Official Statistics

Surveillance of influenza and other seasonal respiratory viruses in the UK, winter 2022 to 2023

Updated 5 June 2024

Main findings

This report describes surveillance findings for seasonal influenza, other seasonal respiratory viruses such as respiratory syncytial virus (RSV) and the monitoring of human cases of avian influenza and Middle-East respiratory syndrome coronavirus (MERS-CoV).

Across the UK, influenza activity was concentrated in a relatively short period, and relatively early within the typical seasonal range. High levels of influenza activity were seen across most surveillance systems from week 48 2022 to week 3 2023, where activity through multiple indicators showed a rapid increase in activity, followed by a rapid decline in activity and returning to low levels of activity for the remainder of the season. Activity was higher than levels observed during the 2021 to 2022 season, with 2022 to 23 the first since the COVID-19 pandemic was declared in which there have not been significant non-pharmaceutical control measures in place: these also suppress influenza transmission.

During the 2022 to 2023 season, influenza A dominated, with influenza A(H3N2) the predominant subtype in Great Britain (England, Scotland and Wales) and A(H1N1)pdm09 predominant in Northern Ireland. Influenza type B Victoria lineage circulated at low levels becoming more prominent in 2023 as influenza A declined. All sequenced influenza B were Victoria lineage; there were no confirmed detections of wild-type influenza B Yamagata lineage.

In primary care, GP influenza-like illness (ILI) consultation rates across the UK were above baseline intensity levels between week 51 2022 and week 1 2023: the first time since the start of the COVID-19 pandemic. Similarly in syndromic surveillance (England), peak rates of 111 calls and emergency department attendances for ILI were higher than most previous seasons between weeks 49 of 2022 and week 1 2023, though this pattern in 111 calls was not observed in other nations. Note that while ILI is a relatively specific indicator, signifying an illness considered by the clinician to be most likely due to influenza virus, all respiratory clinical indicators using signs and symptoms or syndromes can also reflect the activity of SARS-CoV-2 and other respiratory pathogens.

In England and Scotland, the hospitalisation rate peaked during week 51 2022 and was above the baseline activity threshold between week 43 2022 and week 9 2023 in England. Peak hospitalisation rates were higher than previous influenza seasons; improved virological case ascertainment is likely to have contributed to this, with increased influenza testing compared with pre-COVID-19 seasons. Total hospitalisations in England were higher than previous influenza seasons. Total hospitalisations were also high in Scotland, although lower than the 2017 to 2018 season.

Intensive care unit (ICU) or high dependency unit (HDU) influenza admissions also peaked during week 51 2022 and were above the baseline activity threshold between week 46 2022 and week 3 2023 in England. While rates were higher than those observed in 2021 to 2022, total admission rate was lower than those seen in previous seasons.

Provisional end-of-season vaccine effectiveness (VE) against hospitalisation was higher in children (point estimates 60% to 70% across nations providing estimates) than in adults (point estimates around 30% to 35%) with broad confidence intervals for estimates of vaccine effectiveness against different influenza subtypes. This is consistent with previous seasons. The vaccines were well matched to circulating A(H3N2) and influenza B viruses, with some evidence of ongoing evolution of the A(H1N1)pdm09 viruses.

Vaccine uptake for the 2022 to 2023 season in England, Scotland, Wales and Northern Ireland is summarised in this report. This includes uptake in pre-school aged children, primary school aged children, those eligible in secondary school, those in clinical at-risk groups, pregnant women, those aged 50 years and over and all frontline healthcare workers (HCWs).

Activity from other circulating seasonal respiratory viruses, including RSV, was broadly similar than previous seasons. RSV activity was raised through the summer months of 2022, increasing further from week 40 2022 and peaked in weeks 46 to 50 2022 across the nations before declining to low for the rest of the season.

Novel and emerging respiratory viruses, including MERS-CoV and avian-origin influenza viruses, have continued to result in human cases in affected countries. There have been multiple detections of avian influenza in avian species in the UK in 2022 to 2023 as reported by the Department for Environment, Food and Rural Affairs (DEFRA) and Animal and Plant Health Agency (APHA). The UK reported 2 detections of avian influenza A(H5) virus in poultry workers at a farm in England in May 2023, with no evidence of onward transmission. Both cases were asymptomatic and detected as part of an ongoing enhanced surveillance study of asymptomatic workers exposed to poultry infected with avian influenza. Surveillance and public health measures are recommended in the UK for persons potentially exposed to infected birds and for severe respiratory disease among travellers returning from affected countries.

Surveillance of influenza and seasonal respiratory viruses in the UK

Surveillance of influenza and other seasonal respiratory viruses in the 4 UK nations is undertaken throughout the year by teams within the UK Health Security Agency (UKHSA), Public Health Scotland (PHS), Public Health Wales (PHW) and the Public Health Agency (PHA) Northern Ireland, who are each responsible for monitoring influenza activity for their respective nation. UKHSA undertakes a combination of England and UK functions and collated this annual report in collaboration with the other public health agencies.

Surveillance is also carried out for emerging novel respiratory viruses, including MERS-CoV and for human infections with avian influenza such as influenza A(H5N1), influenza A(H5N6) and influenza A(H7N9).

Weekly outputs on influenza are normally published during the winter season between October (week 40) and May (week 20), the period when influenza typically circulates. Since 2020 these reports have also covered COVID-19 and these reports run weekly year-round.

A variety of data sources are collated to provide information on influenza activity and to provide rapid estimates of influenza-related burden within the community, on the health service and in excess all-cause mortality. In addition, in-season and end-of-season monitoring of seasonal influenza vaccine uptake and vaccine effectiveness is undertaken.

This report describes influenza activity observed in the UK in the period from week 40 2022 (week ending 9 October 2022) to week 15 2023 (week ending 16 April 2023). This includes observations and commentary on influenza activity, activity of other seasonal and novel respiratory viruses and from the influenza vaccination programmes. There was some limited influenza B activity beyond week 15 2023. Surveillance of influenza and other respiratory viruses continues year round with reduced reporting for weeks 21 to 39.

The moving epidemic method (MEM) is used by the European Centre for Disease Prevention and Control (ECDC) to standardise reporting of influenza activity across Europe. It has been adopted by the UK and is presented for GP ILI consultation rates for each UK scheme and for the hospitalisation and ICU admissions rate through the SARI Watch scheme.

During the 2022 to 2023 season, the licensed live attenuated influenza vaccine (LAIV) was, for the first time, offered to all children aged 2 years in all UK nations up to the school-leaving age in the UK with the rollout in England reaching year 11. During the 2022 to 2023 season, influenza vaccination was again offered to individuals between 50 and 64 years old as part of the COVID-19 response, anticipating that influenza could rebound to above typical levels as social contact increased.

Measures put in place to control the COVID-19 pandemic have affected the transmission of influenza and other respiratory viruses during the previous 2 seasons. These measures included public health messaging, social and physical distancing measures, national lockdowns, the wearing of face coverings, hand hygiene and travel restrictions. Changes in healthcare-seeking patterns also affect some influenza indicators directly and indirectly. Although many of these restrictions and interventions were lifted by the 2022 to 2023 influenza season (some measures were reintroduced in December 2021 as Omicron-variant COVID-19 emerged, curtailing influenza transmission), these factors continue to be important to consider when interpreting the influenza surveillance indicators presented in this report, especially when comparing with previous seasons.

Community surveillance

Syndromic surveillance

In England, national UKHSA real-time syndromic surveillance systems include GP in-hours (GPIH) consultations and GP out-of-hours (GPOOH) contacts, emergency department (ED) attendances (Emergency Department Syndromic Surveillance System (EDSSS)) and NHS 111 calls and online assessments. These systems monitor a range of indicators sensitive to community influenza activity, for example NHS 111 ‘cold or flu’ calls and GP in-hours consultations for ILI.

Both clinical coding and healthcare-seeking behaviour have been affected during the COVID-19 pandemic. Syndromic data over the 2019 to 2020, 2020 to 2021 and 2021 to 2022 winter seasons should therefore be interpreted with some caution. Furthermore, during winter 2022 to 2023, syndromic surveillance data should also be interpreted with some caution due to the ‘group A strep incident’ from week 48 2022. During this incident, national media reports of an increase in severe invasive group A streptococcus (iGAS) disease in children led to changes in healthcare-seeking behaviour across the NHS, particularly in children with GAS-type symptoms, which are likely to have been respiratory symptoms.

GP in-hours consultation data presented here represents the findings from a sentinel surveillance system, as reported throughout the most recent 6 influenza seasons. The levels (consultation rates) reported may differ from those presented in previous annual reports, which were based on a dual data feed system with a larger population coverage which ended in March 2021.

During winter 2022 to 2023, GPIH ILI consultation rates increased rapidly from week 47, peaking during week 51. This peak occurred comparable with or earlier than in previous pre-pandemic years but was lower than the 2017 to 2018 season. Similarly, GPIH pneumonia consultation rate also peaked during week 51 2022, at similar levels to those observed in previous seasons but lower than those seen in 2017 to 2018 (Figure 1a and Figure 1b).

Figure 1a: Weekly all age GP in-hours consultations for influenza-like illness (ILI), winter 2017 to 2023, England

Figure 1b: Weekly all age GP in-hours consultations for pneumonia, winter 2017 to 2023, England

ED attendance syndromic surveillance data presented here includes 121 EDs (NHS type 01) that reported data throughout the most recent 6 influenza seasons. Therefore, numbers may differ slightly from those presented in previous annual reports, where a different number of EDs were included.

Acute respiratory infection (ARI) ED attendances increased rapidly from week 44, before peaking in week 49 at the highest level of all years for which comparable data is available (Figure 2). Pneumonia ED attendances followed a similar pattern and peaked at a similar level to the 2019 to 2020 season (Figure 2). ILI ED attendances increased from week 45, peaking during week 51. This peak was higher than in any previous season (Figure 2).

Figure 2a: Weekly all age Emergency Department Syndromic Surveillance System (EDSSS) acute respiratory infection (ARI) attendances winter 2018 to 2023

Figure 2b: Weekly all age EDSSS pneumonia attendances, winter 2018 to 2023

Figure 2c: Weekly all age EDSSS ILI attendances, winter 2018 to 2023

From week 40 2022 onwards, the number of NHS 111 ‘cold or flu’ calls was similar to 2021 to 2022, but higher than the numbers seen at the start of the previous 4 seasons. The 2022 increase from week 45 onwards was earlier than in previous years, peaking during week 51 at a level higher than during the early phases of the COVID-19 pandemic in week 11 2020 (Figure 3). It should be noted that COVID-19 activity caused an increase in the use of cold or flu codes prior to the introduction of COVID-19 specific options, causing the rapid drop in cold or flu calls recorded from week 12 2020 onwards.

Figure 3: Weekly all age England NHS 111 cold or flu calls, winter 2017 to 2023

In Wales, the weekly proportion of cold or flu calls made to NHS Direct Wales remained relatively low and stable throughout the season (Figure 4). The percentage of calls was slightly higher than the 2021 to 2022 season, but remained significantly lower than pre-pandemic seasons.

In Scotland, the weekly proportion of all calls to NHS 24 which mention cold or flu increased steadily between week 44 2022 and week 1 2023, before a sharp decline to low levels where it became steady for the remainder of the season. The percentage of calls was similar to what was reported in 2021 to 2022 but with a later peak and remained lower than those seen in most pre-pandemic seasons (Figure 5).

Figure 4: Proportion of calls for cold or flu (all ages) to NHS-Direct, Wales, 2017 to 2023

Figure 5: Proportion of calls for cold or flu (all ages) through NHS 24, Scotland, 2017 to 2023.

FluSurvey (internet-based surveillance)

FluSurvey, run by UKHSA, provides internet-based surveillance of ILI and COVID-19 symptoms in the UK population. It is part of a European wide initiative (including 11 European countries). On registration, individuals aged 18 years and over complete a baseline profile questionnaire which collects information on demographic, geographic, socioeconomic (household size and composition, occupation, education, and transportation), and health (vaccination, diet, pregnancy, smoking, and underlying medical conditions) data. Subsequently, participants are sent weekly reminders via email to report any symptoms relating to flu or COVID-19 that they may have experienced and their health-seeking behaviour as a result of their symptoms. This creates a fast, reliable and flexible real-time monitoring surveillance system. Recruitment of survey participants remained open throughout the 2022 to 2023 season to enable post pandemic surveillance of disease activity in the community.

