A notable shift in the viral landscape is upon us, as a dominant strain of norovirus, GII.17, appears to be altering the disease’s traditional seasonality and causing an uptick in outbreaks, as suggested by a recent report from the Center for Disease Control (CDC). Historically, the US has seen a surge in norovirus outbreaks beginning around December, marking the onset of the virus’s season. However, there has been a significant change in this pattern.
From 2011 to 2024, less than half of US outbreaks each season were caused by the GII.4 strain of the virus. The last season deviated from this trend and kicked off in October, with the GII.17 strain driving a record number of outbreaks. Despite a return to regular outbreak levels, more than 75% of these incidents were linked to the GII.17 strain.
During the 2022-23 season, GII.17 was responsible for 7.5% of outbreaks and GII.4 for 48.9%. But in the 2024-25 season, GII.17 accounted for a whopping 75.4% of infections, while GII.4 was implicated in only 10.7% of outbreaks. Health professionals express concern that GII.17 may establish itself as the new dominant strain, leading to significant shifts in the pattern of outbreaks.
Countries like England and Japan have already seen a change in the epidemiology of norovirus outbreaks with GII.17 driving a higher number of cases than in previous seasons, and with seasonal peaks occurring later. However, this shift might be temporary. According to Miranda de Graaf, a scientist at Erasmus University Medical Center in the Netherlands, the continued global detection of GII.4 offers hope that GII.17’s dominance might only be brief.
Currently, there is no concrete evidence to suggest that the change in the norovirus season last year was caused by GII.17, as stated by a CDC spokesperson. Despite a surge of GII.17 strains recorded in Asia and Europe in 2014, there was no noticeable shift in seasonality. The agency continues to cautiously monitor norovirus outbreaks as they keep a keen eye on GII.17’s prevalence.