Official Statistics

National norovirus and rotavirus report, week 51 report: data to week 49 (8 December 2024)

Updated 19 December 2024

Applies to England

This report covers the 2-week period between 25 November to 8 December 2024. Data reported here provides a summary of norovirus and rotavirus activity, including enteric virus (EV) outbreaks, in England up to reporting week 49 of the 2024/2025 season.

Please note, a report will not be published in week 52 of 2024 or week 1 of 2025. The next report will be published on Thursday 9 January 2025.

In this report the ‘5-season average’ is calculated from the same period during the 5 seasons of 2016/2017, 2017/2018, 2018/2019, 2022/2023 and 2023/2024. Please 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:

• this season, the increase in norovirus activity has begun earlier than usual and remained high in recent weeks, particularly in adults aged 65 years and over
• total norovirus laboratory reports between weeks 48 and 49 of 2024 almost double the 5-season average for the same 2-week period
• rotavirus reporting has increased in recent weeks but was within expected levels during the 2-week period of weeks 48 and 49
• the number of norovirus outbreaks reported to the Hospital Norovirus Outbreak Reporting System (HNORS) since the start of the 2024/2025 season is 23.7% higher than the 5-season average
• during the 2024/2025 season to date, the majority (89.1%) of samples characterised were norovirus genogroup 2 (GII), of which the most frequently identified genotype was GII.17 (62.2%)

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.

Norovirus

The timing of the typical seasonal increase and peak of norovirus activity varies from one season to the next. This season the increase in reporting has begun earlier when compared with the 5-seasons from which the average is calculated.  

Overall, up to week 49 of the 2024/2025 season, the cumulative number of positive norovirus laboratory reports in England (4,523 laboratory reports) was more than double the 5-season average for the same period (2,057 laboratory reports).

Norovirus activity has remained high this season. Total norovirus laboratory reports during weeks 48 and 49 of 2024 (782 laboratory reports) were almost double (95.5%) the 5-season average (400 laboratory reports) for the same 2-week period (Figure 1). Overall, norovirus laboratory reports between weeks 48 and 49 of 2024 were 20.5% higher than during the previous 2-week period of weeks 46 and 47 of 2024 (649 laboratory reports).

Figure 1. Norovirus laboratory reports in England by week during the 2024/2025 season, compared with the 5-season average

During weeks 48 and 49, reporting remained high across all regions of England and all age groups, with the highest number of reports in adults aged 65 years and over.

It is likely that multiple factors contributed to the observed increase in laboratory reports, such as ongoing changes to the epidemiology following the COVID-19 pandemic or changes in testing and reporting to national surveillance. Further work to understand the drivers of the increased reporting is underway.

Rotavirus

The cumulative number of positive rotavirus laboratory reports in England up to week 49 of the current season (1,168 laboratory reports) was 48.8% higher than the 5-season average for the same period (785 laboratory reports) (Figure 2). This is mostly attributable to higher than usual activity at the beginning of the season between weeks 29 and 40.

Figure 2. Rotavirus laboratory reports in England by week during the 2024/2025 seasons, compared with the 5-season average

Rotavirus activity has increased in recent weeks but has remained within expected levels. The total number of laboratory reports (75 laboratory reports) for the 2-week period of weeks 48 and 49 of 2024 were 16.1% higher than the 5-season average (65 laboratory reports) for the same period. Total rotavirus laboratory reports during this period were 11.9% higher than the previous 2-week period of weeks 46 and 47 of 2024 (67 laboratory reports).

Outbreak surveillance

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

A programme of work has recently commenced to move the collection of outbreak surveillance data from HPZone to the Case and Incident Management System (CIMS). As it will be difficult to properly assure the quality of data during this migration, we have paused the reporting of this section for this release, until this programme is complete. This has affected the August, September, October, November and December releases. We hope to resume reporting outbreak surveillance data in March 2025.

Up to week 49 of the 2024/2025 season, 120 outbreaks have been reported to HNORS (Figure 3), 23.7% higher than the 5-season average (97 outbreaks).

Figure 3. Suspected and confirmed norovirus outbreaks reported to HNORS in England by week of occurrence during the 2024/2025 season, compared with the 5-season average

Overall, 82.5% of outbreaks were laboratory confirmed as norovirus.

During the 5 seasons from which the 5-season average was calculated, overall, 85.0% of outbreaks reported to HNORS were laboratory confirmed as norovirus.

Molecular surveillance for norovirus

Data presented here is provided by UKHSA’s Enteric Virus Unit (EVU) and is used by UKHSA to monitor circulating norovirus variants. Refer to the data sources and methodology sections for more information and for guidance on interpretation of trends.

Of the 709 norovirus positive samples characterised during the 2024/2025 season to date, 89.1% (632 out of 709) were genogroup 2 (GII); 9.4% (67 out of 709) were genogroup 1 (GI) and 1.4% were mixed (10 out of 709). The most frequent norovirus GII genotypes identified were GII.17 (62.2%) and GII.4 (13.7%). The most frequently identified norovirus GI genotype was GI.3 (6.1%). Since April 2024 there has been an increase in the proportion of samples characterised as GII.17, with a similar trend noted in the United States and other European countries (1). During the 2024/2025 season so far this was the most commonly detected norovirus genotype.

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 describe recent trends. Data is reported by season rather than calendar year, in order to capture the winter peak of activity in one reporting period (please see 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, as 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 (please see data sources and methodology and impact of COVID-19 pandemic on surveillance sections for more information on the impact of the pandemic). Throughout the 2019/2020 and 2020/21 seasons a reduction in reporting to national surveillance was observed, followed by unusual activity in the 2021/2022 season such as a greater proportion of outbreaks reported in educational settings than prior to 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 (2).

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 (3). Norovirus activity varies from season to season and therefore differences will be observed between every season.

Worldwide the most commonly detected norovirus genotype is genogroup II-genotype 4 (GII.4). Historically between 1995 and 2013 there have been 5 global GII.4 strain replacements events (4, 5). Since the winter of the 2012/2013 season and prior to the emergence of COVID-19 in England the most frequently detected strain was Norovirus/GII.4/Sydney/2012 or GII.4 Sydney2012-like variants (6) until April 2024 when GII.17 emerged(1).

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 2024 to week 26 2025, July to June. The 2024/2025 season is compared to the 5-season average calculated from the 5-season period of 2016/2017 to 2018/2019 and 2022/2023 to 2023/2024. 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 enteric virus 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 enteric virus 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

The UK Health Security Agency (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 period 2020/2021 is available at National norovirus and rotavirus bulletins 2020 to 2021: management information. Data covering the period 2021/2022 is available at National norovirus and rotavirus bulletins 2021 to 2022: management information.

The COVID-19 pandemic impacted 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 include faecal and lower Gastrointestinal tract specimen types only. Reporting may be subject to differences in regional ascertainment.

HPZone data

HPZone data utilises 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. A programme of work has recently commenced to move the collection of outbreak surveillance data from HPZone to the Case and Incident Management System (CIMS).

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 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 additional characterisation data become available.

Further information and contact details

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

References

1. Chhabra and others. ‘Increased circulation of GII.17 noroviruses, six European countries and the United States, 2023 to 2024’ EuroSurveillance: volume 29, pages 1 to 6 (viewed on 26 September 2024)

2. 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)

3. 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)

4. 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)

5. 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)

6. 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)

Feedback and contact information

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

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

Alternatively, you can contact OSR by emailing regulation@statistics.gov.uk or via the OSR website.

Acknowledgements

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

Official statistics

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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.