemiological analyses by quarter can be found in our quarterly epidemiological commentaries. Data is also reported monthly for MRSA, MSSA, E. coli, Klebsiella spp., P. aeruginosa bacteraemia and CDI.

We are always striving to ensure that routine outputs meet user need as much as possible. If you have any suggestions for changes or additions, please email mandatory.surveillance@ukhsa.gov.uk

Commentary

This document contains national and regional level (sub-ICB) epidemiological commentaries for Gram-negative bacteraemias, MRSA and MSSA bacteraemias and C. difficile infections. Mandatory surveillance data series included in this report start from FY 2007 to 2008 or the earliest full quarter of data collection.

However, ICBs only came into existence on 1 July 2022. These may become less accurate the older the data is. The temporal data before FY 2013 to 2014 contained in this report with regards to ICBs and sub-ICBs has only been provided as an indication of the trend over time for a given ICB or sub-ICB and thus, should be treated with caution.

Alternative presentations of the annual data

Data included in this report is also available on Fingertips. This is a user-friendly application that enables access to local data. It is ideal for both healthcare professionals and the general public alike. Fingertips enables access to these data in a succinct format. Much of the mandatory surveillance data is available in graphical format, facilitating easy understanding of important trends and geographical differences.

Please note that there is a one-month delay between the publication of our statistics on GOV.UK and the publication of the same data on Fingertips. For example, data published on GOV.UK at the start of April 2022 would be available on Fingertips in May 2022.

Glossary

Average

Scientifically speaking, this is a measure of location. It is a way of describing data and helps to distribute any inequalities in the data across the whole series. There are 3 main mathematical measures which can be used to calculate an ‘average’ value: the mean, mode and median. Each of these methods has their own strengths and weaknesses.

Bacteraemia

The presence of bacteria in blood.

Bias

Bias is the systematic deviation of either results or inferences from the real situation.

Confidence interval (CI)

Confidence intervals indicate the likely range in which an estimated parameter (such as a mean or rate) is likely to fall. For most scientific studies, it is impractical or impossible to measure every single member of a population and therefore the true population mean cannot be determined. Instead, a representative sample is taken, and the sample mean is used as an estimate of the population mean. Although the sample is intended to be representative, a different sample from the same population may provide a different result simply by chance. A confidence interval, over unlimited repetitions of the sample, should contain the true value of a parameter (such as the true population mean) no less than its confidence interval. It is usual to calculate the 95% confidence interval. That means that if we were to draw several independent, random samples from the same population and calculate 95% confidence intervals from each of them, then 95% of such confidence intervals would contain the true population mean. If we took 20 samples from the same population and calculated 95% confidence intervals, then 19 of 20 (95%) of these 95% confidence intervals would contain the true population meanwhile 1 of 20 (5%) will not.

Denominator

The lower portion of a rate or ratio. This should reflect the population at risk of developing a disease.

Epidemiology

Study of the occurrence and distribution of events (mostly health-related) in a population.

Gram-negative bacteria

Class of bacteria that do not retain crystal violet stain as used as part of a differential staining technique (called the Gram stain). The Gram stain is used as a way of identifying bacteria and the difference in staining results are due to differences in the bacterial cell wall, which has important implications for antimicrobial usage.

Incidence and incidence rate

New cases of a disease occurring in a study population. An incidence rate is then the number of new cases that occur in a defined population in a defined period of time.

ICU bed-days

This is reported by units as the total number of bed days each month from patients who have spent more than 2 nights in the ICU, that is, bed days from a patient’s third night in the ICU and onwards are reported.

NHS ICBs

An administrative unit of the NHS. NHS England has 4 administrative regions: North of England, Midlands and East of England, London and South of England. Below these regions are 42 administrative geographies referred to as ICBs.

