As temperatures drop, norovirus cases increase and more of its RNA ends up in sewage. This year, wastewater samples in the United States show norovirus levels have already passed the previous two seasonal peaks.
The “Ferrari of viruses” is having a banner season. Norovirus, which races through cruise ships, homes, and long-term care facilities, is experiencing a remarkable winter surge in the Northern Hemisphere, sending large numbers of people racing to the bathroom and many others to the hospital, and in rare cases, proving fatal. In the United States, for example, 91 outbreaks of the intestinal virus occurred in the first week of December 2024, far above the previous maximum, 65, for the same week between 2010 and 2024. And levels of its genes in U.S. wastewater are an order of magnitude above last year.
“The early data for the early part of the season is certainly supporting that we’re going to have a pretty intense norovirus year,” says Lisa Lindesmith, who studies the virus at the University of North Carolina (UNC) at Chapel Hill. Some of the surge may be due to a new variant of the virus, unfamiliar to many people’s immune systems, and the resumption of cruises and other gatherings that the COVID-19 pandemic interrupted. And there’s no vaccine anywhere in sight: The most advanced candidate just failed a key trial and others won’t be ready for several years.
Norovirus thrives in cold climes, causing explosive diarrhea and vomiting that typically only last for a day. But several weeks after people recover, they can still shed the virus, and it can remain infectious for long periods on surfaces. It’s notoriously resistant to many disinfectants, and studies in adult volunteers have shown just a trace of virus is enough to sicken a person. Oysters are also a source of infection, because the filter-feeding mollusks concentrate the virus from contaminated water in their tissues. U.S. health officials issued several warnings about infected oysters in December, and France has banned oyster harvesting in certain regions because of norovirus outbreaks.