Official Statistics

National norovirus and rotavirus report, week 23 report: data to week 21 (data up to 26 May 2024)

Updated 8 August 2024

Applies to England

This monthly report covers the 4-week period between 29 April to 26 May 2024. Data reported here provides a summary of norovirus and rotavirus activity, including enteric virus (EV) outbreaks, in England up to reporting week 21 of the 2023/2024 season.

In this report the ‘5-season average’ is calculated from the same period during the 5 seasons of 2015/2016, 2016/2017, 2017/2018, 2018/2019 and 2022/2023. Refer to data sources and methodology for more information and for guidance on interpretation of trends.

Main points

The main messages of this report are:

• since an unseasonal rise in norovirus activity which peaked in week 17 of 2024, norovirus laboratory reports have decreased week-on-week but activity has remained high. The total number of norovirus laboratory reports between weeks 18 to 21 of 2024 was double the 5-season average for the same 4-week period. However, it was 23% lower than the total for the previous 4-week period in 2024, weeks 14 to 17

• rotavirus reporting has increased in recent weeks, with activity during weeks 18 to 21 of 2024 28% higher than the 5-season average for the same 4-week period

• reported EV outbreaks have decreased in recent weeks and remain 23% lower than the 5-season average for the total period of weeks 18 to 21 of 2024. The majority of outbreaks were reported in care home settings

• in recent weeks, the number of norovirus outbreaks reported to the Hospital Norovirus Outbreak Reporting System (HNORS) has decreased, however total reports were 33% higher than the  5-season average during weeks 18 to 21

• the most commonly identified GII.4 norovirus strain so far this season is Norovirus/GII.4/Sydney/2012-like variants. Since April 2024, there has been an increase in the proportion of samples characterised as GII.17

Laboratory surveillance

Data presented here is derived from the Second-Generation Surveillance System (SGSS). Refer to the data sources and methodology section for more information and for guidance on interpretation of trends.

Overall, up to week 21 of the 2023/2024 season, the cumulative number of positive norovirus laboratory reports in England (9,024 laboratory reports) was 42% higher than the 5-season average for the same period (6,338 laboratory reports).

While norovirus activity has remained high, reporting has started to decrease in recent weeks. Total norovirus laboratory reports during weeks 18 to 21 of 2024 (962 laboratory reports) were double the 5-season average (471 laboratory reports) for the same 4-week period (Figure 1).  However, norovirus laboratory reports between weeks 18 and 21 of 2024 were 23% lower than the previous 4-week period of weeks 14 to 17 of 2024 (1,242 laboratory reports).

Since an unseasonal rise in norovirus activity which peaked in week 17 of 2024, norovirus laboratory reports have decreased week-on-week. This downward trend in reporting has been observed across all 9 regions of England and all age groups, although adults aged 65 years and over remain the most affected age group.

It is likely that multiple factors contributed to the observed increase in laboratory reports, such as lower than average temperatures during this period facilitating transmission pathways, ongoing changes to the epidemiology following the COVID-19 pandemic or changes in testing and reporting to national surveillance.

Figure 1. Norovirus laboratory reports in England by week during the 2023/2024 season, compared with 5-season average

The cumulative number of positive rotavirus laboratory reports in England up to week 21 of the current season (2,663 laboratory reports) was 7% higher than the 5-season average for the same period (2,494 laboratory reports) (Figure 2).

Rotavirus activity has increased in recent weeks. The total number of laboratory reports (500 laboratory reports) for the 4-week period of weeks 18 to 21 of 2024 was 28% higher than the 5-season average (390 laboratory reports) for the same period. Rotavirus laboratory reports between weeks 18 and 21 of 2024 were 37% higher than the previous 4-week period of weeks 14 to 17 of 2024 (366 laboratory reports).

Figure 2. Rotavirus laboratory reports in England by week during the 2023/2024 seasons, compared with the 5-season average

Outbreak surveillance

Data presented here is derived from HPZone and the Hospital Norovirus Outbreak Reporting System (HNORS). Please see data sources and methodology for more information and for guidance on interpretation of trends.

Up to week 21 of the 2023/2024 season, the cumulative number of EV gastroenteritis outbreaks reported to HPZone was 31% lower than the 5-season average, with 2,526 vs 3,654 outbreaks, respectively (Figure 3).