A total of 2,490 participants were recruited and completed at least one survey with an average weekly participation of 1,984 (79.7%), contributing 55,628 responses across the 2022 to 2023 season.

There were more participants aged 65 years and over (50.6%) compared with other age groups (0 to 18 years (1.0%), 19 to 45 years (9.2%), 46 to 64 years (39.3%)). There was a higher proportion of female participants compared with male participants (66.4% versus 33.6%). The majority (2,581 or 89.7%) of participants were resident in England, 177 participants were from Scotland, 19 participants were from Northern Ireland and 92 from Wales.

Any participant completing the symptoms questionnaire during the reporting week was included in the analysis. The European Centre for Disease Control (ECDC) ILI case definition was applied of sudden onset of symptoms and at least one of: fever, malaise, headache or muscle pain and at least one of: cough, sore throat, shortness of breath. The total number of self-reported ILI episodes was 1,404 (2.5%). Overall, self-reported ILI activity was slightly higher than that observed in the previous season and peaked during week 52 (Figure 6). Social mixing with people outside of participants’ households increased further compared with 2021 to 2022 as COVID-19 control measures reduced. Some drops in social mixing were seen during seasonal holidays, also aligning to the ILI peak. Further analysis of the 2022 to 2023 flu survey will be available on the FluSurvey website in the summer.

Figure 6: Weekly ILI incidence per 1,000 reported through FluSurvey

Please note that in the 2019 to 2020 season FluSurvey reporting began in week 46. Please note there was no FluSurvey update in week 49 2022.

Figure 7: FluSurvey participants’ self-reported number of social contacts outside the household

FluDetector (internet-based surveillance)

UKHSA works with University College London (UCL) to assess the use of internet-based search queries as a surveillance method for ILI in England. This is part of work on early-warning surveillance systems for influenza, through the Engineering and Physical Sciences Research Council (EPSRC) Interdisciplinary Research Collaboration (IRC) project i-sense.

Combining natural language processing and machine learning techniques, a non-linear Gaussian process model was developed by UCL to produce real-time estimates of ILI. The supervised model, trained on historic national data from the Royal College of General Practitioners (RCGP) scheme, produces daily ILI estimates based on the proportion of ILI related search queries within a 10% to 15% sample of all queries issued, and is extracted daily from Google’s Health Trends Application Programming Interface.

Estimated rates of ILI were above the baseline threshold level of 19.6 per 100,000 between week 48 2022 and week 52 2022, reaching a peak of 40.7 per 100,000 in week 51. Overall estimated rates were higher than those seen in 2021 to 2022, and intermediate to the 2017 to 2018 and 2018 to 2019 influenza seasons (Figure 8).

Figure 8: Daily estimated ILI Google search query rates per 100,000 population

Acute respiratory infection incidents

Here we present data on ARI incidents in different settings that are reported to UKHSA health protection teams (HPTs) and entered onto the HPZone case and incident management system. Incidents are suspected outbreaks of acute respiratory infections linked to a particular setting. All suspected outbreaks are further investigated by the HPT in liaison with local partners.

The ARI definition includes presentations of both of ILI and other acute viral respiratory infections (AVRI). Causal pathogens can include influenza A and B, RSV, adenovirus, rhinovirus, parainfluenza, human metapneumovirus (hMPV) and SARS-CoV-2.

Between week 40 2022 and week 15 2023, a total of 8,260 incidents in closed settings were reported in the UK, the majority of which were SARS-CoV-2. The 2022 to 2023 season saw a decrease in the total number of incidents largely due to a decrease in the number of reported SARS-CoV-2 incidents. The number of influenza and other respiratory virus incidents increased compared with the previous season, although levels remained low compared with pre-pandemic seasons.

England

In England, there were a total of 6,049 ARI incidents in closed settings reported between week 40 of 2022 and week 15 of 2023. Of these, 5,006 (82.8%) were reported from care homes, 166 (2.7%) from educational settings, 477 (7.9%) from hospital settings and 400 (6.6%) from other settings. Where information on virological testing results was available, 2,828 were confirmed as SARS-CoV-2, followed by 454 influenza incidents, 32 RSV incidents and 24 incidents linked to other respiratory viruses (Table 1).

Table 1: The number of incidents in England by institution and virus type between week 40 2022 and week 15 2023.

Setting	Influenza	SARS-CoV-2	RSV	Other respiratory viruses	No organism reported
Care home	413	2,525	28	24	2,016
Hospital	22	132	3	0	320
Educational settings	3	15	1	0	147
Prison	6	48	0	0	39
Workplace settings	0	1	0	0	2
Other	10	107	0	0	187
Total	454	2,828	32	24	2,711

There were 454 influenza incidents reported in England during this period. By subtype, 432 were influenza A(not subtyped), 8 were influenza A(H3), 7 were influenza B, 4 were influenza A(H1N1) and 3 were influenza (untyped). The majority of influenza outbreaks occurred in care home settings, with the highest number in a week observed during week 51 2022 (Figure 9).

Figure 9: The number of total influenza outbreaks by week and setting, 2022 to 2023, England

Scotland

In Scotland, there were a total of 741 ARI outbreaks reported between week 40 2022 and week 15 2023. An outbreak is defined as at least 2 cases (laboratory confirmed and or suspected) of any acute respiratory illness within 48 hours in any setting. Of these, 668 (90.2%) were reported from care homes, 45 from other settings (6.1%), 24 (3.2%) from educational settings and 4 from prisons (0.5%). Virological results indicate 561 outbreaks were SARS-CoV-2, 66 were influenza A(not subtyped), 9 were influenza(not subtyped), 8 were RSV, 5 were seasonal coronavirus, 4 were rhinovirus, and 1 was enterovirus. The remaining 87 outbreaks did not have an organism reported.

Only outbreaks captured through HPZone are reported, therefore the total number of outbreaks may be an underestimate. Please note that the denominator for different settings will vary significantly, as does the propensity to report outbreaks between different settings. For these reasons, comparisons between settings are not advised.

Wales

During the 2022 to 2023 influenza season in Wales there were 708 outbreaks of influenza, ILI or acute respiratory illness (ARI) reported to Public Health Wales HPTs, compared with 1,266 reported outbreaks during the 2021 to 2022 season. Outbreaks were reported from 2022 week 40 to 2023 week 15.

The number of outbreaks reported from residential homes was 668 (94%), 23 outbreaks (3%) were reported from other community settings, 9 (1%) were reported from school or nursery settings, 5 (0.7%) were reported from prisons and 3 (0.4%) were reported from hospitals. Of these outbreaks, 683 were SARS-CoV-2, of which 681 were confirmed, and 2 were suspected. Of the confirmed SARS-CoV-2 incidents, 5 had only one reported case. Of the other outbreaks, 17 were influenza A; there were 3 outbreaks of both SARS-CoV-2 and influenza A and 3 outbreaks of influenza-like-illness with no respiratory result. Where influenza was detected in an outbreak, 14 were influenza A(untyped), 4 were influenza A(H3) and 2 were influenza A(H1N1).

Northern Ireland

In Northern Ireland, there were a total of 762 ARI outbreaks reported to the Public Health Agency between week 40 2022 and week 15 2023, compared with a total of 1,058 ARI outbreaks in the 2021 to 2022 season and 1,025 ARI outbreaks in the 2020 to 2021 season. In the 2022 to 2023 season, 379 (49.7%) reported outbreaks were from care homes, 309 (40.6%) from hospitals, and 74 (9.7%) from other settings. Virological results were available for 762 confirmed respiratory outbreaks of which 729 were SARS-CoV-2, 29 influenza A(not subtyped), 2 influenza/SARS-CoV-2 co-infections, 1 influenza B and 1 RSV.

Further information on ARI incidents during the 2022 to 2023 season is available in the weekly influenza and COVID-19 surveillance report and information on ARI incidents occurring in previous seasons is available in previous annual flu reports.

Primary care consultations

England

Weekly rates of GP consultations for influenza-like illness (ILI) through the RCGP scheme surpassed the 2022 to 2023 season moving epidemic method (MEM) baseline threshold of 11.47 per 100,000 during week 49 2022 and remained in within the medium MEM threshold until week 1 2023 before returning below the baseline threshold from week 3 2023 (Figure 10).

The ILI rate for 2022 to 2023 was higher than that observed in the 2021 to 2022 season. GP ILI consultation was comparable to the 2018 to 2019 and 2019 to 2020 seasons but below 2017 to 2018, when the ILI rate was above the MEM baseline threshold level for 14 weeks (Figure 10).

Figure 10: Weekly all age GP influenza-like illness rates for 2022 to 2023 and past seasons, England (RCGP)

Scotland

Weekly GP consultations for ILI surpassed the baseline MEM threshold of 19.8 per 100,000 between week 51 2022 and week 1 2023 in Scotland. Overall, the ILI consultation rate was higher than levels observed in 2021 to 2022 but below activity seen in the 2017 to 2018 season (Figure 11).

Figure 11: Weekly all age GP influenza-like illness rates for 2022 to 2023 and past seasons, Scotland

Wales

Weekly GP consultations for ILI in Wales surpassed the baseline MEM threshold of 10.97 per 100,000 between weeks 49 2022 to week 2 2023. The ILI consultation rate remained within the baseline MEM threshold for the rest of the season. Overall seasonal ILI consultation was higher than levels observed in 2021 to 2022 but below the activity in 2017 to 2018 (Figure 12).

Figure 12: Weekly all age GP influenza-like illness rates for 2022 to 2023 and past seasons, Wales

Northern Ireland

Weekly GP consultations for ILI in Northern Ireland surpassed the baseline MEM threshold of 11.3 per 100,000 between week 51 2022 and week 1 2023. Rates in the current season were higher than observed in the 2021 to 2022 and 2020 to 2021 seasons, but lower than seen in the 2017 to 2018 season (Figure 13).

Figure 13: Weekly all age GP influenza-like illness rates for 2022 to 2023 and past seasons, Northern Ireland

Secondary care surveillance

In England, the Severe Acute Respiratory Infection (SARI) Watch surveillance system was established in 2020 to report the number of laboratory-confirmed influenza, COVID-19 and RSV cases admitted to hospital and critical care units (ICU or HDU) in acute NHS trusts. This has replaced the UK Severe Influenza Surveillance System (USISS) mandatory and sentinel data collections for influenza surveillance used in previous seasons, and the COVID-19 hospitalisations in England surveillance system (CHESS) collections for COVID-19 surveillance. Aggregate level data is submitted by acute NHS trusts on a weekly basis. A week is based on the ISO week system running from Monday to Sunday.

The weekly rate of new admissions of influenza cases is based on the catchment population of those NHS trusts who made a return in that week. Surveillance is usually between week 40 (around October) to week 20 (late May) in the following year. However, since the pandemic, influenza and RSV surveillance were week 39 (comprising one-year surveillance) to detect out of season rebounds.

The MEM method has been applied to the SARI Watch hospital and ICU or HDU admissions for confirmed influenza to assess the impact of influenza activity throughout the season. The MEM thresholds are based on data from the 2016 to 2017 to the 2021 to 2022 seasons (data from 2020 to 2021 was excluded due to the COVID-19 pandemic).

Trends in influenza hospitalisation and critical care admission should be interpreted in the context of testing practices for acute respiratory infections. In recent years there has been wider implementation of rapid molecular point of care tests for influenza in hospital settings. From a public health surveillance perspective, it is important to consider a step change in influenza case ascertainment in more recent years.

On 16 February 2023, UKHSA issued a reminder to acute NHS trusts that influenza A samples from critical care should be subtyped in line with existing guidance. This was in the context of vigilance over avian influenza and concern that severe cases presenting in ICU or HDU settings may be missed. The new testing direction may impact on the ratio of subtyped to unsubtyped in surveillance data.

On 29 June 2023, the following amendments were issued to the ‘Secondary care surveillance’ section of the report.

Previously published cumulative RSV hospital admission rates in England were based on a sum of weekly cases divided by the sum population of all reporting trusts. This has been replaced with cumulative rates, which are based on a sum of weekly rates. This approach allows for the fact that not all sentinel trusts were involved in data reporting throughout the entirety of the season.

Figures 14, 17, 20 and 23 were adjusted to account for the leap week in 2020. Finally, the number of ECMO admissions in the 2017 to 2018 season was updated to include an additional case added retrospectively. These amendments do not affect the epidemiological interpretation of the data.

