Will the upcoming norovirus season be just as early and active as the past year?
While the answer may be anyone’s guess right now, federal health officials recently warned that the dominant strain of the contagious virus — the leading cause of vomiting, diarrhea, and foodborne illness in the U.S. — has changed between the 2022-2023 and 2024-2025 seasons.
“GII.17 has caused 75 percent of all norovirus outbreaks during the 2024–25 season so far, thereby replacing GII.4 as the predominant norovirus outbreak strain in the United States,” Centers for Disease Control researchers wrote in an article published this month by the agency’s Emerging Infectious Diseases journal.
This past season also started at the earlier date of October, as opposed to December. GII.17 also drove a record wave of outbreaks.
But, what all of this might mean going forward needs further research.
“Additional sequence analysis of complete GII.17 genomes and identification of cross-protective neutralizing antibodies of GII.17 compared with GII.4 viruses could help clarify whether GII.17 viruses will persist. Continued surveillance is needed to determine if this genotype remains the dominant genotype, as well as whether the norovirus season continues to start earlier than previous years,” they wrote.
A spokesperson for the agency told CBS News that there are “currently insufficient historic data to predict whether norovirus GII.17 will remain the dominant genotype and lead to an earlier onset of the norovirus season later this year.”
They pointed to a surge of GII.17 strains in Asia and Europe in 2014 that was followed by no reports of a change to seasonality, and noted that there has been no clear evidence to prove that GII.17’s emergence was the cause of a change to the norovirus season last year.
Still, this past season reached the worst levels in a decade. There were 91 suspected or confirmed outbreaks during the first week of December, which Yale Medicine said exceeded the number of outbreaks during the same week in any year since 2012. The majority were the GII.7 strain. In years when there is a new strain of the virus, there can be 50 percent more norovirus illness.
Cases continued to tick up months into this year. By May 7, there were 2,571 outbreaks. During the same time last year, there were only 1,358.
“The total number of outbreaks reported during the 2024 to 2025 seasonal year is above the range reported during the same period during the 2012 to 2020 and 2021 to 2024 seasonal years,” the CDC said.
Cases have fallen markedly since January, and are now at low levels. While the outbreaks occur throughout the year, they are the most common from November to April as people head indoors and it’s easier to spread norovirus through infected particles. Anyone who consumes raw shellfish is also at risk of contracting it.
Infection can be deadly largely among adults aged 65 and up, but anyone can get sick. Children younger than 5 years old and people with weakened immune systems are more likely to develop severe infections. There are 900 deaths on average each year and between 19 and 21 million illnesses. There’s no specific treatment for norovirus, but most people recover with a period of up to three days.
“The norovirus can spread so quickly, but also, as we already talked about, norovirus can be very — it’s usually very fast. So yeah, if people are taking care of themselves, we leave them alone,” explained Dr. Joanna Bisgrove, a family physician at Rush University Medical Center. “But if it keeps going, we’re like, maybe this isn’t norovirus, and we need to do other things.”