Mean

The arithmetic mean is often what people think of when they say, ‘average value’. The mean is calculated by summing all of the values in a series ( ) and then dividing by the number of values included in the series ( ). Mathematically, this is described by the following formula:

mean = (a1 + a1 + … + an) / n

A real-world example would be if you wanted to calculate the mean amount spent on food shopping over a 4-week period (that is, the average amount per week) having spent £51 in week one, £59 in week 2, £67 in week 3 and £52 in week 4:

mean cost of food per week = (£51 + £59 + £67 + £52) / 4 = £57.25

Median

The median of a series of numbers is the mid-point of that series. This provides a measure of an average value that is not overly affected by a few extreme values. The median of the following set of numbers [1, 2, 3] is 2, while the median of the set of numbers [1, 1, 1, 2, 10, 15, 16, 20, 100, 105, 110] is 15. To calculate the median value, the set of numbers needs to be arranged in order of magnitude, the median is the number that is exactly in the middle. If there is an even number of values in a set, then the median value is the arithmetic mean of the 2 central values.

Mode

Is the most frequent value in a set of data (numbers or text values), in the following set of numbers [1, 1, 1, 2, 10, 15, 16, 20, 100, 105, 110] the mode is 1 as it was included in the set 3 times, while the other numbers were only included once.

Rate ratio

Is the ratio between 2 rates. For example, if the rate of MRSA bacteraemia was 2 per 100,000 population in a year among men, and 4 per 100,000 population in a year among women, the rate ratio would be 2.0. The rate would be 2 times higher among women than men.

Secular trends

Changes over long periods of time.

Methods

Inclusion criteria for reporting to the surveillance system

MRSA bacteraemia

The following positive blood cultures must be reported to UKHSA, for the mandatory MRSA surveillance: all cases of bacteraemia caused by S. aureus resistant to methicillin, oxacillin, cefoxitin or flucloxacillin.

MSSA bacteraemia

The following positive blood cultures must be reported to UKHSA, for the mandatory MSSA surveillance: all cases of bacteraemia caused by S. aureus which are susceptible to methicillin, oxacillin, cefoxitin, or flucloxacillin, that is, not subject to MRSA reporting.

E. coli bacteraemia

The following E. coli positive blood cultures must be reported to UKHSA: all laboratory confirmed cases of E. coli bacteraemia.

C. difficile infection

Any of the following defines a C. difficile infection in patients aged 2 years and over and must be reported to the UKHSA:

• diarrhoea stools (Bristol Stool types 5 to 7) where the specimen is C. difficile toxin positive
• toxic megacolon or ileostomy where the specimen is C. difficile toxin positive
• pseudomembranous colitis revealed by lower gastro-intestinal endoscopy or Computed Tomography
• colonic histopathology characteristic of C. difficile infection (with or without diarrhoea or toxin detection) on a specimen obtained during endoscopy or colectomy
• faecal specimens collected post-mortem where the specimen is C. difficile toxin positive or tissue specimens collected post-mortem where pseudomembranous colitis is revealed or colonic histopathology is characteristic of C. difficile infection

Methods

Methods of reporting data on the HCAI Data Capture System (DCS)

The HCAI DCS is a web portal designed by UKHSA to collect an enhanced data set.

Trusts using the website have access to all the data they have entered, which enables them to assess their burden of these HCAIs. This can be compared to a regional and national aggregate total also available to trusts from the website. Clinical Commissioning Groups (sub-ICBs), local authorities, ICBs, and Directors of Public Health (DPH) are also able to register as users, allowing them to access data specific to their patients.

The data set to be collected is described in the mandatory HCAI surveillance protocol and in the case capture user guide available on the same site. Case unlocks can be requested by reporting organisations using the process described in the Unlock Requests User Guide. Revisions to data is covered by a data specific revisions and correction policy.

An R package for working with data downloaded from the DCS can be found on GitHub.

Deadline for entering data

All cases reported by the NHS with specimen dates during the previous month must be entered onto the website by the fifteenth of the following month. The previous month’s data must then be signed off by the trust’s Chief Executive Officer (CEO) by the fifteenth of every month. For example, data concerning specimens collected in October must be entered and signed off by 15 November.

ICB attribution process

All cases of bacteraemia and C. difficile infection are attributed to an ICB, regardless of onset.