Hospitalisations

Through SARI Watch, a total of 8,751 hospitalised confirmed influenza cases were reported by 45 participating sentinel NHS acute trusts in England from week 40 2022 to week 15 2023. This compares with 1,093 hospitalised cases in the previous season (from week 40 2021 to week 15 in 2022) reported by 41 trusts, the total increasing to 1,710 cases over a one-year surveillance period to week 39 2022. In the 2020 to 2021 season the number of cases were 40 from week 40 2020 to week 15 2021 reported by 58 trusts, increasing to 64 cases over a one-year period to week 39 2020.

In the pre-pandemic seasons, surveillance was from week 40 to week 20 in the following year. In 2019 to 2020 the number of hospitalised cases of confirmed influenza was 4,916 reported by 21 sentinel trusts. In the 2018 to 2019 this was 5,675 cases reported by 23 sentinel trusts. In 2017 to 2018 there were 10,080 cases reported by 26 sentinel trusts.

Note that since the last report, retrospective updates for previous seasons are included.

The cumulative admission rate between week 40 and week 15 was 87.25 per 100,000 trust catchment population in 2022 to 2023. This compares to a cumulative admission rate between week 40 and week 15 of 9.03 per 100,000 in 2021 to 2022, 0.30 per 100,000 in 2020 to 2021, 47.22 per 100,000 in 2019 to 2020, 51.06 per 100,000 in 2018 to 2019 and 83.13 per 100,000 in 2017 to 2018.

The number of influenza hospitalisations in acute NHS trusts in England including admissions to critical care (ICU or HDU) was estimated by applying the weekly influenza hospitalisation rate to the latest England population. The result per week was summed. Between week 40 2022 to week 15 2023, the estimated number of admissions due to confirmed influenza in England was 49,300 (95% CI: 48,200 to 50,300). This compares with an estimated 8,400 cases in England in 2021 to 2022 (one year to week 39) and 300 in 2020 to 2021 (one year to week 40). In the pre-pandemic seasons from week 40 to week 20 in the following year, the estimated number was 26,700 in 2019 to 2020, 29,800 in 2018 to 2020 and 47,200 in 2017 to 2018.

These estimates are based on case confirmation by polymerase chain reaction (PCR) or molecular point of care. There is the assumption of trust representation in terms region and trust type. However, among 45 reporting trusts in the sentinel scheme in 2022 to 2023, there was under representation of small acute trusts as well as trusts from the Midlands and North East.

In the 2022 to 2023 season, hospital admission rates for confirmed influenza started to increase several weeks earlier than in previous pre-pandemic seasons. For the first time since the COVID-19 pandemic, influenza hospitalisation rates breached the medium (2.62 to less than 8.68 per 100,000 trust catchment population) and high (8.68 to less than 14.74 per 100,000 trust catchment population) impact ranges. The peak was the highest observed since surveillance began which also breached the very high impact range (14.74 per 100,000 trust catchment population). The rate returned to baseline levels continuously from week 9 2023 (Figure 14).

The unusual shape of the epidemic wave in the 2022 to 2023 season (characterised by early increases, sharp increases, a high peak followed by steep decreases) could partly be due a step change in case ascertainment as well as potentially reflecting a rebound season in a population with higher-than-usual susceptibility, with higher transmission leading to more hospital activity, and the higher impact of A(H3N2) on older adults.

COVID-19 control measures resulted in unprecedented low activity in 2020 to 2021 (one surveillance year from week 40 2020 to week 39 2021). In 2021 to 2022 (one surveillance year), influenza hospitalisations rebounded later (in week 10 2022) although the rate was similar to activity at the start of typical influenza season. The rate peaked at 1.15 per 100,000 in week 14 2022 taking it from baseline level to the low impact range before returning to baseline. Influenza admissions continued to be seen throughout summer 2022.

Weekly confirmed influenza hospitalisation rates to sentinel hospital trusts in England since winter 2010 are shown in Figure 17.

Figure 14: Weekly overall influenza hospital admission rates per 100,000 trust catchment population (with thresholds) and number of influenza hospital admissions to sentinel trusts, SARI Watch, England

Figure 15 shows the distribution of 8,751 sentinel influenza hospital admissions reported up to week 15 2022 by subtype/type and age group. Overall, 608 were influenza A(H1N1)pdm09, 648 were influenza A(H3N2), 6,250 were influenza A(not subtyped) and 1,245 were influenza B (Figure 15).

Influenza B became a more common cause of admission than influenza A from week 5 2023 as the overall number of influenza A hospitalisations decreased considerably since the peak in week 51 2022. A late season rise in influenza B hospitalisations mainly affected those aged 15 to 44 years.

It should be noted that the high proportion of influenza A that is unsubtyped may reflect increase in the use of rapid nucleic acid amplification tests (NAATs) used at the point of care as well as longer standing use of laboratory NAATs that do not include subtyping. Overall, of the total influenza A hospitalised cases in 2022 to 2023, 83% (6,250 out of 7,506) were not subtyped, ranging from 64% (58 out of 91) in 6 to 11 months to 91% (800 out of 878) in those aged 85 years and over.

The subtyping data is used for public health surveillance and allows a more complete picture of circulating subtypes affecting severe cases in particular. Hospital trusts are encouraged to perform influenza subtyping locally where possible or regionally and to integrate results into local information systems.

Figure 15: Cumulative number of influenza hospital admissions by influenza subtype/type and age group, SARI Watch sentinel surveillance, England, week 40 2022 to week 15 2023

Figure 16: Rate of influenza hospitalisations by age group, SARI Watch sentinel surveillance week 40 2022 to week 15 2023

Figure 17: Weekly all-age rate of confirmed influenza hospital admissions to sentinel trusts, 2010 to 2023, England

Scotland

Scottish influenza hospital admissions are calculated by linking the Rapid Preliminary Inpatient Dataset (RAPID) emergency or non-injury hospital admissions to positive Electronic Communication of Surveillance in Scotland (ECOSS) test results. RAPID captures all hospital admissions. The ECOSS data set provides all laboratory test data for respiratory viruses (including SARS-CoV-2, influenza, and RSV) in Scotland. Linkage is performed by linking the Community Health Index (CHI) number with the admission date to the ECOSS test results and test date.

Cases counted in the hospital admission totals for these pathogens are defined as those admitted to hospital, appearing in RAPID and testing positive for influenza (appearing in ECOSS with a test/specimen date) within 14 days prior or 48 hours post RAPID admission date.

A total of 6,693 hospitalised confirmed influenza admissions was reported from 14 regional NHS boards in Scotland from week 40 2022 to week 15 2023. This compares to 553 hospital admissions in the previous season (from week 40 2021 to week 15 in 2022), the total increasing to 1,140 admissions over a one-year surveillance period to week 39 2022. In the 2020 to 2021 season, the number of admissions from week 40 2020 to week 15 2021 was 62, the total increasing to 91 admissions over a one-year period to week 39 2021.

In the pre-pandemic seasons, surveillance was from week 40 to week 20 in the following year. In 2019 to 2020, the number of hospital admissions of confirmed influenza was 3,741. In 2018 to 2019, there were 4,482 admissions. In 2017 to 2018, there were 9,158 admissions.

Figure 18 shows the distribution of 6,693 influenza hospital admissions reported up to week 15 2023 by subtype/type. Overall, 482 were influenza A(H1N1)pdm09, 975 were influenza A(H3N2), 4,975 were influenza A(not subtyped) and 261 were influenza B.

Figure 18. Weekly number of patients admitted to a hospital as an emergency positive for influenza, by influenza subtype, Scotland, week 40 2022 to week 15 2023

The hospital admission rate for patients who tested positive for influenza reported in week 51 and 52 2022 (24.5 per 100,000 and 23.8 per 100,000 respectively) were higher than those reported for any week since the 2016 to 2017 season (Figure 19).

Figure 19. Weekly hospital admission rate (including ICU/HDU) for influenza, Scotland, 2016 to 2023

ICU or HDU admissions

Through SARI Watch, a total of 1,681 critical care (ICU or HDU) confirmed influenza admissions were reported from 108 NHS acute trusts across England from week 40 2022 to week 15 2023. This compares with 182 admissions in the previous season (from week 40 2021 to week 15 in 2022) reported by 114 trusts, the total increasing to 316 admissions over a one-year period to week 39 2022. In the 2020 to 2021 season the number of admissions were 9 from week 40 2020 to week 15 2021 reported by 129 trusts, increasing to 15 admissions over a one-year period to week 39 2020.

In the pre-pandemic seasons, surveillance was from week 40 to week 20 in the following year. In 2019 to 2020 the number of critical care cases of confirmed influenza was 1,671, 3,017 cases in 2018 to 2019 and 3,247 cases in 2017 to 2018.

ICU or HDU admission rates for confirmed influenza started to increase earlier compared with pre-pandemic seasons. The rate crossed the threshold for the medium impact range (0.18 per 100,000 trust catchment population) for the first time since the 2019 to 2020 season. The rate peaked in week 51 2022 at 0.65 admissions per 100,000 (Figure 20), occurring 2 to 3 weeks earlier than the peaks in the 2018 to 2019 and 2017 to 2018 seasons.

Of the 1,681 influenza ICU or HDU admissions reported up to week 15 2022, 143 were influenza A(H1N1)pdm09, 121 were influenza A(H3N2), 1,208 were influenza A (not subtyped) and 209 were influenza B (Figure 21). Influenza B admission numbers exceeded influenza A admissions later in the season (from week 10 2023) as influenza A admissions decreased.

A high proportion of influenza A remain unsubtyped. Of all influenza A ICU or HDU cases in 2022 to 2023, 82% (1,208 out of 1,472) were not subtyped, ranging from 71% in 6 months or younger to 88% in 55 to 64 years.

The surveillance on influenza ICU or HDU admissions includes a separate collection for influenza fatalities in ICU or HDU as an indication of severity near real time for use in conjunction with other severity indicators such as excess mortality. There were 139 fatalities reported among influenza cases admitted to ICU or HDU reported from to week 40 2022. Of these, 9 were influenza A(H1N1)pdm09, 11 were influenza A(H3N2), 97 were influenza A(not subtyped) and 22 were influenza B. Patients aged 65 years or over made up the largest group in ICU and HDU influenza fatal case reporting (47% of fatal cases).

Weekly confirmed influenza ICU or HDU admission rates in England since winter 2011 are shown in Figure 23.

Figure 20: Weekly influenza ICU or HDU admission rates per 100,000 trust catchment population (with thresholds) and number of influenza ICU or HDU admissions, SARI Watch, England

Figure 21: Cumulative number of influenza ICU or HDU admissions by influenza subtype and age group, SARI Watch, England, week 40 2022 to week 15 2023

Figure 22: Rate of influenza ICU or HDU admissions by age group, week 40 2022 to week 15 2023

Figure 23: Weekly all-age rate of confirmed influenza ICU or HDU admissions, 2011 to 2023, England

Scotland

There were 350 confirmed influenza admissions to ICU or HDU in Scotland between week 40 2022 and week 15 2023. Among these there were 263 influenza A(not subtyped), 54 influenza A(H3N2), 24 influenza A(H1N1)pdm09 and 10 influenza B (Figure 24).

Patients admitted to ICU or HDU with recently confirmed influenza are identified from the Scottish Intensive Care Society Audit Group (SICSAG) that collects detailed patient level data on all patients in ICU or HDU across Scotland. Additional information on patients with laboratory confirmed influenza is collected through ICU enhanced surveillance forms. All patients that are admitted to ICU or HDU with a positive influenza test result within a period of 14 days before the ICU or HDU admission and the ICU or HDU discharge date are included. Where the discharge date is missing, any patients with a positive influenza test result within a period of 14 days before ICU or HDU admission and 7 days after ICU or HDU admission are included.

Figure 24a: Weekly number of laboratory-confirmed influenza ICU or HDU cases, Scotland, 2022 to 2023

Figure 24b: Weekly crude rate of admissions, Scotland, 2021 to 2023

Wales

Between week 40 2022 and week 15 2023 there were 103 influenza admissions to ICU or HDU in Wales. Of these, 40 were influenza A(H3N2), 39 were influenza A(unknown subtype), 21 were influenza A(H1N1)pdm09 and 3 were influenza B.

Northern Ireland

Critical care data for the 2022 to 2023 season is not available.

