The UK Health Security Agency (UKHSA) South West is warning of a potential second wave of norovirus.

According to the latest UKHSA data, norovirus cases continue to rise across the country, with laboratory reports at their highest levels since the agency began reporting data in this way in 2014.

Laboratory-confirmed cases in the two weeks from February 3 to 16, 2025, were 29.4 per cent higher than the previous fortnight and more than double the five-season average (168 per cent) for the same period.

Figures show that the impact is particularly severe in hospitals and care homes, with cases highest among hospitalised patients and care home residents aged 65 and over.

Cases usually start to decline around this time of year as the weather gets warmer, but UKHSA says it is ‘too soon’ to conclude whether norovirus has peaked this season.

The agency states that the increased activity this season is linked to the recently emerged GII.17 genotype. However, the latest data shows that a different but commonly seen genotype—GII.4—is now increasing. Before the emergence of GII.17, GII.4 was the most frequently detected genotype each winter.

While GII.17 remains dominant, accounting for 59 per cent of cases, its prevalence has dropped from 76 per cent since November. Meanwhile, the GII.4 strain has seen a sharp rise, now representing 29 per cent of cases compared to just 10 per cent three months ago.

This means that people who have already had norovirus this season may catch it again, as infection with one genotype does not provide full immunity against the other. However, there is currently no evidence that either GII.17 or GII.4 causes more severe illness.

Dr Toyin Ejidokun, Consultant in Health Protection at UKHSA South West, said:

"Norovirus levels are still exceptionally high and, with multiple genotypes spreading at the same time, people could end up getting infected more than once this season.

“We are seeing the biggest impacts in health and social care settings, such as hospitals and care homes. Symptoms of norovirus can be more severe in older adults, young children and those who are immunocompromised. If you have diarrhoea and vomiting, please do not visit hospitals and care homes or return to work, school or nursery until 48 hours after your symptoms have stopped. Do not prepare food for others, as you can still pass on the virus during this time.

“Unlike some other viruses, norovirus is not killed by alcohol gels. Wash your hands with warm soapy water and clean surfaces with bleach-based products where possible to help stop infections from spreading.”

While the GII.17 genotype is likely driving the surge in cases due to a lack of previous immunity, UKHSA says the higher numbers may also reflect improved testing capabilities and changing patterns of infection since the COVID-19 pandemic.

Experts estimate that reported cases represent only a small fraction of actual infections. For every case reported to national surveillance, approximately 288 cases occur in the community, suggesting around 3 million cases annually in the UK.