In weeks 18 to 21 of 2024, the total number of reported EV outbreaks remained lower than the 5-season average for the same 4-week period (23% lower, with 218 versus 282 reports, respectively). During this 4-week period, the majority of reported EV outbreaks (all suspected or confirmed as norovirus) occurred in care home settings (70%, Figure 3). The total number of EV outbreaks reported was 11% lower than the previous 4-week period of weeks 14 to 17 (282 outbreaks).

During the 5 seasons from which the 5-season average was calculated, overall, 64% of all reported outbreaks attributed to EVs (norovirus, rotavirus, sapovirus and astrovirus) occurred in care home settings, 20% in educational settings, 11% in hospital settings and 5% in ‘other’ settings. Of the outbreaks attributed to EVs, 99% were reported as suspected and confirmed norovirus outbreaks. Only 14% of reported EV outbreaks were laboratory confirmed as norovirus during the previous 5 seasons.

Figure 3. Enteric virus gastroenteritis outbreaks reported to HPZone in England by setting during the 2023/2024 season, compared with the 5-season average

Up to week 21 of the 2023/2024 season, 290 outbreaks have been reported to HNORS (Figure 4), 25% lower than the 5-season average for the same period (384 outbreaks). Over the 4-week period of 18 to 21, reporting decreased each week. However, due to very high levels of reporting in week 18 the number of outbreaks reported during this period was 33% higher than the 5-season average for the same period (31 outbreaks compared to 23 outbreaks).

Overall, 96% of outbreaks were laboratory confirmed as norovirus.

During the 5 seasons from which the 5-season average was calculated, overall, 80% of outbreaks reported to HNORS were laboratory confirmed as norovirus.

Figure 4. Suspected and confirmed norovirus outbreaks reported to HNORS in England by week of occurrence during the 2023/2024 seasons, compared with the 5-season average

Molecular surveillance for norovirus

Data presented here is provided by UKHSA’s EVU and is used by UKHSA to monitor circulating norovirus variants. Please see data sources and methodology for more information and for guidance on interpretation of trends.

Of the 1,306 norovirus positive samples characterised during the 2023/ 2024 season to date, 86% (1,119 out of 1,306) were genogroup 2 (GII); 13% (175 out of 1,306) were genogroup 1 (GI) and 1% were mixed (12 out of 1,306). The most frequent norovirus GII genotypes identified were GII.4 (37%) and GII.17 (25%), the most frequently identified norovirus GI genotypes were GI.3 (6%) and GI.6 (4%). The most commonly identified GII.4 norovirus strain so far this season is Norovirus/GII.4/Sydney/2012-like variants. Since April 2024 there has been an increase in the proportion of samples characterised as GII.17.

Data sources and methodology

The Second-Generation Surveillance System (SGSS) is the national laboratory reporting system, recording positive laboratory reports of norovirus and rotavirus.

The Hospital Norovirus Outbreak Reporting System (HNORS) is a web-based scheme for reporting suspected and confirmed norovirus outbreaks in Acute NHS Trust hospitals, and captures information on the disruptive impact these outbreaks have in hospital settings.

HPZone is a web-based case and outbreak management system used by health protection teams (HPTs) to record outbreaks they are notified of and investigate. In England, suspected and confirmed EV outbreaks (norovirus, rotavirus, astrovirus and sapovirus) are reported as ‘gastroenteritis’ outbreaks. Please note this was not available for this time period.

Norovirus characterisation data is produced by the EVU and is used to monitor the diversity of circulating strains of norovirus in England.

Background information

No single surveillance system fully captures national changes in norovirus or rotavirus activity. Therefore, this report presents data from 4 systems which collectively describes recent trends. Data is reported by season rather than calendar year, in order to capture the winter peak of activity in one reporting period. Refer to data sources and methodology for more information and for guidance on interpretation of trends.

Norovirus activity and the timing of the peak in reporting can vary considerably and differ from one season to the next. Transmission is mostly via contact with an infected person, but norovirus can also be spread by consumption of food contaminated with the virus or contact with contaminated surfaces. Norovirus transmission is influenced by many factors including, but not limited to, whether the person has been infected with norovirus recently, community contact patterns and the time of the year. Infections peak in the colder months and therefore norovirus is most prevalent during winter.