UKHSA’s HCAI DCS does not currently request NHS organisations to record patient ICB details for any of bacteraemia or C. difficile infection cases. To obtain this data an extract, comprising patient NHS number, date of birth, patient forename, patient surname and sex are submitted to NHS Digital, via Demographics Batch Services (DBS) on a daily basis and matched in a 2-stage algorithm using a combination of the provided patient details, to identify patient GP registration details and patient residential postcode.

Overview of sub-ICB attribution

The sub-ICB for each case is attributed, in the following order:

• if the patient’s GP practice code is available (and is based in England), the case will be attributed to the ICB at which the patient’s GP is listed
• if the patient’s GP practice code is unavailable but the patient is known to reside in England, the case is attributed to the ICB catchment area in which the patient resides
• if both the patient’s GP practice code and patient post code are unavailable or if a patient has been identified as residing outside England, then the case is attributed to a ICB based upon the postcode of the Head Quarters of the acute trust that reported the case

Note that the retrospective attribution of cases to an ICB may become less accurate the older the data is. Therefore, the data contained in this report for time periods prior to FY 2013 to 2014 should be treated with caution and only used as an indication of the trend over time for a given ICB.

Algorithms for apportioning cases

Please note that the algorithm applied for the determining onset of bacteraemia versus CDI infection uses a different number of days between specimen collection and admission to apportion cases, but the principle is the same. All cases of bacteraemia and CDI infection are either hospital-onset or community-onset based on the algorithms below.

It is not possible for UKHSA to change the onset status of a case, as this process is based on the data entered by the acute trust and the algorithm is applied to the entire data set not on a case-by-case basis; a case may only change from one category to another if the relevant case details are incorrect and require amendment by the trust.

In addition to onset, all cases are also attributed to a ICB (see above). Thus, all hospital-onset and community-onset cases will be attributed to an ICB. The apportioning algorithm changed slightly with the launch of the new DCS.

Bacteraemia: hospital-onset

Any NHS patient specimens taken on the third day of admission onwards (for example day 3 when day 1 equals day of admission) at an acute trust (including cases with unspecified specimen location) for Inpatients, Day-patients, Emergency Assessment, or unspecified patient category.

Records with a missing admission date (where the specimen location is acute trust or missing and the patient category is Inpatient, Day-patient, Emergency Assessment, or unspecified) are also included.

Bacteraemia: community-onset

Any NHS patient specimens not apportioned to the above. This will typically include the following groups:

• any acute trust specimens taken on either the day of admission or the subsequent day (for example, days 1 or 2, where day 1 equals day of admission)
• any specimens from patients attending an acute trust who are not Inpatients, Day patients or under Emergency Assessment (such as non-admitted patients)
• any specimens from patients attending an identifiable healthcare location except an acute trust, and will typically include GP, nursing home, non-acute NHS hospital and private patients

ICU-associated

Hospital-onset infections that are considered associated with a patient’s stay in ICU are unavailable directly from Mandatory surveillance. Instead, these are calculated by linking records from Mandatory surveillance with those reported to the ICCQIP surveillance. Hospital-onset infections that are considered ICU-associated are defined as infections with a specimen date more than 2 days (or more than 48 hours where times are submitted) after the ICU admission date (where ICU admission is day 1) and with an ICU specimen date within ≤ 2 days of the mandatory surveillance specimen date recorded for the same patient (see analysis of data for linkage methods). Only a subset of ICUs across England routinely report data to the ICCQIP surveillance system and so ICU associated data must be interpreted with caution.

CDI: hospital-onset

Any NHS patient specimens taken on the fourth day of admission onwards (for example, day 4 when day 1 equals day of admission) at an acute trust (including cases with unspecified specimen location) for inpatients, day-patients, emergency assessment, or unspecified patient category. Records with a missing admission date (where the specimen location is acute trust or missing and the patient category is inpatient, day-patient, emergency assessment, or unspecified) are also included.