RSV

England collates data on confirmed hospitalised RSV cases through the SARI Watch surveillance system. For RSV, this is a sentinel surveillance system.

Between week 40 2022 and week 15 2023, a total of 2,435 confirmed RSV cases (2,285 hospitalised to lower level of care and 150 admitted to ICU or HDU) were reported from 35 participating sentinel trusts.

The cumulative admission rate between week 40 and week 15 was 32.18 per 100,000 trust catchment population in 2022 to 2023. This compares to 19.19 per 100,000 in 2021 to 2022 for the same interval (31.18 per 100,000 based on one-year surveillance to week 39). The rate was 0.31 per 100,000 in 2020 to 2021 up to week 15 2021 (30.57 per 100,000 based on one-year surveillance to week 39). In the pre-pandemic seasons (from week 40 to week 20 in the following year the cumulative rates were: 48.23 per 100,000 in 2019 to 2020, 41.66 per 100,000 in 2018 to 2019 and 45.29 per 100,000 in 2017 to 2018.

It is important to note the seasonal displacement of RSV transmission observed after the easing of COVID-19 control measures. RSV hospitalisations rebounded in the summer of 2021 following unprecedented low activity in 2020 to 2021. The overall hospital admission rate peaked in week 31 2021 at 2.77 per 100,000 followed by small decreases causing prolonged activity into the following season. The decline of the RSV hospitalisation rate continued into the winter of 2021 to 2022. The rate rebounded in the summer of 2022 but not as strongly as the previous summer, peaking in week 23 2022 to 0.81 per 100,000 followed by a prolonged period of slow declines. By the 2022 to 2023 season however, RSV hospitalisations resumed a seasonal pattern peaking in week 47 2022 to 3.12 per 100,000, although this peak was lower than in pre-pandemic seasons.

For comparison, peak weekly admission rates for RSV (lower level of care and ICU or HDU) were observed at 5.28 per 100,000 in 2019 to 2020, 4.67 per 100,000 in 2018 to 2019 and 4.06 per 100,000 in 2017 to 2018 (Figure 25).

Figure 25: Weekly hospitalised RSV case rate per 100,000 trust catchment population, England, 2018 to 2023

Children under the age of 5 years are most affected by RSV with substantially high hospitalisation rates. Those aged 65 years and over have the next highest burden.

In the 2022 to 2023 season, the RSV hospitalisation rate for children aged under 5 years peaked at 35.93 per 100,000 in week 47 2022 (Figure 26). The epidemic curve in 2022 to 2023 for those under 5 years followed a typical seasonal pattern in contrast to the 2 preceding seasons. In 2020 to 2021 and 2021 to 2022, the burden was displaced to the summer following the easing of COVID-19 restrictions (thus allowing some other respiratory viruses to circulate). In 2020 to 2021, the rate in this age group peaked at 36.34 per 100,000 in week 31 2021 followed by a prolonged period of slow decreases through the winter. In 2021 to 2022 the rate started to surge from week 20 2022 but this was at lower level and prolonged until a more conventional seasonal pattern resumed in 2022 to 2023.

For comparison to pre-pandemic years, the RSV hospitalisation rate in those under 5 years peaked at 56.73 per 100,000 in week 51 of the 2019 to 2020 season and at 67.37 per 100,000 in week 48 in 2018 to 2019, and at 56.38 per 100,000 in week 46 in 2017 to 2018.

In 2022 to 2023, from week 40 to week 15 inclusive, there were 1,393 RSV hospitalisations (lower level of care or ICU or HDU admissions) to SARI Watch sentinel trusts in those aged under 5 years. Of these, 67.7% (943) were in children aged under 1 year, including 741 aged 6 months or younger.

Figure 26: Rate of RSV hospitalisation in those aged under 5 years per 100,000 trust catchment population by week of admission and season, sentinel data from acute NHS trusts, England, 2018 to 2023

The rate for those aged 65 years and over peaked at 4.64 per 100,000 in week 51 2022, later than the peak among children aged under 5 years. For those aged 85 years and over the rate peaked at 16.56 per 100,000 in week 51 2022. In pre-pandemic seasons the rate for the elderly (65 years and over) was 6.93 per 100,000 in 2019 to 2020, 6.07 per 100,000 in 2018 to 2019 and 3.77 per 100,000 in 2017 to 2018. These also peaked later than the peaks for children aged under 5 years.

Scotland

Scottish RSV hospital admissions are calculated by linking the Rapid Preliminary Inpatient Dataset (RAPID) emergency or non-injury hospital admissions to positive ECOSS test results. RAPID captures all hospital admissions. The ECOSS data set provides all laboratory test data for respiratory viruses (including SARS-CoV-2, influenza, and RSV) in Scotland. Linkage is performed by linking the Community Health Index (CHI) number with the admission date to the ECOSS test results and test date.

Cases counted in the hospital admission totals for these pathogens are defined as those admitted to hospital, appearing in RAPID and testing positive (appearing in ECOSS with a test/specimen date) within 14 days prior or 48 hours post RAPID admission date.

A total of 3,182 hospitalised confirmed RSV admissions were reported from 14 regional NHS boards in Scotland from week 40 2022 to week 15 2023. This compares to 1,014 hospital admissions in the previous season (from week 40 2021 to week 15 in 2022), the total increasing to 2,649 admissions over a one-year surveillance period to week 39 2022. In the 2020 to 2021 season, the number of admissions from week 40 2020 to week 15 2021 was 161, the total increasing to 1,665 admissions over a one-year period to week 39 2021. 

In the pre-pandemic seasons, surveillance was from week 40 to week 20 in the following year. In 2019 to 2020, the number of hospital admissions of confirmed RSV was 4,184. In 2018 to 2019, there were 3,632 admissions. In 2017 to 2018, there were 3,593 admissions.

Figure 27. Weekly hospital admission rate (including ICU and HDU) per 100,000 for RSV, Scotland, 2018 to 2023

The trend in the spread of laboratory-confirmed RSV admissions from 2022 to 2023 does not follow the typical seasonal pattern observed in previous respiratory winter seasons.

Between week 40 2022 and week 15 2023 the highest rate of RSV hospital admissions for those under the age of 5 occurred in week 46 2022 (59.9 per 100,000). In the previous season, the peak was higher and occurred in week 40 2021 (66.9 per 100,000).

Figure 28. Weekly hospital admission rate (including ICU and HDU) per 100,000 for RSV in children under 5 years, Scotland, 2021 to 2023

ECMO admissions

The UKHSA collects data on every adult patient admitted to a severe respiratory failure (SRF) centre, for extra corporeal membrane oxygenation (ECMO) or other advanced respiratory support, whether or not the primary cause is known to be infection related. There are 7 SRF centres in the UK (6 in England and 1 in Scotland) participating in the UKHSA ECMO surveillance module. One of the SRF centres joined the surveillance from the 2022 to 2023 season. Surveillance is all year round.

Between week 40 2022 and week 15 2023 there were 106 admissions to SRF centres requiring ECMO in the UK (Figure 29). Of these, 39 were for laboratory-confirmed influenza (19 influenza A (not subtyped), 8 influenza A(H3N2), 6 influenza A(H1N1)pdm09 and 6 influenza B cases). There were 4 influenza admissions to SRF centres in 2021 to 2022. There were no influenza admissions in 2020 to 2021.

In pre-pandemic seasons, there were 37 influenza admissions to adult SRF centres in 2019 to 2020, 110 in 2018 to 2019 and 62 in 2017 to 2018.

Figure 29: Laboratory confirmed ECMO admissions in adults (COVID-19, influenza and non-COVID-19 confirmed) to Severe Respiratory Failure centres in the UK, week 16 2021 to week 15 2023

Microbiological surveillance

Influenza

The Respiratory Datamart system began during the 2009 influenza pandemic to collate all laboratory testing information in England. It is now used as a sentinel laboratory surveillance tool, monitoring all major respiratory viruses in England. Sixteen laboratories in England reported data for this season.

Overall influenza positivity showed an increased level of activity between week 40 of 2022 and week 15 of 2023, with peak positivity much higher than that seen in the 2021 to 2022 season and comparable to (slightly exceeding) recent pre-pandemic seasons. The majority of detections were influenza A, with increased late detections of influenza B observed. Of the detected influenza A viruses that were subtyped, the majority were influenza A(H3N2).

A gradual increase in overall influenza positivity was observed from week 40 of 2022, rising to 31.8% in week 51 of 2022 before declining. Early season positivity (weeks 40 to 46) was above that of pre-pandemic seasons. Influenza B positivity showed a late-season increase to levels between 0.9% and 1.7% between week 52 2022 and week 15 2023. Comparatively, in the 2021 to 2022 season, detections of all influenza through Respiratory DataMart were much lower, never reaching more than 5.8% of samples tested. This compares to peak positivity of 25.3% in 2019 to 2020 and 28.6% in 2018 to 2019 season (Figure 30).

Figure 30a: Weekly influenza swab percentage positivity through Respiratory Datamart sentinel laboratories in England, 2017 to 2023

Figure 30b: Weekly number of influenza detections by subtype through Respiratory Datamart sentinel laboratories in England, 2022 to 2023

The weekly count of confirmed cases by each major type or subtype of influenza through Respiratory DataMart from winter 2010 onwards is shown in Figure 31.

Figure 31: Weekly number of influenza detections by subtype through Respiratory Datamart in England, with overall percentage positivity, 2010 to 2023

Scotland

In Scotland, overall influenza positivity is reported through non-sentinel sources via ECOSS. There have been high numbers of influenza diagnoses recorded to date this season with a total of 14,094 samples testing positive for influenza between week 40 of 2022 and week 15 of 2023. Overall influenza positivity peaked at 33.6% in week 51 of 2022 (Figure 32).

Swab positivity for influenza is an estimate of underlying national positivity, as the actual number of negative tests for influenza was not available for the 2022 to 2023 season.

Figure 32a: Weekly ECOSS influenza positivity by influenza subtype from week 40 of 2022 to week 15 of 2023, Scotland

Figure 32b: Weekly number positive by influenza subtype from week 40 of 2022 to week 15 of 2023, Scotland

Respiratory syncytial virus (RSV)

From week 40 2022 to week 15 2022, RSV detections reported through the Respiratory DataMart surveillance system in England showed a similar seasonality as in pre-COVID pandemic seasons but at slightly lower levels. Positivity peaked at 12.4% in week 47 2022. This compares with positivity peaks of 13.4% in 2019 to 2020 and 21.5% in 2018 to 2019 (Figure 33).

Surveillance of RSV is ongoing with further data of current trends published in the weekly national influenza and COVID-19 surveillance report.

Figure 33: Weekly overall RSV swab percentage positivity and number of detections through Respiratory DataMart in England, 2018 to 2023

In Scotland, RSV was the second most commonly detected non-influenza respiratory pathogen detected through ECOSS for the 2022 to 2023 season (up to week 15 2023), contributing to 30.5% of all positive samples.

Other seasonal respiratory viruses

Of the other respiratory viruses monitored through the respiratory DataMart system, the highest levels of positivity were observed with rhinovirus throughout the season. Rhinovirus positivity was highest at the beginning of the season and then decreased and fluctuated between 8% and 18% positivity through the rest of the season, with lower activity levels seen between week 51 2022 and week 3 2023 (Figure 34).

Consistent with previous seasons, low levels of adenovirus were observed throughout the season with no clear seasonality seen, although positivity was slightly elevated between weeks 1 and 8 of 2023. Parainfluenza positivity remained relatively low overall for 2022 to 2023 season. It started to increase slowly from week 5 2023, and by the data cut-off time for this report (week 15 2023) it had been showing increased activity with a peak of 6.5% in week 14 which was slightly higher and earlier than the 2021 to 2022 season peak of 4.7% in week 24 2022. Human metapneumovirus (hMPV) activity in 2022 to 2023 was generally lower than that in the 2021 to 2022 season and peaked at 5.2% in week 52 2022, compared with the peak of 10.3% in week 49 2021 in the 2021 to 2022 season (Figure 34).

Figure 34a: Weekly number of positive samples and proportion positive for adenovirus, DataMart sentinel laboratories, England, 2018 to 2023.

Figure 34b: Weekly number of positive samples and proportion positive for parainfluenza, DataMart sentinel laboratories, England, 2018 to 2023.

