This winter’s wave of norovirus infections has reached levels that are now more than double last season’s peak, in figures published Monday by the Centers for Disease Control and Prevention tracking the notorious stomach bug.
Nearly 28% of norovirus tests run over the week of the New Year’s holiday came back positive for the highly contagious virus, which is the leading cause of foodborne illness like vomiting and diarrhea in the U.S.
That is more than double the 13.52% of tests coming back positive reached during the peak of last season’s wave in March, according to data from public health laboratories around the country tallied by the CDC.
Labs usually test samples of sick people’s stool for the virus, which can be diagnosed up to 10 days after symptoms begin. The virus can also be found in other samples, including contaminated food or drinks that can spread the virus.
Rates of norovirus in that CDC system have reached levels at or above last season’s peak in all regions of the country. Norovirus test positivity rates look to be the worst in the Midwest, in a grouping of states spanning Kansas through Michigan.
Since most people sick with norovirus get better without needing to go to the doctor, a majority of cases go unreported in the U.S. Instead, health authorities and experts use other measurements, such as the rate of positive tests, to track trends of the virus.
Data from WastewaterSCAN’s sewer sampling also suggests norovirus rates in recent weeks have been highest in the Midwest as well as the Northeast. Figures published by private testing company BioFire Diagnostics are also above previous seasonal peaks for norovirus.
Why are norovirus cases so high in 2025?
While norovirus rates always worsen during the colder months, in recent years most trends tracking norovirus did not reach their peak until March or April.
That’s different from seasons leading up to the COVID-19 pandemic, which scrambled many usual seasonal patterns of germs. During that time, norovirus outbreaks often reached peak levels as early as December and January.
Experts say this year’s early and steep surge of cases is being driven by a new strain of norovirus called GII.17[P17], which has displaced a previous strain that had dominated previous waves of the virus in the U.S. for a decade.
That new strain has been spotted everywhere around the country, including in many cruise ship outbreaks, a CDC official told CBS News. Lower population immunity to GII.17[P17] could explain this year’s unusual wave of the virus.
More than 7 in 10 outbreaks have been linked to this new norovirus strain this season, according to the latest CDC figures.