CDI: community-onset

Any NHS patient specimens not apportioned to the above. This will typically include the following groups:

• any acute trust specimens taken on either the day of admission or the subsequent day (for example, days 1, 2, 3 where day 1 equals day of admission)
• any specimens from patients attending an acute trust who are not inpatient, day-patient or under emergency assessment (for example, non-admitted patients)
• any specimens from patients attending an identifiable healthcare location except an acute trust, which will typically include GP, nursing home, non-acute NHS hospital and private patients

Prior trust exposure algorithm

From April 2017, reporting trusts were asked to provide information on whether patients with CDI had been admitted to the reporting trust within the 12 weeks (84 days) prior to the onset of the current case. This allows a greater granulation of the healthcare association of cases. This was extended to all other data collections in April 2019.

Cases are split into one of 6 groups for CDI and 5 groups for the bacteraemias.

CDI prior trust exposure categories are:

1. HOHA: date of onset is greater than 2 days after admission (where day of admission is day 1).
2. COHA: is not categorised HOHA and the patient was most recently discharged from the same reporting trust in the 28 days prior to the specimen date (where day 1 is the specimen date).
3. COIA: is not categorised HOHA and the patient was most recently discharged from the same reporting trust between 29 and 84 days prior to the specimen date (where day 1 is the specimen date).
4. COCA: is not categorised HOHA and the patient has not been discharged from the same reporting organisation in the 84 days prior to the specimen date (where day 1 is the specimen date).
5. Unknown: the reporting trust answered ‘Don’t know’ to the question regarding previous discharge in the 3 months prior to CDI case.
6. No Information: the reporting trust did not provide any answer for questions on prior admission.

Bacteraemia prior trust exposure categories

April 2019, the HCAI DCS has included questions relating to prior trust exposure to the same acute trust reporting Gram-negative bacteraemia cases. Please note that for bacteraemias prior trust exposure is only asked of the most recent 4 weeks (28 days) rather than 12 weeks as per CDI. These additional mandatory items were developed to assist the UK governments ambition to reduce healthcare-associated Gram-negative bacteraemia and CDI from a 2019 baseline to threshold levels. This supports the intention to reduce Gram-negative BSI by 50% by March 2024.

Cases since April 2019 have also been categorised as:

1. HOHA: date of onset is greater than 2 days after admission (where day of admission is day 1).
2. COHA: is not categorised HOHA and the patient was most recently discharged from the same reporting trust in the 28 days prior to the specimen date (where day 1 is the specimen date).
3. COCA: is not categorised HOHA and the patient has not been discharged from the same reporting organisation in the 28 days prior to the specimen date (where day 1 is the specimen date).
4. Unknown: the reporting trust answered ‘Don’t know’ to the question regarding previous discharge in the month prior to the current episode.
5. No Information: the reporting trust did not provide any answer for questions on prior admission.

Prior trust denominator

In this report prior trust categories use different denominator data when calculating rates:

• HOHA: the infection occurred within hospital and is healthcare associated, hospital overnight bed-days are used as a denominator as the patient has already been admitted to hospital
• COHA: the infection occurred within the community but is healthcare associated, hospital overnight bed-days and hospital day-only are used as a denominator; the addition of day only is to account for community cases who have not been admitted and may initially present as day-only
• COCA: the infection occurred within the community and is community associated; population data is used in the rate calculation

Analysis of data

Time to onset calculations

To describe time to onset of an episode (bacteraemia or CDI) among inpatients, the number of days between the date of admission to an NHS acute trust and the date of positive specimen were calculated. This was only performed for patients who were admitted to an acute trust (defined as either an inpatient, day patient or emergency assessment, for example patients who should have an admission date to an acute trust) and for those whose specimen was taken on or after the date of admission also at an NHS acute trust.

The number of days between the date of admission and the date of specimen was then grouped into meaningful categories of the number of days.

Denominator data

Trust denominators NHS acute trust-level population data does not currently exist in England as NHS acute trusts do not treat patients within defined geographical boundaries. Therefore, a suitable proxy for population is required to calculate hospital-onset and HOHA rates. The occupied overnight bed-days (from the national KH03 data set) provides the daily average overnight bed occupation for a specific time period: FY 2007 to 2008 to FY 2009 to 2010 and by quarter for FY 2010 to 2011 to FY 2021 to 2022. This data set is an open access return published by NHS England and provides a measure of clinical activity in each trust, which is used as a proxy measure of the patient population.