Figure 34c: Weekly number of positive samples and proportion positive for rhinovirus, DataMart sentinel laboratories, England, 2018 to 2023.

Figure 34d: Weekly number of positive samples and proportion positive for human metapneumovirus (hMPV), DataMart sentinel laboratories, England, 2018 to 2023.

In Scotland, the pattern of non-influenza respiratory pathogens detected through ECOSS for 2022 to the 2023 season (up to week 15, 2022), was more comparable to pre-COVID-19 pandemic seasons than the 2020 to 2021 and 2021 to 2022 seasons.

Rhinovirus was the most commonly detected non-influenza respiratory pathogen detected through non-sentinel sources (ECOSS) for the 2022 to 2023 season (up to week 15, 2022), contributing to 33.2% of positive samples. RSV was the second most common (30.5% of positive samples) followed by adenovirus (14.1% of positive samples), hMPV (8.3% of positive samples), coronavirus (non-SARS-CoV-2) (8.2% of positive samples), and parainfluenza (5.6% of positive samples).

In Wales, of the 54,280 testing episodes in hospital patients that had a sample collected from a hospital setting, all were tested for influenza and RSV, with 54.7% (29,682 out of 54,280) tested for all respiratory pathogens and 27.7% (n=15,014/54,280) tested for influenza, RSV and SARS-CoV-2 only. The most commonly detected non-influenza respiratory pathogens were SARS-CoV-2 (20.9%, 8,197 out of 54,280) and rhinovirus (15.1%, 5,942 out of 39,266).

Other detected causes of respiratory infection included:

• adenovirus (7.3%, 2,862 out of 39,266)
• enterovirus (3.3%, 1,292 out of 39,266)
• seasonal coronavirus (4.5%, 1,772 out of 39,266)
• hMPV (4.2%, 1,667 out of 39,266)
• parainfluenza (3.3%, 1,296 out of 39,266)
• RSV (6.75%, 3,665 out of 54,280)
• Mycoplasma pneumoniae (0.01%, 3 out of 39,266)

Seventy-one per cent (10,734 out of 15,014) of patients tested for all 9 routinely screened pathogens were negative, and from the patients that were only tested for influenza, RSV and SARS-CoV-2, 43.9% (17,219 out of 39,266) were negative for all 3 pathogens.

Preceding, co- and secondary respiratory infections with SARS-CoV-2 and influenza

The surveillance of preceding, co- and secondary infections with COVID-19 in England began as part of UKHSA’s response to the COVID-19 pandemic. Regular reporting in the National flu and COVID-19 surveillance report began in late 2020. The workstream aims to provide rapid detection and timely surveillance of bacterial, viral and fungal pathogens acquired in conjunction with COVID-19. This workstream was expanded to include the surveillance of pathogens acquired in conjunction with influenza in 2022.

Seasonal respiratory virus co-infections are defined as a patient with a sample testing positive for the viral organism and another sample testing positive for either SARS-CoV-2 or influenza within one day before or after the viral organism positive date. Preceding and secondary infection definitions for these organisms are defined as the pathogen occurring 2 to 28 days pre- or post- SARS-CoV-2 or influenza infection. Please note that undertesting of pathogens may result in an underestimate of preceding, co- and secondary infection cases. Furthermore, children receiving LAIV in the preceding 14 days prior to swabbing may result in positive influenza detection in the absence of infection. Full preceding, co- and secondary infection definitions are available in Appendix 1 of the National flu and COVID-19 surveillance graphs packs.

Seasonal respiratory viral pathogens acquired in conjunction with SARS-CoV-2

A total of 5,790 pre-, co- and secondary infections were detected across all seasonal respiratory viruses among persons with COVID-19 in the 2022 to 2023 season in England, via the Respiratory DataMart and SGSS systems. Of which, 3,879 were defined as co-infections, 951 as preceding and 960 as secondary to a COVID-19 infection. These numbers are an increase to the previous 2021 to 2022 season (Figure 35).

Between week 40 2022 and week 15 2023, 1,926 co-infections of influenza A and SARS-CoV-2 and 296 co-infections of influenza B and SARS-CoV-2 were identified in England. A total of 527 preceding and 461 secondary influenza A infections were reported in comparison with 50 preceding and 44 secondary influenza B infections. These numbers are a considerable increase on last season, where between week 40 2021 and week 14 2022, 86 influenza A co-infections and 35 influenza B co-infections were recorded. Over the same period, there were 56 preceding and 82 secondary influenza A infections, and 23 preceding and 18 secondary influenza B infections recorded.

Between week 40 2022 and week 15 2023, the most frequent viral organisms (preceding, co- or secondary infections) identified from respiratory specimens among persons with COVID-19 were influenza A, RSV and influenza B (Figure 36). Influenza A was the most commonly detected pre-, co- and secondary infection accounting for 50.3% of all pre-, co- and secondary infections, followed by RSV which accounted for 20.0%.

Figure 35: Weekly number of respiratory viral specimens, by timing of diagnosis, in persons with COVID-19 diagnosed in England, 2021 to 2023

Figure 36: Most frequent respiratory viral specimens, by timing of diagnosis, in persons with COVID-19 diagnosed in England between week 40 2022 and week 15 2023

Seasonal respiratory viral pathogens acquired in conjunction with influenza

Please note, the base infection is any type of influenza (A, B, or both) for all viral preceding, co- and secondary infections, except for influenza B where the base infection is influenza A.

A total of 4,436 pre-, co- and secondary infections were detected across all seasonal respiratory viruses among persons with influenza in the 2022 to 2023 season in England, via the Respiratory DataMart and SGSS systems. Of which, 3,196 were defined as co-infections, 681 as preceding and 559 as secondary to an influenza infection. These numbers are an increase compared with the previous 2021 to 2022 season (Figure 37).

Between week 40 2022 and week 15 2023, 593 respiratory viral co-infections of influenza B and influenza A were identified in England. A total of 38 preceding and 47 secondary influenza B infections with influenza A were reported. These numbers remain low but are an increase on last season, where between week 40 2021 and week 14 2022, 397 co-infections of influenza B and influenza A were recorded. There were 2 preceding and 2 secondary influenza B infections with influenza A recorded.

Between week 40 2022 and week 15 2023, the most frequent viral organisms (preceding, co- or secondary infections) identified from respiratory specimens among persons with influenza were RSV, influenza B and rhinovirus (Figure 38). RSV was the most commonly detected pre-, co- and secondary infection accounting for 38.1% of all pre-, co- and secondary infections, followed by influenza B which accounted for 15.3%.

Figure 37: Weekly number of respiratory viral specimens, by timing of diagnosis, in persons with influenza diagnosed in England, 2021 to 2023.

Figure 38: Most frequent respiratory viral specimens, by timing of diagnosis, in persons with influenza diagnosed in England between week 40 2022 and week 15 2023

Please note the baseline infection is any type of influenza (influenza A or B or both) for all viral preceding/co-/secondary infections except for influenza B where the baseline infection is influenza A.

Sentinel GP-based swabbing

In England, influenza activity through the GP sentinel swabbing scheme in collaboration with the RCGP was higher overall in comparison with the previous season, with a total of 1,385 positive samples between week 40 of 2022 and week 15 of 2023. Activity began to increase in week 45 of 2022. Influenza A(H3N2) accounted for the majority of positive influenza specimens (Figure 39).

Figure 39: Number of influenza positive samples and weekly positivity (%) by influenza type, England, week 40 2022 to week 15 2023.

Broader virological swab positivity is depicted in Figure 40. For non-flu pathogens detected through the RCGP sentinel swabbing scheme, rhinovirus was the most detected respiratory pathogen between week 40 2022 and week 15 2023 in England (32.4% of total positive specimens). Influenza comprised 19.0% of the total respiratory pathogens detected in the swabbing scheme. This was followed by SARS-CoV-2 at 11.6% of total positive specimens, and RSV at 10.4% of total positive specimens. Other respiratory pathogens include seasonal coronavirus (7.9%), hMPV (7.2%), adenovirus (7.1%) and enterovirus (4.4%).

Figure 40 : Number of positive samples by pathogen, England, week 40 2022 to week 15 2023

In Scotland, the Community Acute Respiratory Infection (CARI) surveillance programme tests for a range of respiratory pathogens:

• SARS-CoV-2
• influenza A and B
• RSV
• adenovirus
• coronavirus (non-SARS CoV-2)
• HMPV
• rhinovirus
• parainfluenza
• Mycoplasma pneumoniae

Between week 40 of 2022 and week 15 of 2023 there were 1,478 detections of influenza through GP sentinel swabbing, 763 of which were influenza A(H3N2), 399 were influenza A(H1N1)pdm09, 197 were influenza A(not subtyped), 112 were influenza B and 7 were co-infections of influenza A(H1N1)pdm09 and influenza A(H3N2).

For non-flu pathogens, rhinovirus was the most commonly detected respiratory pathogen detected through sentinel sources (CARI) for the 2022 to 2023 season (up to week 15, 2023), contributing to 24.4% of positives for non-influenza respiratory pathogens. RSV was the second most common non-influenza pathogen (8.5% of positive samples) followed by SARS-CoV-2 (7.9% of positive samples), coronavirus (non-SARS-CoV-2) (6.9% of positive samples), HMPV (6.2% of positive samples), adenovirus (5.8% of positive samples), parainfluenza (4.0% of positive samples) and Mycoplasma pneumoniae (0.2% of positive samples). Figure 41 includes individual pathogens and total samples, in addition to coinfections as some samples tested positive for more than one pathogen (ranging from 0.4% to 13.6% coinfections weekly within the season).

Figure 41: Number of CARI samples and positives by pathogen, Scotland, week 40 2022 to week 15 2023

In Northern Ireland, 39 specimens tested positive for influenza during the 2022 to 2023 season through GP sentinel swabbing. Sixteen tested positive for influenza A(H1N1)pdm09, 12 for influenza B, 10 for influenza A(H3N2) and one for influenza A(not subtyped).

In Wales, 383 specimens tested positive for influenza during the 2022 to 2023 season through GP sentinel swabbing. Of these, 232 were influenza A(H3N2), 97 were influenza A(H1N1)pdm09, 29 were influenza A(not subtyped) and 25 were influenza B.

Virus characterisation

UKHSA characterises the properties of influenza viruses through one or more tests, including genome sequencing (genetic analysis) and haemagglutination inhibition (HI) assays (antigenic analysis). This data is used to compare how similar the currently circulating influenza viruses are to the strains included in seasonal influenza vaccines, and to monitor for changes in circulating influenza viruses. The interpretation of genetic and antigenic data sources is complex due to a number of factors, for example, not all viruses can be cultivated in sufficient quantity for antigenic characterisation, so that viruses with sequence information may not be able to be antigenically characterised as well. Occasionally, this can lead to a biased view of the properties of circulating viruses, as the viruses which can be recovered and analysed antigenically may not be fully representative of majority variants, and genetic characterisation data does not always predict the antigenic characterisation.

Between week 40 2022 and week 15 2023, the UKHSA Respiratory Virus Unit have genetically characterised, by sequencing of the haemagglutinin (HA) gene, 2563 influenza A viruses (1694 A(H3N2) and 869 A(H1N1)pdm09 viruses) and 147 influenza B viruses.

The 1,694 influenza A(H3N2) viruses genetically characterised, all belong in the genetic subclade 3C.2a1b.2a.2. The Northern Hemisphere 2022 to 2023 influenza A(H3N2) vaccine strain (an A/Darwin/9/2021-like virus) also belongs in this 3C.2a1b.2a.2 genetic subclade. Sequencing of the haemagglutinin (HA) gene shows that these A(H3N2) viruses belong in subclade 3C.2a1b.2a.2; defined by the amino acid changes Y159N, T160I(-CHO), L164Q, G186D, D190N, F193S and Y195F in the HA gene. Approximately 60% of characterised A(H3N2) viruses have the additional HA substitution H156S (3C.2a1b.2a.2a), of which two-thirds had D53G (subclade 3C.2a1b.2a.2a.1 and 3C.2a1b.2a.2a.2), one-third had D53N (subclade 3C.2a1b.2a.2a.3) and a small proportion (<5%) had D53S. Approximately 40% of viruses that retained H156 all clustered into the group with E50K, F79V, I140K amino acid substitutions (subclade 3C.2a1b.2a.2b). These groups within the 2a.2 subclade circulated continuously over the season, with limited change in proportion over time.

