Following a winter with higher-than-average reports of norovirus cases, the UK Health Security Agency (UKHSA) is warning the public about a potential second wave of infections. Norovirus is an important foodborne pathogen that can cause gastroenteric symptoms and fever, and it is often linked to commodities like berries or food prepared at restaurants.

Data for the 2024–2025 fall and winter season showed that norovirus laboratory reports were significantly higher than the five-season average, associated with a recently emerged GII.17 genotype. Now, UKHSA is reporting that, from February 3–16, 2025, lab-confirmed cases of the virus were 29.4 percent higher than the previous two weeks, and more than double the five-season average (168 percent) for the same two-week period. The new wave of increased activity is due to an uptick of infections by a different, common genotype (GII.4).

Although GII.17 remains dominant, accounting for 59 percent of cases, its prevalence has dropped from 76 percent of cases in November. Infections caused by GII.4 has risen to 29 percent of cases, from just 10 percent three months ago. Prior to the emergence of GII.17, genotype GII.4 was typically the most commonly detected norovirus each winter.

Since this potential second wave of norovirus is caused by a different genotype than was dominant in the fall and winter, UKHSA warns that people who already caught the stomach bug in recent months are subject to catch it again. However, there is no indication that illness caused by one genotype is more severe than the other. The agency states that, although it is likely the GII.17 genotype has driven up norovirus cases in recent months due to a lack of previous immunity, the increase in lab-confirmed cases may also reflect UKHSA’s improved testing capabilities and changing patterns of infection since the COVID-19 pandemic.