Genotypes and transmission routes of noroviruses causing sporadic acute gastroenteritis among adults and children, Japan, 2015-2019.

Noroviruses (NoVs) are major causes of acute viral gastroenteritis at all ages worldwide. The molecular epidemiology of sporadic cases remains poorly understood, especially in adults. Additionally, no studies have analyzed the transmission route in sporadic acute gastroenteritis. In this study, we investigated cases of very mild sporadic NoV acute gastroenteritis in adults (medical staff) who do not visit the outpatient clinic and child outpatients. We also evaluated genotype differences between adults and children and possible transmission routes in adults during 5 years. The number of NoV positives were 58 in adults and 124 in children. In adults, the NoV positivity rate in this study was higher (64.4%) than that in previous reports of outpatients (10%) and inpatients (5%) in the United State. This finding suggested that the NoV positivity rate might be high in adults with very mild acute gastroenteritis. In adults, human-to-human transmission rates from children and food-borne transmission (raw oysters) were 21.6% (11/51) and 19.6% (10/51), respectively. Among adults, GII.2, GII.4, and GII.17 were the predominant genotypes, with rates of 32.7%, 30.9%, and 21.8%, respectively. Among children, GII.4 and GII.2 were the predominant genotypes, with rates of 45.5% and 40.6%, respectively. GII.17 was only detected in 0.8% (1/123) of children. Trends in NoV genotypes are expected to differ depending on the patient's age. Investigating sporadic cases including the patient's background (age and transmission route) may be helpful to monitor the trend of NoV strains, forecast prevalent NoV GII genotypes, and develop NoV vaccines.