KH03 data can be found on the NHS England website.

Where data for trusts were missing, data for the same quarter in the preceding year were used. These included the following trusts and periods:

1. Moorfields Eye Hospital NHS Foundation Trust (RP6)
   KH03 data was missing for FY 2007-2008. Last recorded data from 2006 to 2007 were used as a proxy.
   
2. Moorfields Eye Hospital NHS Foundation Trust (RP6)
   KH03 data was missing for FY 2007-2008. Last recorded data from 2006 to 2007 were used as a proxy.
   
3. The Rotherham NHS Foundation Trust (RFR)
   KH03 data was missing for FY 2009 to 2010, quarter 1 FY 2010 to 2011 to quarter 1 of FY 2011 to 2012. Last recorded data from April 2008 to March 2009 were used as a proxy.
   
4. Sheffield Teaching Hospitals NHS Foundation Trust (RHQ)
   KH03 data was missing for quarter 1 FY 2010 to 2011 to quarter 1 FY 2011 to 2012. Data from 2009 to 2010 were used as a proxy.
   
5. The Princess Alexandra Hospital NHS Trust (RQW)
   Data was missing from quarter 1 FY 2014 to 2015. Data from quarter 1 FY 2013 to 2014 were used as a proxy. Data was missing from quarter April 2014 to March 2015 and quarter 3 April 2013 to March 2014 data was used as a proxy.

In Annual Epidemiological Commentaries published prior to 7 July 2016, April to June 2014 to October to December 2014 quarterly KH03 figures for one acute trust (Lincolnshire Hospitals NHS Trust (RWD)) had a percentage change of more than 20% compared to the previous quarter and the same quarter in the previous year. As a result, it was replaced with the KH03 data of the same quarters in the previous year (April to June 2013 to October to December 2013). However, UKHSA has reviewed its policy for processing KH03 data. All data irregularities identified are now flagged with colleagues at NHS England (data owners of the KH03 data set). Until we receive confirmation that any identified change in the occupied overnight bed-days for an acute trust is anomalous, UKHSA will use the data as published in the KH03 data set. This affects all reports published since 1 December 2015. In order for the KH03 data used to calculate rates included in this report to be consistent over the full-time period, previously amended KH03 data for trust RWD for FY 2014 to 2015 has been altered to reflect that published in the KH03 data set. Please note that this could lead to slight differences in trust-apportioned or assigned rates when compared with publications prior to 1 December 2015.

Other affected organisations and time periods include:

• Shrewsbury and Telford Hospital NHS Trust (RXW) FY 2009 to 2010
• Imperial College Healthcare NHS Trust (RYJ) April to June 2012

ICU denominators

Each quarter units submit infection rate denominator data for each month in the previous quarter to the ICU DCS directly. These are:

• the total number of occupied bed-days for each unit per month (defined as patient bed-days)
• the total number of occupied bed-days for each unit per month, restricted to patients who have spent more than 2 nights in the ICU (where ICU admission is day 1) (defined as ICU bed-days)
• the total number of occupied bed-days for patients with a CVC in situ (defined as CVC days)
• the total number of occupied bed-days for patients with a CVC in situ restricted to patients who have spent more than 2 nights in the ICU (defined as ICU-CVC days)
• the total number of blood culture sets taken by each unit per month

ICU bed-days are used as the denominator for the rates of ICU-associated hospital-onset infections.

Units may submit data on denominators in daily or monthly reports. If only monthly totals have been provided by a unit, then this data is used. If only daily denominator data is submitted, the ICU DCS calculates a monthly value as:

Monthly value = (sum of daily values of metric / total days denominator data has been entered) x number of days in the month

If a unit has both daily data and total monthly data entered (as opposed to monthly calculated values from the daily data) then the entered monthly data is used in rate calculations unless a unit had informed in writing that the daily data aggregated to monthly totals should be used instead of the entered monthly data.

