Official Statistics

National norovirus and rotavirus report, week 23 report: data up to week 21 (28 May 2023)

Updated 10 August 2023

Applies to England

This monthly report covers the 4-week period between 1 to 28 May 2023. Data reported here provide a summary of norovirus and rotavirus activity (including enteric virus (EV) outbreaks) in England up to reporting week 21 of the 2022 to 2023 season. The next monthly report will be published on 13 July 2023.

Please note HPZone Enteric Virus (EV) outbreak data for the current reporting period is unavailable. For more information, see Outbreak surveillance section.

The main messages of this report are:

1. Norovirus activity has decreased in recent weeks, norovirus laboratory reports during weeks 18 to 21 of the 2022 to 2023 season were comparable to the 5-season average of the same period prior to the emergence of COVID-19 (2014/2015 to 2018/2019).
2. Rotavirus activity has decreased and during weeks 18 to 21, 2023, laboratory reports of rotavirus were 10% lower than the 5-season average (2014/2015 to 2018/2019) for the same 4-week period pre-COVID-19.
3. Norovirus outbreaks reported to HNORS have continued to decrease and were below the 5-season average in recent weeks.
4. During the 2022 to 2023 season to date the majority (84%) of samples characterised were norovirus Genogroup 2 (GII) of which the most frequently identified strain was genotype GII.4 (57%).

Background

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 interpretation of trends section for more details).

The COVID-19 pandemic led to many changes that likely had a negative effect on surveillance indicators, but which also likely resulted in reduced norovirus and rotavirus (and other EV) transmission. The reasons for the reduction in reporting over the pandemic are considered to be multifactorial (data covering this time period is available at National norovirus and rotavirus bulletins 2021 to 2022: management information and please see impact of COVID-19 pandemic on surveillance section for more details).

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).

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.

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 (3, 4). Since the winter of the 2012 to 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 (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.

Laboratory surveillance

Data presented here is derived from the Second-Generation Surveillance System (SGSS). Please see data sources and reporting caveats for more information, for guidance on interpretation of trends and the impact of the COVID-19 pandemic.

A sustained period of unusually low activity from March of the 2019 to 2020 season continued into the 2020 to 2021 season (data covering this period is available at National norovirus and rotavirus bulletins 2021 to 2022: management information), not returning to the overall seasonal trend observed pre-pandemic until the latter part of the 2021 to 2022 season. Since week 52, 2022 norovirus laboratory reports have consistently exceeded the pre-COVID-19 5-season average (Figure 1).

This season the cumulative number of positive norovirus laboratory reports in England up to week 21 (7,970 laboratory reports) is 30% higher than the 5-season average for the same period prior to the pandemic (2014/2015 to 2018/2019) (6,139 laboratory reports).

Norovirus activity over the 4 week period of weeks 18 to 21, 2023 (473 laboratory reports) decreased compared with activity during weeks 14 to 17, 2023 (791 laboratory reports). Total norovirus laboratory reports during weeks 18 to 21 were comparable to the 5-season average (482 laboratory reports) for the same 4 week period (Figure 1).

Figure 1. Norovirus laboratory reports in England by week during 2021 to 2022 and 2022 to 2023 seasons, compared with 5-season average

Similar to norovirus, there was a notable reduction in rotavirus activity from March 2020 onwards. Reported rotavirus activity remained low throughout the whole 2020 to 2021 season (data covering this period is available at National norovirus and rotavirus bulletins 2021 to 2022: management information) and into the first half of the 2021 to 2022 season. Reporting returned to levels comparable to the 5-season average of the same period pre-COVID-19 in 2022 (Figure 2), generally following the historical pre-COVID-19 seasonal trend from week 13 onwards.

The cumulative number of positive rotavirus laboratory reports in England to week 21 of the current season (2,470 laboratory reports) was 9% lower than the 5-season average (2014/2015 to 2018/2019, post-vaccine and pre-COVID-19) for the same period (2,704 laboratory reports).

Rotavirus activity has decreased in recent weeks and overall rotavirus activity for the 4-week period of weeks 18 to 21, 2023 (441 laboratory reports) was 10% lower than the 5-season average (488 laboratory reports) for the same period.

Figure 2. Rotavirus laboratory reports in England by week during 2021 to 2022 and 2022 to 2023 seasons, compared with 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 reporting caveats for more information, for guidance on interpretation of trends and the impact of the COVID-19 pandemic.

The HPZone EV outbreak data required to update figures 3 and 4, and the accompanying text, was not available at the time of the production of this report. Therefore, data covering the current reporting period (week 18 to 21, 2023) will be included in the next report published on 13 July 2023. Please refer to the previous report for the most recent published HPZone EV outbreak data (National norovirus and rotavirus report, week 19 report: data up to week 17 (30 April 2023)).