The 869 influenza A(H1N1)pdm09 viruses characterised to date this season, all belong in genetic subgroup 6B.1A.5a.2. The Northern Hemisphere 2022 to 2023 influenza A(H1N1)pdm09 vaccine strain (an A/Victoria/2570/2019-like virus) also belongs in genetic subclade 6B.1A.5a, within the 6B.1A.5a.2 cluster. Almost 80% of viruses characterised by sequencing of the haemagglutinin (HA) gene possessed the additional amino acid substitutions of P137S, K142R, D260E, T277A, E356D and N451H (subclade 6B.1A.5a.2a.1). This 80:20 ratio was maintained throughout the season. In community surveillance, 6B.1A.5a.2a.1 viruses were present at less than 90% at the start of the season in October 2022 and accounted for the greatest proportion of viruses (less than 60%) throughout the season. The vaccine strains selected for the Northern Hemisphere influenza vaccine 2023 to 2024 are clade 6B.1A.5a.2a.1 viruses.

The 147 influenza B/Victoria lineage viruses that have been genetically characterised, all belonging in subclade V1A3, within the subgroup V1A3a.2. The Northern Hemisphere 2022 to 2023 influenza B/Victoria lineage vaccine strain (a B/Austria/1359417/2021-like virus) also belongs in this V1A3a.2 subclade/group.

The Respiratory Virus Unit has confirmed by genome sequencing the detection of LAIV viruses in 2 influenza A positive samples and 9 influenza B positive samples collected since week 40 2022, all from children aged between 2 and 16 years of age, consistent with known shedding characteristics of LAIV viruses as detailed in laboratory guidance.

Virus characterisation in Scotland is performed by the West of Scotland Specialist Virology Centre (WoSSVC) throughout the influenza season. This data is published in the weekly National respiratory infection report.

Influenza antiviral susceptibility

Influenza positive samples are genome sequenced and screened for mutations in the virus neuraminidase (NA) and the cap-dependent endonuclease (PA) genes known to confer neuraminidase inhibitor or baloxavir resistance, respectively. The samples tested are routinely obtained for surveillance purposes, but diagnostic testing of patients suspected to be infected with antiviral-resistant virus is also performed.

Influenza virus sequences from samples collected between weeks 40 2022 and 17 2023 have been analysed. Analysis of 1,494 A(H3N2) viruses by sequencing found 2 oseltamivir resistant viruses. One oseltamivir resistant virus with an E119V amino acid substitution present as a mixed population (80% E119V) was collected from an adult, post-oseltamivir treatment, in January 2023. An R292K mutation was detected transiently, in a viral subpopulation (25%), and was undetectable in a sample taken 9 days later, while the E119V mutation was maintained over 19 days. The patient was not treated with zanamivir. A second oseltamivir resistant virus with an E119V amino acid substitution (100% E119V) was collected from an immune compromised adult in February 2023. Follow-up of this case is ongoing. Of 808 A(H1N1)pdm09 NA sequences analysed, one oseltamivir resistant virus with an H275Y amino acid substitution present as a mixed population (80% H275Y) was detected. The sample was collected from an immune compromised adult, post oseltamivir treatment, in December 2022. No viruses with known markers of resistance to neuraminidase inhibitors were detected in 141 influenza B NA sequences analysed.

No viruses with known markers of resistance to baloxavir marboxil were detected in 1,201 A(H3N2), 611 A(H1N1)pdm09 and 106 influenza B PA sequences analysed.

Table 2: Antiviral susceptibility of influenza positive samples tested at UKHSA-RVU.

Subtype	Neuraminidase inhibitors: Susceptible	Neuraminidase inhibitors: Reduced susceptibility	Baloxavir: Susceptible	Baloxavir: Reduced susceptibility
A(H3N2)	1,492	2	1,201	0
A(H1N1)pdm09	807	1	611	0
B/Victoria-lineage	141	0	106	0

Mortality

Influenza-attributable deaths

The FluMOMO model has been used by UKHSA and predecessor organisations to estimate influenza-related mortality in England, adjusting for extreme temperature, and is published in the scientific literature and in previous annual reports. Due to the mortality due to the COVID-19 pandemic from late March 2020, and very little influenza circulation in 2020 to 2021 and 2021 to 2022 winters, FluMOMO was not run in those years.

In winter 2022 to 2023 there was a concentrated period of influenza activity, as well as a spike in all-cause mortality. There was also Omicron-variant COVID-19 circulation and several periods of severe cold weather or cold snaps. To estimate the contribution of these factors to the detected excess mortality, FluMOMO was adapted. A working paper detailing the adapted methodology used provides further information.

Note that these are new methods and the findings should be considered experimental statistics. Furthermore, one of the adaptions to prevent undue influence of COVID-19 on trends required assigning a period of time covering the first 2 COVID-19 waves no weight in the model. This meant that for the 2021 to 2022 winter the excess was not attributed.

Figure 42 shows the weekly number of all-age deaths and attribution to influenza, COVID-19 and extreme temperature from week 40 2012 to week 13, 2023. The model demonstrates the winter 2022 to 2023 spike in mortality is predominantly attributed to influenza (red line), and partly attributed to COVID-19 (blue) and cold weather (green) (Figure 42, Table 3 and Table 4).

Influenza-related mortality for winter 2022 to 2023 is estimated at nearly 15,000 which is higher than that seen in the previous 4 years, but lower than the 2014 to 2015, 2016 to 2017 and 2017 to 2018 seasons (Table 4).

The feature of the 2022 to 2023 winter season was the concentrated period of high activity which supports attribution in the model. While the model also produces estimates of the contribution of cold weather and COVID-19 to all-cause mortality, these estimates are less reliable because Omicron COVID-19 has not shown large spikes and the effects of cold weather are assumed to spread across the week of the cold weather and the following 2 weeks.

For temperature-related mortality, the 5,500 estimate is based on 3 cold weeks (with mean central England temperature below 3°C) and is the highest since 2012 to 2013. This estimate is contingent on the definition of cold used. If the threshold for cold was changed to assign more weeks as cold weeks, it is likely more deaths would be attributed to severe cold weather; however, the aim was to look at extreme cold weather only.

Figure 42. Weekly number of all-age deaths and attribution to influenza (red line), COVID-19 (blue line) and cold weather (green line), England, 2012 to 2023 (up to week 13 2023)

Table 3. Estimated number of deaths associated with influenza, COVID-19 and cold weather by all-age and age 65 years plus, observed through the adapted FluMOMO algorithm with 95% confidence intervals, England, 2022 to 2023 season (week 40, 2022 to week 13, 2023)

Age	Influenza	COVID-19	Cold	Unexplained	Total
All-age	14,623 (14,197-15,049)	10,345 (10,150-10,540)	5,533 (5,262-5,804)	3,029	33,528
65 years +	12,546 (12,134-12,962)	9,820 (9,628-10,013)	5,136 (4,871-5,406)	2,974	30,476

Table 4: Estimated number of all age deaths associated with influenza, COVID-19 and cold weather observed through the adapted FluMOMO algorithm with 95% confidence intervals, England, 2012 to 2013 season to 2022 to 2023 (up to week 13, 2023)

Year*	Influenza	COVID-19	Cold	Unexplained	Total
2012 to 2013	9,021 (8,643-9,399)	-	5,748 (5,541-5,955)	7,694	22,463
2013 to 2014	167 (140-194)	-	0 (-10-10)	-4,824	-4,656
2014 to 2015	29,965 (29,404-30,526)	-	1,452 (1,371-1,533)	-1,073	30,344
2015 to 2016	12,223 (11,999-12,447)	-	88 (61-115)	320	12,630
2016 to 2017	17,769 (17,533-18,005)	-	597 (545-649)	2,084	20,450
2017 to 2018	22,419 (21,965-22,873)	-	3,215 (3,066-3,364)	6,580	32,212
2018 to 2019	5,144 (4,966-5,322)	-	1391 (1,303-1,479)	-5,180	1,354
2019 to 2020	8,800 (8,608-8,992)	-	0 (-10-10)	52,456	61,256
2020 to 2021	-	-	-	-	-
2021 to 2022	104 (66-142)	25,971 (25,647-26,295)	0 (-14-14)	-5,269	20,806
2022 to 2023	14,623 (14,197-15,049)	10,345 (10,150-10,540)	5,533 (5,262-5,804)	3,029	33,528

Note that Unexplained is negative if the estimated excess from influenza, COVID-19 and cold weather is more than the observed total excess above the baseline.

*Each season runs from week 40 on one year to week 20 of the next.

Vaccination

Seasonal influenza vaccine uptake in adults

Although all countries of the UK use standardised specifications to extract vaccine uptake data from IT information systems in primary care (GP system suppliers), there are some differences in extraction specifications, so comparisons should be made with caution.

England

In England, the uptake of seasonal influenza vaccine is monitored by UKHSA throughout the season based on weekly and monthly extracts from GP system suppliers via ImmForm for the cohorts primarily delivered via the GP practice.

Cumulative uptake on influenza vaccinations administered up to 28 February 2023 was reported from 97.1% (6,257 out of 6,447) of GP practices in England in 2022 to 2023.

Comparative data is up to 28 February 2022 where vaccine uptake was reported from 97.1% (6,355 out of 6,542) of GP practices in England in 2021 to 2022.

This season saw a vaccine uptake of 79.9% in those aged 65 years and over (compared with 82.3% in 2021 to 2022) and 49.1% for those aged 6 months to under 65 years of age with one or more underlying clinical risk factors (excluding pregnant women without other risk factors and carers), compared with 52.9% in 2021 to 2022 (Table 5).

Vaccine uptake in pregnant women was 35.0%, compared with 37.9 % in 2021 to 2022. In the 2022 to 2023 season, the programme was extended for a third season to include all 50 to under 65 year olds not at risk. Vaccine uptake in this group was 40.6%, but data between seasons for this age group is not directly comparable with the 2020 to 2021 season due to the current season eligibility starting 6 weeks later than last season.

Scotland

In Scotland, the uptake of seasonal influenza vaccine is estimated by PHS throughout the season, using a combination of:

• aggregated data returns submitted by staff in all the territorial NHS boards on a weekly basis for those eligible in the following cohorts:

  • 6 months to 2 years at risk
  • 2 to 5 years not at school
  • primary and secondary school pupils
  • pregnant women

• weekly extracts of data downloaded from the national clinical data store (NCDS), which contains individual-level data for adults eligible for flu vaccine, and for one NHS board, the data relating to the childhood schools’ and nurseries’ programmes

At this time, the vaccine uptake data reported is not comparable with previous years which reflects the improvements in data recording but also changes in how uptake has been determined in some cohorts.

By the end of week 15 2023, a total of 1,843,781 influenza vaccines were delivered to adults currently resident in Scotland corresponding to a 63.7% uptake. Among adults aged 65 years and older, 85.5% have been vaccinated against influenza during the current vaccination programme. Among at-risk groups, 56.9% have been vaccinated against influenza. Figures (numerators and denominators) are representative of the current living Scottish population of each respective eligible cohort. Deaths and leavers from Scotland have been removed.

Data is presented in Scotland’s weekly national respiratory report.

Wales

In Wales, the uptake of seasonal influenza vaccine is monitored on a weekly basis by PHW throughout the season based on automated weekly extracts of Read coded data using software installed in all general practices in Wales collected through the Audit+ Data Quality System. Cumulative uptake data on influenza vaccinations administered were received from 99% of GP practices in Wales in 2022 to 2023 (2 GP practices were excluded due to data quality issues and may be included at a later date). This showed a vaccine uptake of 76.3% in those aged 65 years and over (compared with 78.0% in 2021 to 2022) and 44.2% for those aged 6 months to under 65 years of age with one or more underlying clinical risk factors, compared with 48.2% in 2021 to 2022. Uptake in all patients aged 50 to 64 years old was 42.0%. Overall uptake in pregnant women was 60.0% compared with 78.5% in 2021 to 2022. In Wales, vaccine coverage in pregnant women is measured differently using a survey of pregnant women giving birth each year during February. In addition, as elsewhere in the UK, data is also automatically collected from general practices for women with pregnancy-related Read codes, this data reports uptakes of 46.7% in pregnant women at risk and 34.2% in healthy pregnant women.