Denominator data is cross-checked for plausibility with specific validation rules and are considered missing when units submit positive blood culture data but no denominator data for the same time period. Denominator data that breaches validation rules (for example, a greater number of ICU bed-days than patient bed-days) or which is missing (even though data on positive blood cultures is submitted for the same period) is verified with the units who may update the records. Missing denominator data and denominator data that continues to breach validation rules or erroneous data (notified by the unit) is deleted and is imputed from other data entered by the same unit. Positive blood culture data (numerator data) is never imputed. Each denominator metric, that is patient bed-days, ICU bed-days, CVC days, ICU-CVC days and blood cultures per month, is imputed individually.

Data for a given missing denominator metric is imputed following a 3 step hierarchical process whereby if data is still missing after the current step, the next step is started. Missing data is replaced with:

• non-imputed data provided by the unit for the same month that is missing but for the previous year
• non-imputed data provided by the unit for the most recent previous month
• non-imputed data provided by the unit for the most recent future month (this is possible because units provide data in quarters)

After imputation, data for given denominator metric in any particular month where there is a more than 100% increase or 50% decrease compared to the previous month is flagged, along with the previous month. The flagged values are checked with the unit, then assessed in the context of all provided data for that metric by the unit and the values dropped if it does not fit in with the rest of the time series. The data is then reimported, rechecked to confirm no unusual values. If the first imputation used historic data as per the hierarchical selection process, this may not reflect capacity or activity change within a unit, then one of the later imputation selection steps may be used to select data for imputation.

IMD and ethnicity numerator data

We acquired IMD and ethnicity data through data linkage.

We established each case’s IMD by identifying the postcode of residence at the time of infection, linking this to the lower layer super output area (LSOA) of residence, and this to its 2019 IMD decile. Deciles were converted into quintiles.

We identified each case’s high-level ethnic group using the Office for Health Improvement and Disparities (OHID) COVID-19 Health Inequalities Monitoring for England (CHIME) tool, which relies on NHS England Hospital Episodes Statistics records. We linked these data to our surveillance records using NHS number and date of birth.

IMD and ethnicity denominator data

We used Office of National Statistics (ONS) mid-year populations by LSOA (total and by age; 2018 to 2020) and linked these to IMD deciles. Deciles were converted into quintiles. Populations were then converted to financial year level. As mid-year populations for 2021 onwards were not available at the time of production of this report, we used last observation carried forward, with 2020 data as proxy.

We used 2021 Census populations by low-level ethnic group by year of age. We summed these populations by high-level ethnic group. As 2018 to 2020 populations by ethnic group were unavailable, we estimated them by multiplying the 2021 populations by the ratio of English population in a given year and the English population in 2021. This makes the assumption that the ethnicity distribution in England has not changed substantially since 2018. We then converted from calendar to financial year.

As C. difficile infection numerators only include people aged 2 years and over, we also produced IMD and ethnicity populations restricted to this age group.

Mortality rate

Mortality rate is a widely used outcome for assessing risk of death. Mortality rate is the number of deaths divided by the population at risk. This reflects the incidence of all-cause deaths following these infections in the population.

Case fatality rate

Case fatality rate (CFR) is the number of deaths as a percentage of all reported cases. This provides a measure for comparing survivability of different infections.

This chapter presents data on all-cause mortality, and therefore includes deaths that may not be directly attributable to the infections.

Rate calculations: ICB rates

All cases are attributed to a ICB and using this data we calculate rates per 100,000 population for each ICB. Therefore, to calculate rates for ICBs, the following equation is applied:

Rate, per 100,000 population = (number of new cases attributed to ICB / financial year population) x 100,000

We use the ONS mid-year population estimated data for relevant time periods, adjusted as described above. For instance, for FY 2010 to 2011 mandatory surveillance data we have used mid-year April 2010 to March 2011 population estimates.

