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Noroviruses, which are often mistakenly called `stomach flu’, are single-stranded RNA viruses that are both highly contagious and are able to evolve rapidly. New strains tend to emerge every 4 or 5 years
Victims usually experience nausea, frequent vomiting & diarrhea, and stomach pain – and may also suffer from headache, fever, and body aches.
The illness generally runs its course over 1 to 3 (very long) days, and most people fully recover. But among those who are aged or infirmed, the virus can take a heavy toll.
According to the CDC, in the United States each year the norovirus produces:
The `standard’ mode of norovirus transmission is considered to be the fecal-oral route, but limited airborne transmission is suspected as well. Complicating matters, alcohol based hand sanitizers are ineffective against this virus.
In 2015, in CID Study: Airborne Norovirus In Healthcare Facilities, a study looked for - and found - norovirus in ambient air samples taken from 8 hospitals, both inside and outside of an infected patient’s room.
While the act of vomiting (see Vomiting Larry And His Aerosolized Norovirus) could account for this airborne infusion of the virus, the usual receptacle is a toilet, followed by a power flush (see Nature: Another Toilet Plume Study To Consider) which may also help disperse the virus through the air.
While norovirus infections can occur anytime, they tend to be most common between November and April, and can increase the impact of an already busy winter cold/flu/COVID season.
This year, the UK has reported an unusually harsh noro season, and in recent weeks the dominant strain (GII.17) has begun to give way to an older variant (G11.4), raising concerns over a ` second wave'.
UKHSA warns of potential second norovirus wave
People who have already had the virus this winter could be at risk again, as new data shows shift in circulating strains.
From: UK Health Security Agency Published 27 February 2025
The latest UK Health Security Agency (UKHSA) data shows norovirus cases continue to rise across the country, with laboratory reports at the highest levels since reporting data this way began in 2014.
Laboratory confirmed cases in the 2 weeks from 3 to 16 February 2025 were 29.4% higher than the previous fortnight and more than double the 5-season average (168.0%) for the same 2-week period. The impact is particularly severe in hospitals and care homes, with cases highest among people aged 65 and over. Cases usually start to decline around this time of year as the weather gets warmer, but it is too soon to conclude whether or not norovirus has peaked this season.
The increased activity this season is associated with the recently emerged GII.17 genotype. However, the latest data shows that a different, but commonly seen genotype (GII.4) is now increasing. Prior to the emergence of GII.17, GII.4 is the genotype that most commonly detected and increased each winter. While the GII.17 genotype remains dominant, accounting for 59% of cases, its prevalence has dropped from 76% since November. Meanwhile, the GII.4 strain has sharply risen, now representing 29% of cases compared to just 10% three months ago.
This means that people who have already had norovirus this season may catch it again, as having one genotype does not fully protect against the other. However, at present there is no indication that either GII.17 or GII.4 leads to more severe illness.
Common symptoms of norovirus include:
- nausea and vomiting
- diarrhoea
- high temperature
- abdominal pain
- aching limbs
Some people, particularly young children, older adults and those with weakened immune systems are more likely to develop severe symptoms, which can cause dehydration. Anyone with these symptoms should drink plenty of fluids.
Amy Douglas, Lead Epidemiologist at UKHSA, said:Norovirus levels are still exceptionally high and now with multiple genotypes spreading at the same time, people could end up getting infected more than once this season.While it is likely the GII.17 genotype has driven up norovirus cases this season due to a lack of previous immunity, the higher numbers we are seeing may also reflect UKHSA’s improved testing capabilities and changing patterns of infection since the COVID-19 pandemic. Norovirus also spreads more easily in lower temperatures as people spend more time indoors and typically peaks during winter months.
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. And don’t prepare food for others, as you can still pass on the virus during this time.
Alcohol gels do not kill norovirus. Wash your hands with soapy warm water and clean surfaces with bleach-based products where possible to help stop infections from spreading.
UKHSA 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.