Northern Ireland

In Northern Ireland, the uptake of seasonal influenza vaccine is monitored by the PHA of Northern Ireland. From 2021 to 2022 onwards, influenza vaccine uptake has been determined using data extracted from regional Immunisation Information System developed by the Department of Health (DoH) Digital team; known as the Vaccine Management System (VMS). Caution should be used when considering the 2021 to 2022 and 2022 to 2023 influenza vaccination uptake rates in comparison with previous seasons, due to the introduction of the new VMS involving new methods of recording and extracting influenza vaccine data. In the population aged 65 years and over uptake was 83.0% (compared with 57.7% in 2021 to 2022). Uptake was 51.4% in the population of 50 to 64 year olds in 2022 to 2023, compared with 43.5% in 2021 to 2022. The first year of rollout in this age group was 2020 to 2021. Note that the VMS does not currently collect data on the specific clinical risk conditions for influenza vaccination. Uptake in pregnant women (counted as mothers who delivered during the flu vaccine programme) was 29.8% in 2022 to 2023 compared with 45.9% in 2021 to 2022.

Healthcare workers

In England, vaccine uptake among all frontline healthcare workers (trusts and GP practices) was 49.9%. In trusts, vaccine uptake was 49.4% (from 94.3% of trusts responding), a decrease from 60.5% vaccine uptake in 2021 to 2022. In GP practices, vaccine uptake was 66.5% (from 14.6% of GP practices), a decrease from 74.1% vaccine uptake in 2021 to 2022.

In Scotland, in 2022 to 2023, the combined uptake among health and social care workers was 46.9%. Please note in the 2021 to 2022 season, healthcare workers and social care workers were reported separately.

In Wales, uptake reached 46.2% in 2022 to 2023 compared with 56.0% in 2021 to 2022.

In Northern Ireland, uptake in frontline healthcare workers including social care was 37.5% in 2022 to 2023 compared with 49.8% in 2020 to 2021. Uptake in 2021 to 2022 is not comparable as it was not possible to disaggregate frontline and non-frontline staff. It should be noted that data sources differ between 2020 to 2021 and 2022 to 2023.

Table 5a: Vaccine uptake in target groups in England

Target groups	Number vaccinated	Denominator	% uptake
65 years and over	8,563,437	10,723,554	79.9
6 months to under 65 years at risk	4,098,547	8,350,452	49.1
Pregnant - no risk	182,511	549,692	33.2
Pregnant - at risk	36,188	75,691	47.8
Pregnant - all	218,699	625,383	35.0
Frontline healthcare workers	596,415	1,195,888	49.9

Table 5b: Vaccine uptake in target groups in Scotland

Target groups	Number vaccinated	Denominator	% uptake
65 years and over	951,476	1,113,408	85.5
6 months to under 65 years at risk	470,883	827,849	56.9
Pregnant - no risk	Not available	Not available	Not available
Pregnant - at risk	Not available	Not available	Not available
Pregnant - all	Not available	Not available	Not available
Frontline healthcare workers	152,330	325,024	46.9

Table 5c: Vaccine uptake in target groups in Wales

Target groups	Number vaccinated	Denominator	% uptake
65 years and over	525,902	689,651	76.3
6 months to under 65 years at risk	200,211	452,847	44.2
Pregnant - no risk	5,859	17,107	34.2
Pregnant - at risk**	1,082	2,316	46.7
Pregnant - all	6,941	19,423	37.7
Frontline healthcare workers ***	44,557	96,505	46.2

Table 5d: Vaccine uptake in target groups in Northern Ireland

Target groups	Number vaccinated	Denominator	% uptake
65 years and over	273,125	329,235	83.0
6 months to under 65 years at risk	Not available	Not available	Not available
Pregnant - no risk	Not available	Not available	Not available
Pregnant - at risk	Not available	Not available	Not available
Pregnant - all	3,127	10,495	29.8
Frontline healthcare workers	16,838	44,901	37.5

1. The pregnant women at risk are included in the under 65 years at risk category.

2. The frontline healthcare workers category excludes social care workers for England, Wales, Scotland and Northern Ireland.

3. In Wales, vaccine coverage in pregnant women is measured differently using a survey of pregnant women giving birth each year during January.

Influenza vaccine (LAIV) programme for children

England

The influenza vaccine uptake in 2 and 3 year olds in England is monitored by UKHSA throughout the season, through weekly and monthly extracts from GP system suppliers via ImmForm. Cumulative vaccine uptake on influenza vaccinations administered up to 28 February 2023 was reported from 98.4% (6,339 out of 6,442) of GP practices in England in 2022 to 2023.

Comparative data is up to 28 February 2022 where vaccine uptake was reported from 97.1% (6,347 out of 6,538) of GP practices in England in 2021 to 2022. This season saw a vaccine uptake for all GP-registered 2 year olds of 42.3% (compared with 48.7% in 2021 to 2022) and was 45.1% in 3 year olds (compared with 51.4% in 2021 to 2022) in England.

The combined uptake for 2 and 3 year olds was 43.7% compared with 50.1% in 2021 to 2022.

In the 2022 to 2023 season, the influenza vaccine programme for school-aged children was focused on the year groups reception to year 9 (aged 4 to 14 years).

The programme was mainly delivered via a school-based route, with one area (Isle of Scilly) delivering vaccinations through general practice. Vaccine uptake was monitored through manual returns by local teams for their responsible population.

An estimated 3,502,566 children in school years reception to year 9 (age 4 to 14 years old) in England received at least one dose of influenza vaccine during the period 1 September 2022 to 28 February 2023. With an estimated total target population of 6,747,523, the overall uptake was 51.9%.

Vaccine uptake in all primary school age children (age 4 to 11 years old) was 56.3% compared with 57.4% in 2021 to 2022 (Table 6). Vaccine uptake in secondary school age children (age 11 to 14 years old) was 41.9% compared with 45.5% in 2021 to 2022 (Table 6).

Vaccine uptake in children of school age reception to year 9 generally decreases with increasing age. This trend has been seen in the 4 previous years where comparable data is available (Table 6 and Figure 43).

Table 6: Percentage influenza vaccine uptake in children of school age: reception to year 11 (age 4 to 14 years old) in England.

Age group	2022 to 2023 Percentage vaccine uptake	2021 to 2022 Percentage vaccine uptake
Reception (age 4 to 5 years)	56.7	56.7
Year 1 (age 5 to 6 years)	56.4	58.7
Year 2 (age 6 to 7 years)	57.5	58.6
Year 3 (age 7 to 8 years)	57.3	57.8
Year 4 (age 8 to 9 years)	56.2	57.2
Year 5 (age 9 to 10 years)	55.6	56.1
Year 6 (age 10 to 11 years)	54.2	55.8
Year 7 (age 11 to 12 years)	45.2	48.2
Year 8 (age 12 to 13 years)	40.7	45.4
Year 9 (age 13 to 14 years)	39.6	42.0
All primary school age (age 4 to 11 years)	56.3	57.4
All secondary school age (age 11 to 14 years)	41.9	45.5
All eligible school age (age 4 to 14 years)	51.9	53.8

Figure 43: Percentage influenza vaccine uptake for children in school years reception to year 6 (age 4 to 10 years rising to 11 years old) (primary school age), collected between 1 September 2022 to 31 January 2023

Overall vaccine uptake for all children of school age reception to year 9 (age 4 to 14 years old) by local authority (not shown here) ranged from 38.5% (391,568 out of 1,016,832) in London to 58.8% (365,556 out of 621,328) in the South West. Uptake by year group and local authority ranged from:

• 21.9% to 77.9% in reception
• 21.2% to 77.1% in year 1
• 21.2% to 86.8% in year 2
• 21.2% to 83.0% in year 3
• 19.9% to 79.6% in year 4
• 19.2% to 79.5% in year 5
• 17.1% to 74.7% in year 6
• 0.7% to 70.6% in year 7
• 0.7% to 67.7% in year 8
• 0.7% to 66.3% in year 9

Further detail on final influenza vaccine uptake data in all cohorts (GP patients, school-aged children and frontline healthcare workers) in England is now publicly available.

Scotland

As previously described, the uptake of seasonal influenza vaccine in children is estimated by PHS throughout the season, using a combination of:

• aggregated data returns submitted by staff in all the territorial NHS boards on a weekly basis for those eligible in the following cohorts:

  • 6 months to 2 years at risk
  • 2 to 5 years not at school
  • primary and secondary school pupils

• weekly extracts of data downloaded from the national clinical data store (NCDS), which contains individual-level data relating to the childhood schools’ and nurseries’ programmes for one NHS board

The estimated uptake in preschool children (2 to under 5 years old, not yet in school) was 56.4%; this compares with 57.2% in 2021 to 2022. For primary school children the estimated uptake was 74.7% compared with 74.6% in 2021 to 2022. In secondary school children there was an estimated uptake of 61.0% compared with 62.1% in 2021 to 2022. Note, caution should be taken when comparing uptake percentage between the seasons due to differences in data collection methods.

Wales

In Wales, immunisations for 2 and 3 year olds were delivered through general practices, apart from one health board where the majority of 3 year olds were immunised through nursery school immunisations sessions (uptake in these nursery school sessions was 50.6%). National uptake of influenza vaccine in 2 and 3 year olds decreased in 2022 to 2023. Uptake of influenza vaccine for children aged 2 years was 42.9% (compared with 47.0% in 2021 to 2022), for 3 year olds it was 44.7% (compared with 48.2% in 2021 to 2022). For the whole group of children aged 2 and 3 years, uptake was 43.8% (compared with 47.6% in 2021 to 2022).

The childhood influenza programme in Wales includes all primary and secondary school children. Uptake in primary school children decreased this season. Children aged 4, 5, 6, 7, 8, 9 and 10 years, received their vaccinations in school immunisation sessions and uptake was 62.7%, 65.0%, 65.1%, 64.2%, 64.5%, 63.9% and 62.7% in each of these groups respectively. For the group as a whole, uptake was 63.8% (compared with 68.7% in 2021 to 2022). Uptake in secondary school children also decreased this season. Children aged 11, 12, 13, 14 and 15 years, received their vaccinations in school immunisation sessions and uptake was 62.3%, 55.3%, 52.9%, 51.4% and 49.8% in each of these groups respectively. For the group as a whole, uptake was 54.4% (compared with 58.2% in 2021 to 2022).

Northern Ireland

Caution should be used when considering the 2021 to 2022 and 2022 to 2023 influenza vaccination uptake rates in pre-school children in comparison with previous seasons, due to the introduction of the new VMS involving new methods of recording and extracting influenza vaccine data. Influenza vaccinations administered by trust school nursing teams are recorded in the Child Health System (CHS), similar to previous seasons.

In 2022 to 2023, the childhood influenza vaccination programme continued to include all pre-school children aged 2 to 4 years old, all primary school aged children (year groups 1 to 7) and post primary school children in year groups 8 to 12. The former group was offered vaccination through primary care, while the latter 2 groups were offered vaccination through school health teams. The vaccination uptake rate in 2022 to 2023 for pre-school children aged 2 to 4 years old was 33.0% (compared with 25.4% in 2021 to 2022). The vaccination uptake rate for children in primary school (aged approximately 4 to 11 years old) was 70.6% (compared with 72.7% in 2021 to 2022). In 2021 to 2022, Northern Ireland expanded the vaccination programme from all year 8 children (introduced in 2020 to 2021) to include post-primary school children (years 8 to 12) with an uptake rate of 63.8%. In 2022 to 2023, uptake within this group was 60.2%. These year groups were vaccinated through school clinics.

Vaccine effectiveness

In England for the 2022 to 2023 season, as in previous seasons, influenza VE was estimated using a test-negative study design. VE is presented against influenza in those requiring hospitalisation following an emergency care attendance in England. Infection and admission data was collected through the Respiratory DataMart surveillance scheme and the Emergency Care Data Set (ECDS). Vaccination status of study participants was obtained through data linkage with the National Immunisation Management Service (NIMS).

In children aged 2 to 17 years, the overall adjusted influenza vaccine effectiveness (aVE) in 2022 to 2023 was 65% (95% CI: 52% to 75%) against all laboratory confirmed influenza. In adults aged 18 to 64 years, the overall aVE was 32% (95% CI: 13% to 47%) against all laboratory confirmed influenza (Table 7). In adults aged 65 years and over, the overall aVE was 28% (95% CI: 15% to 39%). The overall VE largely reflects protection against A(H3N2), as influenza A predominated, and the dominant subtype was A(H3N2).

