Epidemiologic patterns in published norovirus outbreak reports: A comparison of genogroups I and II

Noroviruses (NoVs) are a significant cause of foodborne and waterborne outbreaks of acute gastroenteritis worldwide. Despite the development of sensitive diagnostic and detection methods, questions remain regarding the risk factors associated with NoV outbreaks. The goal of this study was to examine the association between NoV genogroup and outbreak transmission mode, season and setting in outbreaks reported in the literature. The data for this analysis were obtained from published reports of NoV outbreak investigations and compiled in a database. A total of 250 outbreaks reported from the US, Europe, Asia and Africa in the past three decades had sufficient detail to be included in the database. NoV genogroup information was available in 81% of the published outbreak reports we reviewed. Genogroup II (GII) NoVs were involved in 70% of published NoV outbreaks. About 12% of published outbreaks involved both GI and GII NoVs. GI outbreaks were associated with significantly higher attack rates (median 57%) compared to GII outbreaks (median 29%). Higher attack rates were also observed for foodborne outbreaks and outbreaks involving restaurants and food catering. A higher proportion of GI outbreaks (39%) occurred in restaurant settings while GII outbreaks were more frequent (24%) in healthcare settings. Our analyses of seasonality revealed significant differences in the distribution of GI and GII outbreaks; 56% of reported GII outbreaks occurred in the winter whereas no seasonal trends were detected for the GI outbreaks. These data highlight trends in NoV outbreaks and assist in the identification of future NoV outbreaks.