Throughout the 2021 to 2022 season up to week 50 of the 2022 to 2023 seasons, reports of suspected and confirmed norovirus outbreaks in hospitals (captured by HNORS) were substantially lower than the 5-season average. Up to week 21 of the 2022 to 2023 season, 255 outbreaks have been reported (Figure 5), a 48% decline in reporting compared with the pre-COVID-19 5-season average for the same period (490 outbreaks). Overall, 95% of outbreaks (241) were laboratory confirmed as norovirus.

The weekly number of outbreaks reported to HNORS has remained below the 5-season average across the majority of the 2022/2023 season to date. However, between weeks 2 and 10 of 2023 reported outbreaks increased, coinciding with high levels of norovirus laboratory reports. Over the 4-week period of weeks 18 to 21 2023 reported outbreaks have decreased compared with the previous 4 week period (weeks 14 to 17).

Figure 5. Suspected and confirmed norovirus outbreaks reported to HNORS in England by week of occurrence during the 2021 to 2022 and 2022 to 2023 seasons compared with the 5-season average

Molecular surveillance for norovirus

Data presented here is provided by UKHSA’s Enteric Virus Unit (EVU). The reduction in norovirus reporting to national surveillance during the 2019 to 2020 and 2020 to 2021 seasons lead to a period of low referral of norovirus-positive samples for characterisation, therefore this data is not presented. Please see data sources and reporting caveats for more information, for guidance on interpretation of trends and the impact of the COVID-19 pandemic.

During the 2021 to 2022 season, 572 norovirus positive samples were characterised of which 90% were genogroup 2 (GII), 8% were genogroup one (GI) and 2% were mixed. The 3 most frequent norovirus GII genotypes identified were GII.4 (48%), GII.3 (13%) and GII.2 (10%) and the most frequently identified norovirus GI genotypes were GI.3 (3%) and GI.6 (3%). The most common GII.4 norovirus strain identified was Norovirus/GII.4/Sydney/2012-like variants.

Of the 866 norovirus positive samples characterised during the 2022/2023 season to date, 84% (727 out of 866) were genogroup 2 (GII); 14% (125 out of 866) were genogroup 1 (GI) and 2% were mixed (14 out of 866). The 3 most frequent norovirus GII genotypes identified were GII.4 (57%), GII.7 (8%) and GII.6 (7%) and the most frequently identified norovirus GI genotypes were GI.2 and GI.3 (both 4%). The most commonly identified GII.4 norovirus strain so far this season is Norovirus/GII.4/Sydney/2012-like variants.

Data sources

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.

Reporting caveats

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 2021 to week 26 2022, July to June.

The 2021 to 2022 and 2022 to 2023 seasons are compared with the 5-season average calculated from the 5-season period of 2014 to 2015, to 2018 to 2019. The 2019 to 2020, 2020 to 2021 and 2021 to 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 to 2020 season and the continued impact into the 2020 to 2021 and 2021 to 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 has 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 has been undertaken and demonstrate the quality of this data is now comparable with the data collected before the pandemic and therefore reporting can 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 to 2021 and 2021 to 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 impacted activity across many gastrointestinal pathogen surveillance indicators for England in 2020 and 2021, and reduced norovirus reporting continued into 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 EV 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. Therefore, trends for the 2019 to 2020, 2020 to 2021 and 2021 to 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

Please note this was not available for this time period.

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.

Over the 5 seasons prior to the emergence of COVID-19 (2014/2015 to 2018/2019) an average of 86.1% of gastroenteritis outbreaks reported to HPZone were attributed to EVs (norovirus, rotavirus, sapovirus and astrovirus), 1.8% to other causative agents and 12.0% were of unknown cause. Of the outbreaks attributed to EVs, 98.4% were reported as suspected and confirmed norovirus outbreaks.

During the previous 5 seasons prior to the emergence of COVID-19 (2014/2015 to 2018/2019) 62.7% of all reported outbreaks attributed to EVs (norovirus, rotavirus, sapovirus and astrovirus), occurred in care home settings, 18.7% in educational settings, 13.0% in hospital settings and 5.6% in ‘other’ settings. Of the outbreaks attributed to EVs, 98.4% were reported as suspected and confirmed norovirus outbreaks. Only 13.7% of reported EV outbreaks were laboratory confirmed as norovirus during the previous 5 seasons.

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. During the 5 seasons prior to the emergence of COVID-19 (2014/2015 to 2018/2019) 73.6% of outbreaks reported to HNORS were laboratory confirmed as norovirus.

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 are subject to a reporting delay, and the numbers reported in any week may rise further additional characterisation data become available.

References

1. Atchison and others. ‘Rapid declines in age group–specific rotavirus infection and acute gastroenteritis among vaccinated and unvaccinated individuals within 1 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 2012–2013’, 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.

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.

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