Emerging norovirus GII.4 Sydney[P31] causing acute gastroenteritis outbreak in children in Japan, during COVID-19, 2021

Norovirus (NoV) is the most common agent causing outbreaks and sporadic cases of acute gastroenteritis among all ages, especially children under 5 years old. During the coronavirus disease 2019 (COVID-19) pandemic, NoV infection has decreased drastically in Japan due to school closures and no outbreak related to NoV infection had been reported. In mid-September 2021, NoV outbreak occurred in kindergarten and nursery schools in Maizuru, Kyoto prefecture, Japan. Twenty-six stool samples collected from patients who were diagnosed of NoV gastroenteritis from the outbreak by an immunochromatographic (IC) kit at a pediatric outpatient clinic in Maizuru city during 3 weeks from September 13 to October 8, 2021 were examined for the presence of NoV GII by reverse transcriptase-polymerase chain reaction (RT-PCR), genome sequencing, and phylogenetic analysis. All 26 samples were confirmed positive to NoV GII and their genotypes were identified as GII.4 Sydney[P31]. The amino acid substitutions in open reading frame1 (ORF1) and ORF2 genes were found when compared with previously detected sporadic NoV GII.4 Sydney[P31] strains isolated in Japan. The clinical characterization of infected children was described. Most of the children were mild cases and vomiting was the most frequent clinical symptom. This study reported a recent emergence of NoV GII.4 Sydney[P31] causing acute gastroenteritis outbreak in children in Japan during the COVID-19 pandemic and suggests a need for further monitoring of NoV GII.4 variants.