Since the 2019/2020 season norovirus activity in England has been more variable compared to historical trends, likely due to multiple impacts of the COVID-19 pandemic. Refer to data sources and methodology for more information on the impact of the pandemic. Throughout the 2019/2020 and 2020/2021 seasons a reduction in reporting to national surveillance was observed. This was followed by unusual activity in the 2021/2022 season such as a greater proportion of outbreaks reported in educational settings than before the pandemic. In the 2022/2023 season norovirus activity returned to pre-pandemic levels and in early 2023 norovirus reporting peaked at a level twice as high as that experienced in the decade prior to the emergence of COVID-19.

Following the introduction of the rotavirus vaccine in July 2013 the total number of laboratory-confirmed rotavirus infections each season has remained low compared to the pre-vaccine period. A 77% decrease in laboratory-confirmed rotavirus infections in infants was observed in the first season following vaccine introduction (1).

The UK Health Security Agency (UKHSA) routinely undertakes norovirus characterisation as part of national surveillance to monitor the diversity of circulating strains. This molecular surveillance enables detection of novel strains or emergence of existing strains that could lead to a strain replacement event and which have previously been associated with a temporal shift in norovirus activity (2). Norovirus activity varies from season to season and therefore differences will be observed between every season.

The most commonly detected norovirus genotype worldwide is genogroup II- genotype 4 (GII.4). Historically, between 1995 and 2013 there have been 5 global GII.4 strain replacements events (3, 4). Since the winter of the 2012/2013 season and before the emergence of COVID-19 in England, the most frequently detected strain was Norovirus/GII.4/Sydney/2012 or GII.4 Sydney2012-like variants (5).

To enable effective molecular surveillance, it is crucial that samples are obtained from suspected norovirus cases or outbreaks for laboratory confirmation and then norovirus-positive samples are referred on to the Enteric Virus Unit (EVU) for characterisation.

Interpretation of trends

In order to capture the winter peak of activity in the reporting period the norovirus and rotavirus season runs from week 27 in year 1 to week 26 in year 2, that is, week 27 of 2023 to week 26 of 2024, July to June. The 2023/2024 season is compared to the 5-season average calculated from the 5-season period of 2015/2016 to 2018/2019 and 2022/2023. The 2019/2020, 2020/2021 and 2021/2022 seasons are not included in this calculation due to the adverse impact of the emergence of COVID-19 on surveillance part way through the 2019/2020 season and the continued impact into the 2020/2021 and 2021/2022 seasons. In years with a week 53 (2015 and 2020) data is combined with week 52 data to avoid distortion of the figure.

Under-ascertainment is a recognised challenge in EV surveillance with sampling, testing and reporting criteria known to vary by region. In addition, samples for microbiological confirmation are collected in a small proportion of community outbreaks. Therefore, this report provides an overview of EV activity across England and data should be interpreted with caution.

All surveillance data included in this report is extracted from live reporting systems, is subject to a reporting delay, and the number reported in the most recent weeks may rise further as more reports are received. Therefore, data pertaining to the most recent 2 weeks is not included.

Impact of COVID-19 pandemic on surveillance

UKHSA relaunched the Official Statistics National Norovirus and Rotavirus Report after it was temporarily suspended due to quality issues with the data from the 4 aforementioned data sources during the COVID-19 pandemic period. Additional analyses of this data were undertaken and demonstrated the quality of this data was comparable with the data collected before the pandemic and therefore reporting was allowed to resume as an Official Statistic. Between December 2020 and October 2022 the report was replaced by the National Norovirus and Rotavirus Bulletin to ensure an overview of norovirus and rotavirus activity in England continued to be available to the public (data covering the periods 2020/2021 and 2021/2022 is available at National norovirus and rotavirus bulletins 2020 to 2021: management information and National norovirus and rotavirus bulletins 2021 to 2022: management information).

