The numbers of norovirus cases in England are the and are still rising, according to the UK Health Security Agency (UKHSA).
In the current winter season, up until , which is the latest data available, there have been more than 4550 lab reports of norovirus in England. The number of reported cases for the two weeks up to 19 February is 14 per cent higher than for the previous two weeks, meaning this spike is coming quite late in winter, although the timing of the season varies from year to year.
There have also been 24 per cent more lab-confirmed cases of norovirus in England this winter than the five-season average before the emergence of covid-19. The majority of cases appear to be in people over 65 in care homes. What is behind the outbreak?
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Norovirus, also known as the winter vomiting bug, is typically a mild condition that causes fever, vomiting and diarrhoea, lasting for two or three days. It affects nearly of the population every year in the UK.
The noroviruses that cause it are spread by an infected person not washing their hands adequately after using the toilet and then passing viruses directly to others or leaving them on surfaces they touch. The viruses can also be spread via an infected person’s vomit.
“A very small amount of virus particles can cause illness, so it is easily transmitted,” says at Columbia University in New York.
Norovirus cases usually spike during winter, says at the London School of Hygiene & Tropical Medicine. “This is because the virus survives much better in cold and damp conditions,” she says. “Additionally, people are much more likely to meet indoors in winter, which also helps spread norovirus due to less ventilation than outdoor settings.”
During the height of the coronavirus pandemic in England, the number of norovirus cases plummeted. Much of this may be because of lockdowns and social-distancing measures reducing transmission of other viruses as well as the coronavirus, says Aiello, but the UKHSA says it could also be due to fewer tests being done for norovirus cases as a result of reduced testing capacity and less access to healthcare.
Norovirus cases were expected to rise this winter in England as people mixed more freely, says O’Reilly, but why they have so quickly surpassed pre-pandemic levels isn’t known.
Cases in Scotland are also than the five-year average before the coronavirus pandemic. Yet such a rise doesn’t seem to have occurred in many other countries. Wales and Northern Ireland haven’t reported the same trend, and in the US, for example, norovirus cases have largely seen before the start of the pandemic.
O’Reilly says the high number of cases in England could be due to reduced immunity to the virus after several winters with fewer-than-usual norovirus cases, but it isn’t clear why that wouldn’t also apply to other countries.
The majority of norovirus cases appearing in over-65s isn’t surprising, says at the University of Florida. People of this age are more likely to live in nursing homes or long-term care facilities, she says.
“These facilities have a high level of contact between residents, visitors and healthcare workers,” says Shapiro. “And then when you combine these environmental conditions with a virus that can easily be spread by contact as well as aerosolisation when someone vomits – it is easier to cause an outbreak.”
It is possible that the number of infections is higher in younger age groups than is being reported, suggest some researchers, because most people won’t go to a doctor if they have norovirus, so samples won’t be collected, whereas sampling is more common in care homes.
“It’s not yet clear to me whether the apparent increase reflects heightened disease activity or better surveillance and reporting of cases among older adults,” says at Emory University in Atlanta, Georgia.
