By Katelyn Jetelina MPH PhD

Tired of respiratory viruses this winter? Not so fast. Norovirus has entered the room.

Norovirus—a virus that causes stomach cramping, intense episodes of vomiting and diarrhea, and sometimes fevers—is taking off in the Northern Hemisphere. If we had a storm system for viruses, this might be deemed a sh… you get it.

What’s happening right now?
Norovirus outbreaks happen any time of the year, but their seasonal pattern is remarkably consistent, peaking in February and March. We don’t really know what is driving this consistency.

People don’t typically go to the doctor for norovirus (they wait it out), so the CDC reports data from specific outbreak investigations in 14 states. This won’t tell us how many people are infected in the U.S.; rather, it tells us the trend. We are currently on trend with last year. But December and January have been less than exciting compared to pre-pandemic years (gray shaded areas). Unfortunately data is lagged.

Another monitoring system—percent positivity reported in some states—shows a slightly different, more up-to-date story. We have already passed last year’s peak. This may mean we will get a bigger wave than usual.

Trends in the U.S. are consistent with England where norovirus is 66% higher than normal, but within historical range. Norovirus activity in people ages 65+, though, are at levels that “haven’t been seen in over a decade”.

There is good news: the virus has not significantly mutated. This is what typically drives previously bad norovirus seasons in the States. The increase in cases now may be driven by typical winter behaviors combined with less population immunity due to the pandemic, like we saw with RSV.

How high will this peak be? We don’t know. But, in pre-pandemic times, we could expect 19-21 million people infected, ~100,000 hospitalizations, and ~900 deaths throughout a season.

How does it spread?
Norovirus is very infectious. On average, one infected person will infect 2-7 other people.

For one, it can live on surfaces for weeks. It’s spread through the fecal to oral route so transmission avenues include:

Direct contact. 6 in 10 infections are through direct contact, like shaking hands or touching door handles, and then putting your hand in your mouth.

Indirectly, like through foods. An infected person can touch food with bare hands that have viral particles on them. You can ingest the food and then get sick.

Aerosolized. If someone throws up in a toilet, for example, viral particles can become aerosoled after flushing. This isn’t the main route of transmission.

Second, people only need a few viral particles to get sick. Once the virus enters the body, norovirus hijacks your cells and turns them into viral factories. It latches on, specifically, to cells in your gut causing less than wonderful symptoms. Before 2018, we didn’t know why norovirus chose the gut, but a new study found it’s because our gut is home to one rare type of cell (tuft cells).

Third, this virus spreads pre-symptomatically and up to 2 weeks after symptoms resolve. In other words, you can spread it even if you don’t have symptoms.

Who is at risk?
Three out of four norovirus outbreaks occur in nursing homes. Restaurants and schools follow. Cruise ship outbreaks usually make the news, but only account for 1% of outbreaks.

Children under 5 years old and adults aged 85 years and older are more likely to have an outpatient or emergency department visit than people of other ages. Of those who die, 90% are persons aged 65 years and older.

How to protect yourself?
It’s tough.

There are no vaccines. Some are under development, but they’re tricky to make because noroviruses constantly mutate and are very diverse.

They’re also one of the hardest viruses to kill because they have a stable coating called a capsid. This means:

Washing hands needs to be thorough. It takes about 30 seconds of vigorous washing and rubbing with hot water and soap. And you must get it all, even under nails.

Quick application of hand sanitizer usually won’t get rid of it.

Bleach-based products on surfaces are the best.

If someone brings it home, to daycare, or to nursing homes, it’s hard to avoid. Some things that can help:

If you’re infected, don’t prepare food.

Use the longest dishwasher or laundry cycle. It can stick to plates even after being washed at restaurants, especially with sticky substances like cheese. Hand washing dishes doesn’t work as well because the water doesn’t get warm enough to kill the virus.

Consider linens contaminated.

Clean surfaces, like doorknobs, thoroughly.

If you get sick, the good news is that it only lasts 24-48 hours. But stay home at least two days after symptoms lift. Also, stay hydrated.

Bottom line
Norovirus is on the rise. Ten percent of the population should expect to get sick, but there are things we can do to reduce risk.

I’m optimistic April will be a much calmer time for viruses. We’re almost there.

Love, YLE

“Your Local Epidemiologist (YLE)” is written by Dr. Katelyn Jetelina, MPH PhD—an epidemiologist, data scientist, wife, and mom of two little girls. During the day she works at a nonpartisan health policy think tank and is a senior scientific consultant to a number of organizations, including the CDC. At night she writes this newsletter. Her main goal is to “translate” the ever-evolving public health science so that people will be well equipped to make evidence-based decisions. This newsletter is free thanks to the generous support of fellow YLE community members.