In subtype-specific analyses across the age groups, low to good aVE was demonstrated against influenza A(H3N2). Low circulation of influenza A(H1N1) led to greater uncertainty in VE against influenza A(H1N1), with low to moderate mid-point estimates, and wide confidence intervals, crossing zero in all age groups. Protection against influenza B was good in children but uncertain in adults aged 18 to 64 years and not able to be estimated in adults aged 65 years and over due to low case numbers.

There was good evidence of protection against hospitalisation by quadrivalent cell-based vaccine (QIVc), the joint first-line recommended vaccine for those aged 18 to 64 years, and the adjuvanted quadrivalent egg-based vaccine (aQIV), joint first line recommended for those aged 65 years and over. There was insufficient usage of recombinant quadrivalent influenza vaccine (QIVr), joint first line recommended vaccine in the 18 to 64 years and 65 years and over age groups, to be able to estimate VE using Respiratory DataMart. For LAIV, the first line recommended vaccine in children aged 2 to 17 years, the aVE was 64% (95% CI: 49% to 75%).

Table 7: Adjusted influenza vaccine effectiveness against hospitalisation following an emergency department visit, using laboratory-confirmed influenza by influenza and vaccine type by age group, 2022 to 2023 season, England

	Ages 2 to 17	Ages 18 to 64	Ages 65 and above
All aVE	65% (52 to 75%)	32% (13 to 47%)	28% (15 to 39%)
Influenza type: A(H1N1)	39% (-47 to 75%)	35% (-39 to 69%)	2% (-79 to 46%)
Influenza type: A(H3N2)	68% (43 to 82%)	25% (-28 to 56%)	39% (14 to 56%)
Influenza type: B	91% (68 to 97%)	42% (-36 to 76%)	Not available
Vaccine type: LAIV	64% (49 to 75%)	Not available	Not available
Vaccine type: QIVc	72% (24 to 89%)	35% (15 to 51%)	30% (-14 to 57%)
Vaccine type: QIVe	Not available	13% (-36 to 44%)	Not available
Vaccine type: aQIV	Not available	Not available	27% (14 to 38%)

Adjusted for week of sample, age group, UKHSA region, clinical risk status, and COVID-19 vaccination status (adults only).

Scotland

In Scotland, in the 2022 to 2023 season, VE was also measured using a test-negative study design. VE is presented against influenza infection among individuals admitted to hospital, as an emergency admission, with a respiratory related diagnosis. The analysis was performed through the data linkage of influenza testing data from the Electronic Communication of Surveillance in Scotland (ECOSS), hospital admission data from Scottish Morbidity Records 01 (SMR01) database and vaccination status of patients from the NCDS. Study period for the analysis was 3 October 2022 to 2 April 2023. Due to limitations in the collection of childhood influenza vaccine data in Scotland, aVE results are only shown for those aged 5 to 15 years.

In children aged 5 to 15 years, the overall aVE in 2022 to 2023 was 71% (95% CI 54% to 82%) against all laboratory confirmed influenza in those admitted to hospital with a respiratory diagnosis. In adults aged 18 to 64 years, the overall aVE was 36% (95% CI 28% to 43%) and in adults aged 65 years and over, overall aVE was 30% (95% CI 21% to 38%).

In subtype-specific analyses across the age groups, moderate to good aVE was demonstrated against influenza A(H3N2). Like England, low circulation of influenza(H1N1) led to greater uncertainty in VE against influenza A(H1N1). Low to good mid-point estimates were reported, but due to wide confidence intervals, crossing zero in children aged 5 to 15 years and adults aged 65 years and over, these results should be interpreted with caution. High aVE was found against influenza B across all age groups; however, like with influenza A(H1N1), these results should be interpreted with caution due to low circulation of influenza B in the study period and wide confidence intervals.

Table 8: Adjusted influenza vaccine effectiveness against infection among individuals admitted to hospital with a respiratory diagnosis, using laboratory confirmed influenza by age group and influenza type, 2022 to 2023 season, Scotland

	Ages 5 to 15**	Ages 18 to 64	Ages 65 and above
All aVE	71% (54 to 82%)	36% (28 to 43%)	30% (21 to 38%)
Influenza type: A	70% (51 to 81%)	34% (25 to 41%)	30% (21 to 38%)
Influenza type: A(H1N1)	68% (-30 to 93%)	56% (30 to 72%)	24% (-25 to 54%)
Influenza type: A(H3N2)	57% (14 to 78%)	33% (3 to 53%)	36% (11 to 54%)
Influenza type: B	91% (23 to 99%)	77% (50 to 90%)	77% (22 to 93%)

Adjusted for time during season, age, sex, deprivation and number of clinical risk groups.

**Data from Ayrshire and Arran, Greater Glasgow and Clyde, Lanarkshire and Tayside is excluded in those aged 5 to 15 years.

Emerging respiratory viruses

Middle East Respiratory Syndrome coronavirus (MERS-CoV) infections

Since the World Health Organization (WHO) first reported cases of Middle East Respiratory Syndrome coronavirus (MERS-CoV) in April 2012, a total of 2,604 laboratory confirmed cases have been reported globally to WHO up to April 2023. This includes 936 fatal cases (case fatality ratio of 36%). In the most recent reporting period between week 40 2022 and week 15 2023, 4 cases have been reported globally by WHO. The majority of these cases have occurred in countries in the Arabian Peninsula. A feature of MERS-CoV is its ability to cause large outbreaks within healthcare settings. Local secondary transmission following importation has been reported from several countries including the UK, France, Tunisia, and the Republic of Korea.

A total of 5 cases of MERS-CoV (3 imported and 2 linked cases) have been confirmed in the UK through ongoing surveillance since September 2012.

In April and May 2014, 2 laboratory confirmed cases transited through London Heathrow Airport on separate flights to the US. Contact tracing of flight contacts did not identify any further cases.

Further information on management and guidance of possible cases is available online. The latest ECDC MERS-CoV risk assessment highlights that risk of widespread transmission of MERS-CoV remains very low.

Avian influenza and other zoonotic influenza

Human influenza A(H5N1) and human influenza A(H5N6)

Since 2003 and up to 24 April 2023, 874 human cases of avian influenza A(H5N1) have been reported worldwide including 458 deaths, giving an overall case fatality ratio of 52.4%. Since 2020, the avian influenza A(H5N1) clade 2.3.4.4b has become widespread in birds, with some limited spillover to non-avian species and rare detections in humans.

In mid-May 2023 UKHSA notified WHO of the detection of avian influenza A(H5) virus in a poultry worker at infected premises in England where poultry was infected with high pathogenicity avian influenza (HPAI) A(H5N1) viruses. Another detection was reported in a second individual at the infected premises. Both detections were later confirmed by additional testing as A(H5N1) clade 2.3.4.4b. Both cases were asymptomatic and detected as part of an ongoing enhanced surveillance study of asymptomatic workers exposed to poultry infected with avian influenza.

Since 2021 there has been an increase in detections of influenza A(H5N6) in humans, with up to 84 laboratory-confirmed cases of human infection from 2014 to 19 May 2023 reported to WHO in the Western Pacific Region. The last case was reported from China, with an onset date of 17 December 2022. The case was hospitalised on 21 December 2022 with severe pneumonia and has since recovered.

Human influenza A(H9N2)

Since 2021 there has been an increase in detections of influenza A(H9N2) in humans, with up to 87 laboratory-confirmed cases of human infection reported from 2015 to 19 March 2023. Of these, 85 were reported from China and 2 were reported from Cambodia. The last case was reported from China, with an onset date of 5 February 2023, and has since recovered. The majority of human cases of avian influenza A(H9N2) have reported exposures to avian species such as backyard poultry or live poultry markets.

Avian influenza summary

Most human cases of avian influenza viruses are exposed through contact with infected poultry or contaminated environments, including live poultry markets or domestically kept birds. Since the viruses continue to be detected in animals and environments, further human cases can be expected. Even though small clusters of H5N1 and H7N9 virus infections have been reported, current epidemiological and virological evidence suggests that these viruses have not acquired the ability to undergo sustained transmission among humans. It is important to ensure that imported cases of suspect avian influenza are detected promptly to ensure public health measures including infection control can be rapidly put in place to minimise any risk of onward transmission.

There have been multiple detections of avian influenza in avian species in the UK in 2022 to 2023 as reported by APHA.

Information on recent acute public health events or potential events of concern including avian influenza can be found on WHO Disease Outbreak News.

Swine influenza

There continue to be some sporadic reports internationally of human infections with swine influenza viruses such as influenza A(H1N1)v and influenza A(H1N2)v. Close contact with infected pigs or attendance at sites such as farms and agricultural fairs continue to pose an exposure risk. There have been no new cases reported from the UK.

Conclusions

During the 2022 to 2023 season, influenza A dominated, with influenza A(H3N2) the predominant subtype in Great Britain and A(H1N1)pdm09 predominant in Northern Ireland. Influenza A activity was concentrated in a relatively short period, and relatively early within the typical seasonal range with some late season influenza B activity at low levels.

High levels of influenza activity were seen across most surveillance systems from week 48 2022 to week 3 2023, where activity through multiple indicators showed a rapid increase in activity, followed by a rapid decline in activity and returning to low levels of activity for the remainder of the season.

Excess mortality associated with influenza was above average for a season, though not into the range of the highest impact seasons when over 20,000 excess deaths are recorded in England. Mortality was likely influenced by this being an A(H3N2) dominated season, which is typically more severe in older adults due to lack of childhood exposure, and by the protection given by the vaccines, which were well-matched to the circulating A(H3N2) viruses.

Influenza-like-illness consultation rates showed consistent activity trends with other indicators, although lower compared with previous years; it should be noted that peak activity weeks coincided with bank holiday weeks and that fewer appointment slots may have reduced ILI consultation rates.

Influenza hospitalisation rates also peaked at weeks 50 to 52 2022, with peak and cumulative activity surpassing previous years substantially. This should be interpreted in the context of more intensive testing leading to improved virological ascertainment of patients hospitalised by influenza compared with norms prior to the COVID-19 pandemic. Peak ICU admission rates in England were only marginally higher than in previous seasons with significant influenza activity, with cumulative admission rates being lower than pre-pandemic seasons. This relative lower utilisation may reflect a number of factors including higher pre-pandemic testing in critical care relative to medical wards, critical care concurrent demands, and this being an A(H3N2) dominated season, rather than an A(H1N1)pdm09 dominated season which would have had higher impact on younger adults and therefore critical care. ECMO admissions due to SARI were mainly caused by influenza.

Influenza activity in the UK in 2022 to 2023 has been broadly comparable to other Northern Hemisphere countries, though relatively higher proportions of A(H1N1)pdm09 were seen in much of Europe.

Vaccines had an important role in reducing morbidity and other impacts of influenza this winter. Vaccine effectiveness against hospitalisation was higher in children (point estimate 60 to 70% across nations providing estimates) than in adults (point estimates around 30 to 35%). There was higher protection against influenza B than A where analysis was possible, and broad confidence intervals for estimates of vaccine effectiveness against different influenza A subtypes.

This has been the first full influenza season since the COVID-19 pandemic was declared in 2020. Previous sero-epidemiological investigation has shown some reduction in population immunity associated with the reduced circulation of influenza from COVID-19 non-pharmaceutical control measures. It is likely the resultant higher-than-usual population susceptibility contributed to the timing, shape and scale of influenza activity observed in 2022 to 2023 (see van Leeuwen and others on medRxiv, January 2023).

Activity from other typical circulating respiratory viruses including rhinovirus, adenovirus, parainfluenza and hMPV, was overall similar to that seen in the previous few seasons. RSV activity was similar to those observed in pre-pandemic seasons.

Surveillance continues in the UK for other respiratory viruses, as well as novel respiratory viruses including MERS-CoV and avian influenza viruses such as influenza A(H7N9), influenza A(H5N1) and influenza A(H5N6), with risk assessments being updated regularly.

Acknowledgements

Compiled by the Influenza surveillance section, Immunisation and vaccine-preventable diseases division, UK Health Security Agency with contributions from:

• Public Health Scotland
• Public Health Wales
• Public Health Agency, Northern Ireland
• Royal College of General Practitioners
• Real-time syndromic surveillance team, UK Health Security Agency
• Co- and secondary infections team, UK Health Security Agency
• Respiratory Virus Unit, Colindale, UK Health Security Agency