The COVID-19 pandemic affected activity across many gastrointestinal pathogen surveillance indicators for England in 2020 and 2021, and reduced norovirus reporting continued into early 2022. The reasons for the reduction in norovirus reporting are considered to be multifactorial. It is likely that the interventions implemented to control COVID-19 led to a reduction in enteric virus transmission. However, when considering the surveillance data reported here, the magnitude of the reduction is unlikely to be wholly attributable to these control measures alone. It is likely that other factors such as, but not limited to, changes in ascertainment, access to health care services and capacity for testing also contributed to the observed reduction due to changes in ascertainment and varied over time. The reduction in norovirus reporting to national surveillance during the 2019/2020 and 2020/2021 seasons also led to a period of low referral of norovirus-positive samples for characterisation. Therefore, trends for the 2019/2020, 2020/2021 and 2021/2022 seasons should be interpreted with caution.

SGSS data

SGSS data is England only. Week number is calculated from specimen date and location is based on laboratory geography. Norovirus data includes faecal and lower gastrointestinal tract specimen types only. Reporting may be subject to differences in regional ascertainment.

HPZone data

HPZone data utilises the week of date of outbreak entry on to HPZone for analyses due to mandatory completion of the field. While this usually reflects the date of notification, batch reporting of outbreaks can occur.

HNORS data

HNORS reporting is voluntary and variations may reflect differences in ascertainment or reporting criteria by region. National guidance recommends closure of the smallest possible unit in hospitals. Therefore, not all outbreaks reported to HNORS result in whole ward closure (some closures are restricted to bays only) and not all suspected cases are tested. Additionally, not all suspected cases are tested for norovirus – often only a proportion of individuals will be tested in any suspected outbreak.

Week number is calculated from the date of first case onset for HNORS data.

From May to October 2019 and during February 2020 the HNORS website was temporarily offline. The reliance on manual data collation during this period may have negatively impacted ascertainment so trends should be interpreted with caution.

Norovirus characterisation data

Norovirus genotype and GII.4 strain characterisation data from the reference laboratory is subject to a reporting delay, and the numbers reported in any week may rise further as additional characterisation data becomes available.

References

1. Atchison and others. ‘Rapid declines in age group–specific rotavirus infection and acute gastroenteritis among vaccinated and unvaccinated individuals within one year of rotavirus vaccine introduction in England and Wales’ The Journal of Infectious Diseases: volume 213, pages 243 to 249 (viewed on 17 October 2022)

2. Allen and others. ‘Emergence of the GII-4 norovirus Sydney2012 strain in England, winter 2012 to 2013’ The Public Library of Science One: volume 2, article e88978 (viewed on 17 October 2022)

3. Allen and others. ‘Characterisation of a GII-4 norovirus variant-specific surface-exposed site involved in antibody binding’ Virology Journal: volume 6, article number 150 (viewed on 17 October 2022)

4. Zakikhany and others. ‘Molecular evolution of GII-4 Norovirus strains’ The Public Library of Science One: volume 7, article e41625 (viewed on 17 October 2022)

5. Ruis C and others. ‘The emerging GII.P16-GII.4 Sydney 2012 norovirus lineage is circulating worldwide, arose by late-2014 and contains polymerase changes that may increase virus transmission’ The Public Library of Science One: volume 6, article e0179572 (viewed on 17 October 2022)

Further information

Further information about norovirus surveillance and rotavirus surveillance is available on GOV.UK.

Our statistical practice is regulated by the Office for Statistics Regulation (OSR). OSR sets the standards of trustworthiness, quality and value in the Code of Practice for Statistics that all producers of official statistics should adhere to. You are welcome to contact us directly by emailing NoroOBK@ukhsa.gov.uk with any comments about how we meet these standards. Alternatively, you can contact OSR by emailing regulation@statistics.gov.uk or via the OSR website.

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UKHSA will next be conducting a formal review of these statistics in summer 2024. Following this review, an implementation plan will be developed to continue to improve the trustworthiness, quality, and value of these statistics. Key continuous improvements made will be highlighted within future releases of these statistics for transparency.

Acknowledgements

We are grateful to all who provided data used in this report, including NHS Infection Control and Prevention staff (HNORS users), UKHSA local health protection teams (HPTs) and UKHSA regional teams (Field Services), and UKHSA Regional Public Health and Collaborating Laboratories.

This report was produced by the Gastrointestinal Infections and Food Safety (One Health) Division, UKHSA.

Please direct any queries or comments to NoroOBK@ukhsa.gov.uk