As Covid-19 cases continue to fall in the United States, cases of another virus are rebounding to pre-pandemic levels.

Outbreaks of norovirus, the bug responsible for the dreaded stomach flu, have been on the rise since January, according to the Centers for Disease Control and Prevention.

From August 2021 through the beginning of last month, the CDC reported 448 norovirus outbreaks. During the same time period the previous year, just 78 were reported.

Since early January, the number of weekly outbreaks has risen from fewer than 10 to more than 50. The data is based on reports from state health departments in Massachusetts, Michigan, Minnesota, Nebraska, New Mexico, Ohio, Oregon, South Carolina, Tennessee, Virginia, Wisconsin and Wyoming.

There’s no data to suggest that the norovirus outbreaks are in any way linked to the omicron variant of the coronavirus.

But it may be possible that the lifting of Covid restrictions is helping the virus spread. Norovirus, which causes vomiting and diarrhea, is highly contagious and spreads from person to person through surfaces.

Dr. Robert Atmar, a professor of medicine and infectious diseases expert at Baylor College of Medicine in Houston, said he suspects the increase in outbreaks may be a reflection of loosened Covid mitigation strategies nationwide.

Masking, for example, is no longer required in most areas of the country. And while norovirus is not spread by respiratory droplets, the increase in cases could reflect a relaxation of infection control in general.

Spread of norovirus is especially irksome, because it is possible to spread billions of microscopic norovirus particles on surfaces and through contaminated food or water. It takes just a few of those particles to make someone sick.

“The concentration of virus in stool samples is pretty high, so it doesn’t take much microscopic contamination to infect people,” Atmar said.

Outbreaks tend to be more common in areas where people congregate in close quarters, such as cruise ships or day care centers.

The best way to avoid norovirus is with good old-fashioned hand hygiene, he said. That is, washing your hands vigorously with soap and water for about 20 seconds after using the bathroom and before preparing food.

When it comes to cleaning surfaces, a diluted bleach solution is best for decontaminating affected areas, he added.

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