Norovirus Outbreaks Have Increased to Pre-Pandemic Levels, CDC Data Show

The easing of COVID restrictions has meant getting back to a sense of normalcy for many people. But it has also brought the return of other illnesses that pandemic precautions were keeping at bay.

Rates of norovirus—sometimes called the stomach bug—are nearing pre-pandemic levels, the Centers for Disease Control and Prevention (CDC) said in its September 23 Morbidity and Mortality Weekly Report (MMWR).

That dip in cases, the CDC said, was likely linked to public health measures in place meant to stop the spread of COVID.

"Norovirus obviously is often transmitted person-to-person, and back in the day, we were social distancing, staying at home, our children were not in school, and all of those things have changed," William Schaffner, MD, professor of infectious diseases at Vanderbilt University Medical Center, told Health. "Now children are back in school, we're traveling, going to religious services, restaurants, going back to business in person and the like. And that just provides opportunities for this very contagious virus to spread."

As we head into fall and winter—peak season for norovirus—here's what to know about the virus, how COVID preventative measures can also help stop the spread of norovirus, and if it's possible to keep cases low, even as we return to normal.

A Fairly Common, Highly Contagious Virus

Each year—particularly from November to April—there are huge numbers of norovirus cases in the U.S. In any given year, the U.S. can see about 2,500 outbreaks, according to the CDC, which can result in anywhere from 19 to 21 million norovirus cases.

In 2012, the CDC created the NoroSTAT Data system to better track norovirus cases in the country. It takes a sampling of annual norovirus outbreaks from 12 states in the U.S.—this information was what revealed the sharp decline, then eventual increase in norovirus cases over the course of the COVID pandemic.

Between August 2019 and July 2020, the MMWR said, the selected states reported 1,056 outbreaks. During COVID, that number plunged to just 343 outbreaks. But this past year's data—between August 2021 and July 2022—norovirus outbreak numbers climbed back up to 992 for the year. Numbers were still lower than average in the summer and fall of last year, but were back to nearly normal levels by early February 2022.

The 2021-2022 data also showed that most norovirus outbreaks were in long term care facilities and were caused by person-to-person transmission, the MMWR said.

Most people who get norovirus will experience acute gastroenteritis, which refers to inflammation of the stomach or intestines. This usually leads to nausea, vomiting, and diarrhea primarily, but people sick with norovirus can also have headaches, fever, or body aches.

Besides dehydration—which can happen if a person is vomiting or having diarrhea very frequently—norovirus does not cause many symptoms that are particularly dangerous. Luckily, for most people, norovirus is not deadly. However, there are about 900 deaths from norovirus annually, which are usually in people over the age of 65.

COVID Precautions Kept Transmission to a Minimum

The norovirus is an incredibly transmissible virus, the CDC said, and the waning of COVID precautions in recent months is likely responsible for the large jump in the number of norovirus outbreaks.

People who are sick with norovirus can shed billions of particles, but it only takes a few of these particles to make another person sick, the CDC said.

"Particles from norovirus can aerosolize (when tiny particles of vomit or diarrhea spray through the air) and then cause infection if they are swallowed, or if someone touches a surface that was contaminated with aerosolized particles and then touches their mouth," Anita Kambhampati, epidemiologist with the Division of Viral Diseases at the CDC and lead MMWR author, told Health.

The norovirus can be spread in a variety of ways, the CDC warns:

• Eating food or drinking liquids that are contaminated with norovirus
• Touching surfaces or objects contaminated with norovirus, then putting your fingers in your mouth
• Having direct contact with someone who has norovirus, including caring for them or sharing utensils

Because most norovirus transmission requires person-to-person contact, without COVID precautions that had people washing their hands frequently, staying home from work when they were sick, and distancing themselves from other people, norovirus can spread quite rapidly.

"This is a tough little virus. It's what we call an unencapsulated virus and that just goes with the way it reproduces," John Sellick, DO, professor of medicine at the University at Buffalo, told Health. "It's evolved to be very hardy in the environment, [it's] very difficult to clean up."

Is a Future With Fewer Norovirus Cases Possible?

Because of how easy it is to spread norovirus, experts say that it's probably not sustainable to keep outbreak numbers as low as they were during the worst months of the COVID pandemic.

Still, some quarantine-era hygiene measures may be useful to keep the spread down this fall and winter, especially as people start spending more time indoors and in close contact with one another.

"We need to give a lot of attention to good hygiene in school circumstances. Children are not the most hygienic part of the population, but anything we can do to encourage good hand hygiene and sanitation of the inanimate environment, which schools are doing already, many of them, in order to help prevent the transmission of COVID. So those things will help and work to support each other," Dr. Schaffner explained.

On a more individual level too, Kambhampati said, people should wash their hands well with soap and water, wash fruits and vegetables, and most importantly, stay home while they're sick and for at least two days afterward being ill.

"This is especially important for those who work in restaurants, healthcare facilities, schools and daycares, or other places where people might be exposed," she added.

There are also some vaccines for norovirus in development right now, experts said, though they may not be immediately available. Besides the fact that norovirus mutates frequently—which as we've seen with COVID, makes it challenging to nail down a vaccine—epidemiologists and public health researchers just have had more pressing things going on recently, Dr. Sellick said.

"With SARS-COV2 we actually have pretty good genetic information, and there's no reason we couldn't get that with norovirus. It's just a question of what resources are available in the public health sphere," he explained.

So in the absence of a vaccine, washing hands is probably the easiest thing people can do to keep outbreaks from growing out of control. And as long as people seek medical care when they need it, norovirus may be unpleasant, but it's not the public health concern that COVID is.

"Although norovirus makes you feel miserable for a period of days, there's very little hospitalization and very little fatality associated with [it]," Dr. Schaffner said. "But 99.9% of people recover without any adverse subsequent consequences."