Rate calculations: Trust rates

We calculate acute trust rates using hospital-onset cases as the numerator. The total occupied bed-days (KH03) data is used as an indicator of the total activity in each trust during the relevant time periods. Since April 2010 to March 2011 KH03 has been published quarterly, prior to this it was published on a financial year basis. The average daily overnight bed occupancy for all acute trusts has been multiplied by the number of days in the relevant time period. The relevant rate per 100,000 bed-days was calculated as follows:

Rate, per 100,000 bed days = (number of new cases reported by trust / (average daily occupancy x number of days in period)) x 100,000

Prior to trust apportioning, all-reports rates were calculated per acute trust. Therefore, for historical purposes to retain the time series, we also calculate an all-reports rate per acute trust. ‘All reported cases’ refers to all bacteraemias or C. difficile infections that were detected by the acute trust that processed the specimen. It is important to note that this does not necessarily imply that the infection was acquired there.

Healthcare associated infections in Wales, Scotland and Northern Ireland

Surveillance for C. difficile infections, Gram-negative, MRSA and MSSA bacteraemias is also performed in Wales, Scotland and Northern Ireland.

Please note that there are several differences between the English mandatory surveillance systems and the systems run by the devolved administration, including case definitions or protocols for diagnosing the infections, definitions re: inpatient episode versus hospital-onset episodes and the way in which data is presented. Therefore, the data provided in the published reports from Public Health Agency Northern Ireland, Public Health Wales and Antimicrobial Resistance and Healthcare Associated Infection (ARHAI) Scotland are not directly comparable with those data published by UKHSA, found in this report and annual tables.

Rate calculations: IMD and ethnicity rates

Observed incidence rates were calculated for each organism and financial year as follows:

rate per 100,000 population = (number of infections in a financial year in a given ethnic or IMD group / population in that ethnic or IMD group) x 100,000

Age-standardised rates were estimated using direct standardisation with the 2013 European Standard Population and Byar’s method and Dobson method adjustment. When counts are less than 10, this method is not reliable, so their rates were not calculated.

These were complete-case analyses. We excluded cases without a known IMD or ethnicity value (respectively 1.5% and 4.4% of all cases) from the calculation of IMD and ethnicity rates. These means that rates stratified by IMD or by ethnicity are slight underestimates.

Reasons for a missing IMD value include:

• the patient’s residence was not in England
• the patient’s residence was in an area that has not been assigned an IMD value yet
• the patient was homeless

Reasons for a missing ethnicity value include:

• the patient had opted not to state their ethnic group on admission to hospital
• the Trust did not record a valid NHS number or date of birth on the DCS
• hospital episode statistics (HES) did not contain an ethnicity value

ICU and mandatory surveillance linkage

ICCQIP surveillance data capture

Since May 2016, units participating in voluntary ICCQIP surveillance report all positive blood cultures (PBC) occurring during a patient’s stay within ICU (PBC from admission date or results reported after discharge but from samples taken during ICU stay) up to a total of 4 organisms per blood culture bottle. Other data include each patient’s NHS number and date of birth and ICU admission date. Units also submit data on the number of occupied ICU bed-days which form the denominators of the infection rates.

Data sets

It is not possible for units to submit ICU data without a patient NHS number or date of birth however, if the NHS number is unknown at the time of data submission, units are advised to enter all 9s. For the purposes of linkage with mandatory surveillance data invalid NHS numbers (all 9s or all 0s or numbers with fewer than 10 digits) and dates of birth (1 January 1900) are removed. Other data excluded from ICU data sets prior to linkage with mandatory surveillance data was PBCs not considered ICU-associated, that is, specimens taken less than 3 days (or 48 hours or less where times were reported) after the ICU admission date (where ICU admission is day one). ICU denominator data is processed according to pre-defined protocols (25).

