New Norovirus Genotype Drove Most Outbreaks in 2024-2025 Season

A research letter from CDC’s Emerging Infectious Diseases has identified the increasing prevalence of a different type of norovirus than usual in the U.S.

Specifically, GII.17 cases and outbreaks were up during the 2023-24 season. However, during 2011-2024, “GII.4 viruses have caused >50% of U.S. outbreaks each season.” During the 2022-23 season, “GII.17 accounted for 7.5% and GII.4 for 48.9% of all outbreaks. The next season (2023-24), the percentage of GII.17 outbreaks increased to 34.3%, whereas GII.4 outbreaks declined to 27.7%. By the 2024-25 season, GII.17 outbreaks had increased markedly to 75.4%, whereas GII.4 outbreaks further decreased to 10.7%. In addition, during 2022-23 and 2023-24, seasonality was primarily driven by GII.4 viruses, showing peak activity in February 2023 and March 2024, whereas during the 2024-25 season, norovirus peaked in January 2025.”

The percentage of GII.17 outbreaks (47.4%) overtook the percentage of GII.4 outbreaks (23.7%) in April 2024, and “from May 2024 through March 2025, GII.17 accounted for >50% of all outbreaks each month.”

These data “highlight a substantial shift in genotype distribution of norovirus outbreaks in the U.S. from 2022 to 2025, with GII.17 emerging as the predominant genotype. That shift coincides with a notable decline in GII.4, which has traditionally been the leading cause of U.S. outbreaks.” Additionally, in 2024-25, the onset of the norovirus season was in early October 2024, when it typically has begun in early December each season.