Abstract

Human norovirus is a major cause of viral acute gastroenteritis worldwide. However, the transition of endemic norovirus genotypes remains poorly understood. The characteristics of natural immunity against norovirus are unclear because few studies have been performed in the natural infection setting. This prospective 10-year sur-veillance study of acute gastroenteritis in the province of Osaka, Japan, revealed that norovirus spread shows temporal, geographic, and age group–specific features in the humans. Genogroup II genotype 4 (GII.4) was detected in most sporadic pediatric cases, as well as in foodborne and nursing home outbreaks, respectively. The dominant genotypes in outbreaks at childcare facilities and schools shifted every season and involved GI, GII.2, GII.3, GII.4, and GII.6. Evidence at both the facility and individual levels indicated that genotype-specific herd immunity lasted long enough to influence the endemic norovirus genotype in the next season. Thus, norovirus circulates through human populations in a uniquely dynamic fashion. Keywords. genotype; herd immunity; longitudinal surveillance; norovirus; reinfection. Norovirus (NoV) is the most common causative agent of viral acute gastroenteritis (AG) worldwide. People of all ages are at risk of NoV infection, and it spreads easily by person-to-person contact via the fecal–oral route.