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Norovirus appears to keep spreading as rate of positive tests exceeds last year's peak

The highly contagious virus' symptoms typically include diarrhea, vomiting, nausea and stomach pain.

Norovirus appears to be at a 12-month high, according to data from the Centers for Disease Control and Prevention.

The rate of norovirus tests coming back positive, averaged over three weeks, was around 17% as of the end of last week. That’s the highest it has been at any time in the last year.

Norovirus is sometimes referred to as the stomach flu, but it is not related to the influenza virus. Rather, it is a highly contagious virus that typically causes gastrointestinal symptoms like diarrhea, vomiting, nausea and stomach pain. Mild fever and aches are possible, too.

Just a few virus particles are enough to make someone sick, and they spread easily via hands, surfaces, food and water. An infected person can transmit the virus for days after they’re feeling better, potentially even up to two weeks, according to the CDC. 

Regionally, the Midwest and South had the highest average test positivity rates for norovirus as of Saturday, both approaching 18%.

The CDC tracks norovirus outbreaks via a network of 14 state health departments. Despite the upward trend, the agency said the numbers of cases and outbreaks fall within the average from 2012 to 2020. In fact, the 254 outbreaks reported between August and mid-February is less than the number during the same period last year, 313.

“Reported norovirus outbreaks and reported cases from both state health departments and clinical laboratories remain within the expected range for this time of year,” Kate Grusich, a CDC spokesperson, said in an email. 

Norovirus rates usually rise in the winter, because most outbreaks in the U.S. happen between November and April. On average, the country sees around 20 million cases per year, with nearly 110,000 hospitalizations and 900 deaths, mostly among those who are 65 and older.

Grusich said measures taken earlier in the pandemic to curb Covid's spread likely helped prevent some norovirus outbreaks as well.

"As pandemic restrictions have relaxed, the number of norovirus outbreaks has returned to levels similar to pre-pandemic years," she said. 

Norovirus is the leading cause of foodborne illness in the country. Food can get contaminated if fruits or vegetables are grown or washed with contaminated water. Oysters, too, pose a norovirus risk if they are harvested from contaminated water. In December, a multistate outbreak was linked to raw oysters from Texas, with nearly 300 norovirus cases reported.

There is no treatment for norovirus, but the CDC recommends drinking lots of fluids to prevent dehydration. The illness typically resolves within a few days.

One reason it's difficult to stop the virus from spreading is that it's relatively resistant to alcohol, according to Craig Wilen, an associate professor of laboratory medicine and immunobiology at the Yale School of Medicine. By contrast, many other viruses can be targeted with alcohol-based solutions like hand sanitizers.

“Part of the challenge is people call it the stomach flu and it is not the flu. It's a completely unrelated virus from the flu and for the flu, ethanol sanitizers work. But for norovirus, you need a bleach or a detergent base,” Wilen said.

Proper hand washing with soap is best, he added.

To disinfect surfaces, Grusich recommended that people "leave bleach disinfectant on the affected area for at least five minutes, then clean the entire area again with soap and hot water."

As with other viruses, immune protection develops after a norovirus infection but fades over time.

It’s not known exactly how long immunity lasts, Grusich said, and “whether you are susceptible to norovirus infection is partly determined by your genes”

"There are many different types of noroviruses," she added. "Infection with one type of norovirus may not protect you against other types."

Studies have found people can get norovirus up to five times over their lives, Grusich said.

Wilen estimated that people can get norovirus again within a year or two, though the symptoms are likely to be most severe the first time.

"There’s some famous old studies where they took people and intentionally infected them with feces from human norovirus patients, and they all get infected two years out, but they’re mostly resistant two months out," he said.

In England, norovirus rates this season have been exceptionally high, according to the UK Health Security Agency. During the weeks between Feb. 6 and Feb., 19, lab reports of norovirus were more than double the five-year average over the same weeks before the Covid pandemic. However, the agency said the numbers were still within the overall historical range.