HPR volume 19 issue 2: news (27 February 2025)
Updated 27 February 2025
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Update on increasing levels of norovirus activity in England, February 2025
Since November 2024 the UK Health Security Agency (UKHSA) has reported increasing norovirus activity in England (1). Activity has remained high throughout the 2024 to 2025 season and is still increasing (2) (Figure 1).
The cumulative total of norovirus laboratory reports since the start of the season (week 27 of 2024), is more than double the 5-season average (9,289 compared to 4,261, +118.0%). The ‘5-season average’ is calculated from the same period during the 5 seasons of:
- 2016/2017
- 2017/2018
- 2018/2019
- 2022/2023
- 2023/2024
These exclude the 3 seasons impacted by the COVID-19 pandemic.
During weeks 6 and 7 of 2025, reports were more than two and a half times (+168.0%) the 5-season average for the same reporting period. The most significant impacts are being seen in health and social care settings, with a notable increase in outbreaks reported in care home settings correlating with a rise in laboratory reports for cases aged 65 years and over. Outbreaks reported to UKHSA in hospital settings are also 39.0% higher than the 5-season average (308 outbreaks compared to 222).
Figure 1. Trends of norovirus laboratory reports in England, 2014/2015 to mid-February of 2024/2025 [note 1]
Note 1: Taking account of data up to week 6 of 2025 (up to Sunday, 16 February)
NPIs are non-pharmaceutical interventions.
During this time, the recently emerged genogroup II- genotype 17 (GII.17) has replaced GII.4 (Sydney/2012-like variants) as the most frequently detected genotype (Figure 2); however, GII.4 detections have been increasing in recent weeks. Up to week 7 of the 2024/2025 season (first 33 weeks) 58.9% of all GII noroviruses genotyped were GII.17 and 29.4% were GII.4. Compared to the data published in November up to week 46 of the 2024/2025 season (first 20 weeks) this represents a 17.1% decrease from 76.0% for GII.17 and a 19.4% increase from 10.0% for GII.4 (Figure 2).
Figure 2. Trends of GII norovirus samples typed as genotype GII.17, GII.4 or another GII in England, by season, 2017/18 to mid-February of 2024/2025 [note 1]
Note 1: Taking account of data up to week 6 of 2025 (up to Sunday, 16 February)
NPIs are non-pharmaceutical interventions.
High levels of reporting likely driven by GII.17 noroviruses has also been observed in the United States. Suspected and confirmed norovirus outbreaks reported to the Centers for Disease Control and Prevention during the 2024/2025 season so far are more than double the same period in the previous season, and well above the range observed at this time of year going back to the 2012/2013 season (3). Up to 31 January 2025 79.0% of outbreaks have been genotyped as GII.17 and only 6.2% as GII.4 Sydney/2012-like variants (4,5).
Norovirus transmission is influenced by many factors including, but not limited to, whether the person has been infected with norovirus recently, and if so what genotype, as well as community contact patterns and the time of the year. Analysis of the available data indicates it is likely that the emergence of GII.17 has resulted in a true increase in norovirus transmission this season, potentially due to a lack of prior immunity to this genotype. However, further work to understand the relative contributions of other factors, such as, increased use of PCR multiplex technology at frontline diagnostic laboratories and changes in reporting are ongoing. Norovirus survives better in the environment at colder temperatures, consequently activity often increases during prolonged periods of low temperatures and peaks in the winter months. Therefore, it is likely that recent cold weather is also contributing to the high levels of activity observed.
The latest data on norovirus is published weekly during the winter months as Official Statistics and UKHSA guidance on how to prevent onward transmission is available at: Norovirus: What to do if you catch it and helping to stop the spread
References
1. UKHSA (2024). ‘Increasing levels of norovirus activity in England, November 2024’. HPR volume 18, number 10.
2. UKHSA (February 2025). ‘National norovirus and rotavirus surveillance reports: 2024 to 2025 season’.
3. Dai YC, Xia M, Huang Q, Tan M, Qin L, Zhuang YL, Long Y, and others (2017). ‘Characterization of antigenic relatedness between GII.4 and GII.17 noroviruses by use of serum samples from norovirus-infected patients’. Journal of Clinical Microbiology: volume 55, number 12, pages 3,366 to 3,373
4. US Centers for Disease Control and Prevention (2025). Norostat Data: about the current norovirus season.
5. US Centers for Disease Control and Prevention (2025). Confirmed norovirus outbreaks submitted by state.
Infection reports in this issue
Hepatitis C (England and Wales): 2024
Vaccine coverage
RSV vaccine coverage report in older adults for catch-up cohorts in England: January 2025