2021-01-272021-01-272016-05-151058-48381537-6591http://dx.doi.org/10.34944/dspace/505227013692 (pubmed)http://hdl.handle.net/20.500.12613/5070© The Author 2016. Published by Oxford University Press for the Infectious Diseases Society of America. Background. Norovirus is an important cause of childhood diarrhea. We present data from a longitudinal, multicountry study describing norovirus epidemiology during the first 2 years of life. Methods. A birth cohort of 1457 children across 8 countries contributed 7077 diarrheal stools for norovirus testing. A subset of 199 children contributed additional asymptomatic samples (2307) and diarrheal stools (770), which were used to derive incidence rates and evaluate evidence for acquired immunity. Results. Across sites, 89% of children experienced at least 1 norovirus infection before 24 months, and 22.7% of all diarrheal stools were norovirus positive. Severity of norovirus-positive diarrhea was comparable to other enteropathogens, with the exception of rotavirus. Incidence of genogroup II (GII) infection was higher than genogroup I and peaked at 6-11 months across sites. Undernutrition was a risk factor for symptomatic norovirus infection, with an increase in 1 standard deviation of length-for-age z score associated with a 17% reduction (odds ratio, 0.83 [95% confidence interval,. 72-.97]; P =. 011) in the odds of experiencing diarrhea when norovirus was present, after accounting for genogroup, rotavirus vaccine, and age. Evidence of acquired immunity was observed among GII infections only: Children with prior GII infection were found to have a 27% reduction in the hazard of subsequent infection (hazard ratio, 0.727; P =. 010). Conclusions. The high prevalence of norovirus across 8 sites in highly variable epidemiologic settings and demonstration of protective immunity for GII infections provide support for investment in vaccine development.1210-1217enCC BY-NC-NDhttp://creativecommons.org/licenses/by-nc-nd/4.0/diarrheaimmunitynorovirusCaliciviridae InfectionsChild, PreschoolCohort StudiesDiarrheaFecesHumansIncidenceInfantInfant, NewbornNorovirusArticle2021-01-27