Newer Strain of Norovirus Dominates US Outbreaks

Nearly 28% of norovirus tests run for the week ending Jan. 4 came back positive for the virus, the U.S. Centers for Disease Control and Prevention (CDC) reported. That is more than double the 13.52% of tests coming back positive reached during the peak of last season’s wave in March, CBC News reported. Rates of norovirus in that CDC system have reached levels at or above last season’s peak in all regions of the country. 

Norovirus on cruise ships also reached higher-than-normal levels in December, with at least six reported norovirus outbreaks on vessels, CBS News reported.

A newer strain of norovirus now makes up a majority of outbreaks nationwide, according to data from the CDC. From Sept. 1 to Dec. 31, 2024, the GII.17 norovirus variant is attributed to 57 outbreaks amounting to 72.2% of U.S. norovirus outbreaks.

The GII.17 strain emerged in 2014 and has spread worldwide, according to Health. However, the strain only rose to dominance in norovirus outbreaks over a year ago; between Sept. 1, 2023, through Aug. 31, 2024, GII.17 accounted for 109 norovirus outbreaks.

On the other hand, the former leading strain, GII.4—which had dominated previous norovirus outbreaks since 2012—was only responsible for 84 outbreaks. From Sept. 1, 2024, through Dec. 31, 2024, GII.4 only accounted for 7% of norovirus outbreaks, the CDC reported.

Still, health officials do not believe the GII.17 strain is more contagious or causes longer or more severe illness, CBS News reported. The robust norovirus season is thriving with the help of numerous strains of norovirus, which also means people can (and do) get norovirus many times in their lives. Americans may have lower immunity to the GII.17 strain though, health officials believe.

The CDC recommends washing hands with soap and water often to prevent norovirus spread.