Norovirus cases surge across US

Norovirus is the leading cause of vomiting and diarrhea from acute gastroenteritis in the United States, according to CDC, and is the top source of foodborne illness in the country, responsible for almost 60% of all cases.

Every year, there are around 2,500 norovirus outbreaks nationwide, which is defined by CDC as when two or more similar illnesses come from a common exposure that is either "suspected or laboratory-confirmed to be caused by norovirus."

Each year, norovirus outbreaks result in around 900 deaths. The virus also drives 109,000 hospitalizations, 465,000 ED visits, and up to 21 million illnesses each year.

Norovirus cases have jumped significantly in recent months. Around 14% of tests for norovirus came back positive during the week ending Nov. 15, which is double the roughly 7% positivity that occurred three months ago, according to CDC.

Electronic health records data from Epic Research shows that Alabama, Nebraska, Oklahoma, Texas, and Wyoming are some of the states currently seeing the highest number of norovirus cases. Epic's data also shows that ED visits have been rising since July.

In addition, norovirus wastewater concentrations have increased nationwide by 69% since October, according to Amanda Bidwell, scientific program manager for the WastewaterSCAN program.

"Right now, we're in the high category for norovirus at the national level," she said.

However, rates are currently lower than last year's spike when positive tests peaked at almost 25% — a spike that was driven largely by a new variant of the norovirus. For more than 30 years, norovirus outbreaks have been driven by the GII.4 variant of the virus, but last year saw the rise of the GII.17 variant, which has become more prevalent in the 2020s.

Robert Atmar, professor in the Department of Medicine at Baylor College of Medicine, said there could be a combination of factors that have led to the dominance of GII.17.

"The two possibilities is that something in the virus changed to make it more transmissible, or there were changes in the population that a larger proportion were susceptible," he said. "And probably it's a combination of both. But whether it's going to persist this year, I think, is one of the big questions in our scientific community."

Craig Wilen, associate professor of laboratory medicine and immunobiology at Yale University, said that GII.17 might not actually be more transmissible, but many people probably have less immunity to it.

"Probably the GII.17 last year was so effective at infecting so many people because we had significantly less immunity to that virus, since it was different enough from viruses that we've previously been infected with," he said.

Atmar said that either norovirus variant has a "50/50 chance" of dominating this year, but he doesn't expect the same level of illnesses as last year because of "increased immunity."

What are the symptoms of norovirus and how does it spread?

The most common symptoms of norovirus are nausea, vomiting, diarrhea, and stomach pain, though some people can also develop a headache, fever, or body aches.

Typically, symptoms hit within 12 to 48 hours after exposure and usually clear on their own within 24 to 72 hours. There currently is no specific treatment or cure for norovirus, but experts say staying hydrated is essential in order to replace lost fluids.

Most people recover from norovirus quickly, but adults 65 and older, young children, and those with weakened immune systems face a higher risk of severe illness or death.

Norovirus primarily spreads through contact with the virus, which is present within the vomit or feces of an infected person. It's possible to continue spreading the virus for up to two weeks after you start to feel better.

How to stay safe from norovirus (and what to do if you're sick)

Norovirus is especially difficult to kill. Bisgrove noted that "getting norovirus off of surfaces is tricky because it is resistant to most disinfectants and hand sanitizers."

In fact, norovirus "can stay on food, countertops and serving utensils for up to two weeks, even at freezing temperatures and until heated to above 140 degrees Fahrenheit," she added.

Wilen said that norovirus is "highly stable in the environment."

"It's highly infectious," he said. "Traditional ethanol-based hand sanitizers don't work as well for this virus as they do for other viruses."

CDC recommends washing your hands with soap and water for at least 20 seconds, especially after using the restroom, changing diapers, before eating or handling food, and before giving medicine to yourself or someone else.

You should also frequently disinfect touched surfaces, like counters, doorknobs, cell phones, and children's toys.

The Environmental Protection Agency has a list of disinfectants that are effective against norovirus, including a solution of 5 to 25 tablespoons of household bleach per gallon of water.

To reduce your risk of getting infected by contaminated food, CDC advises cooking items to the recommended internal temperature, especially shellfish. Fruits and vegetables should also be thoroughly washed before eating.

If you end up getting sick, experts recommend you stay home and avoid gatherings for at least two days after your symptoms subside. You should also avoid close contact with others, not handle food, clean bathrooms thoroughly, and wash laundry in hot water.