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• Norovirus (NoV) was frequent among 1294 children with acute gastroenteritis (AGE). • 24% of AGE outpatient episodes and 18% of AGE hospitalizations were NoV-positive. • The predominant NoV genotype was GII.4, present in 58% of NoV infections. • Brazil, with rotavirus vaccination, had the highest prevalence of NoV infection. Noroviruses (NoVs) cause acute gastroenteritis (AGE) worldwide, affecting children in particular. We aimed to estimate the burden of disease due to NoV among children aged <6 years in Brazil, Chile, Philippines and Thailand. This was a prospective, hospital-based, observational study. Children were recruited over one year between 2014 and 2017. Four cohorts were analysed: community-acquired AGE outpatients and inpatients, nosocomial AGE inpatients, and asymptomatic outpatients. We collected demographic and clinical data, and a stool sample that was tested for NoV. Positive samples were tested for Rotavirus (RV) and NoV-genotyped. Disease severity was assessed by the Vesikari and modified Vesikari scores. Prevalence and incidence of NoV-AGE were estimated by cohort and country. 1637 participants yielded valid laboratory results. The proportion of NoV-positive cases was 23.8% (95% CI 20.8-27.2) in the outpatient cohort, 17.9% (15.0-21.3) in the hospital cohort, 21.4% (12.7-33.8) in the nosocomial cohort and 9.6% (6.9-13.2) in the asymptomatic cohort. Genotype GII.4 was predominant (58%). Less than 4% samples had RV coinfection. In general, NoV-positive subjects had more severe presentations than NoV-negative subjects. NoV caused AGE with substantial burden throughout the studied settings, with higher relative frequency in Brazil where RV vaccination coverage is high. [ABSTRACT FROM AUTHOR]