Data excluded from mandatory surveillance data sets prior to linkage with ICU data were:

• E. coli, MRSA and MSSA cases reported before May 2016
• P. aeruginosa and Klebsiella spp. cases reported before April 2017
• cases which were not hospital-onset

Linkage

Organism specific records from mandatory DCS were linked deterministically using patient NHS number and date of birth to ICU-associated PBC data. Any cases that were reported to ICCQIP surveillance, but not mandatory surveillance were excluded. However, bacteraemia for organisms reported to mandatory surveillance which had not been reported to the ICCQIP surveillance were included. For example, if in the ICCQIP surveillance data Patient A had an ICU-associated episode of Acinetobacter spp. bacteraemia only but this patient had an E. coli infection reported to the mandatory surveillance programme and the E. coli specimen date was while Patient A was in the ICU according to the ICCQIP surveillance data, this mandatory E. coli bacteraemia was included as a hospital-onset ICU-associated E. coli infection.

Analysis

The total number of E. coli, Klebsiella spp., MSSA, MRSA, and P. aeruginosa bacteraemia reported to Mandatory surveillance that were identified as ICU-associated was used as the numerator in the ICU rates and percentages calculations presented in this report. The counts are also separated into bacteraemia that were reported in both surveillance systems, and bacteraemia that were not reported for an ICU patient but were reported to mandatory surveillance. Bacteraemia may be reported to mandatory but not to ICCQIP surveillance for several reasons including:

• the mandatory surveillance cases may have been acquired outside of the ICU and are misclassified as hospital-onset ICU-associated due to the leeway in the mandatory specimen date being within 2 days of the ICU specimen date and not exactly the same
• units may not enter complete organism data for a particular patient when they have polymicrobial infections
• the number of organisms per blood culture bottle is limited to 4 organisms for ICU patients, so if more organisms were cultured from the bottle, it would not be possible to report these to ICCQIP surveillance

Annualised rates of E. coli, Klebsiella spp., MSSA, MRSA, and P. aeruginosa bacteraemia per 1,000 ICU bed-days were calculated for each organism and each financial year as:

Rate, per 1,000 ICU bed days = (number ICU-linked mandatory infections in a financial year / total ICU bed days in a financial year) x 1,000

The percentage of HO E. coli, Klebsiella spp., MSSA, MRSA, and P. aeruginosa bacteraemia cases reported to mandatory surveillance that likely originated in the ICU was calculated for each organism and each financial year as:

(number ICU-linked mandatory bacteraemia in a financial year / total HO bacteraemia in a financial year) x 100

As ICCQIP is a voluntary surveillance system the data reported may be an underestimate and so must be interpreted with caution.

Table 31. Total number of ICU-associated HO bacteraemia where the organism was reported to both Mandatory and ICU Surveillance systems, England, by financial year: April 2016 to March 2023

Financial year	E. coli	Klebsiella spp.	MRSA	MSSA	P. aeruginosa
2016 to 2017	44	NA	2	36	NA
2017 to 2018	142	122	8	101	50
2018 to 2019	124	145	4	105	58
2019 to 2020	129	145	9	92	69
2020 to 2021	155	330	11	142	94
2021 to 2022	154	209	8	143	90
2022 to 2023	106	175	5	99	64
Total across all financial years	854	1,126	47	718	425

Table 32. Total number of ICU-associated HO bacteraemia where the specified organism was reported only to Mandatory but not to ICU surveillance, England, by financial year: April 2016 to March 2023

Financial year	E. coli	Klebsiella spp.	MRSA	MSSA	P. aeruginosa
2016 to 2017	2	NA	0	1	NA
2017 to 2018	3	8	2	4	2
2018 to 2019	5	3	0	5	0
2019 to 2020	6	2	0	3	0
2020 to 2021	7	7	0	7	5
2021 to 2022	9	4	3	8	6
2022 to 2023	5	6	1	4	1
Total across all financial years	37	30	6	32	14

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1. Of note; while interpreting the findings presented in this epidemiology report on CDI, it is important to acknowledge that the data contains 54 false positives. false positives have been traced back to one Trust in the NHS Buckinghamshire, Oxfordshire and Berkshire West ICB region, and thus any data reported in this report for CDI in this region may be slightly inflated. Caution should be exercised while drawing conclusions or making public health decisions based solely on the data presented in this report for the identified region. ↩