A dominant strain of norovirus is emerging, which could disrupt the seasonal pattern and increase the number of outbreaks, a June 2 CDC report suggests.
Here’s what to know:
1. In previous years, the U.S. saw norovirus outbreaks increase around December, which marked the start of the season of the virus. Between 2011 and 2024, GII.4 viruses caused less than 50% of US outbreaks each season.
2. Last year’s season started in October as a new norovirus strain, GII.17, drove a record number of outbreaks. The outbreaks have since returned to normal levels, but GII.17 is still identified in more than 3 in 4 outbreaks in the U.S. During the 2022-23 season, GII.17 accounted for 7.5% of all outbreaks and GII.4 for 48.9%. In the 2024-25 season, GII.17 made up 75.4% of outbreaks and GII.4 made up 10.7%.
3. Some are concerned GII.17 may become the new dominant strain and lead to shifts in the outbreak pattern.
4. In the last year, several countries, including England and Japan, saw GII.17 drive more waves than in previous seasons. These waves also came sooner, peaked weeks later than in previous seasons and remained above those of recent years, CBS News reported June 4.
5. However, GII.4 is still detected around the world, which could mean GII.17’s dominance could be short lived, Miranda de Graaf, a scientist at Erasmus University Medical Center in the Netherlands, told CBS.
6. A CDC spokesperson told CBS there is no clear evidence to prove that GII.17 caused a change to the norovirus season last year. The agency is monitoring norovirus outbreaks in its CaliciNet laboratory database, and said that in 2014, there was a surge of GII.17 strains in Asia and Europe that did not change seasonality.
“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,” the spokesperson said.
