Emergence of norovirus GII.7 and predominance of recombinant strains in pediatric acute gastroenteritis in Japan, 2021-2024.

Noroviruses (NoVs) are a leading cause of acute gastroenteritis (AGE) worldwide, with genetic diversity driven by recombination and genotype replacement. This study investigated the prevalence and molecular characteristics of NoVs among Japanese children with AGE from July 2021 to June 2024. A total of 1,095 stool samples were collected from three prefectures (Kyoto, Osaka, and Shizuoka), and NoV RNA was detected in 456 cases (41.6%). Genogroup GII predominated (96.9%), while GI infections were rare. Genotyping identified eight genotypes, including two GI (GI.1, GI.6) and six GII (GII.2, GII.3, GII.4, GII.6, GII.7, GII.17). GII.4 remained the predominant genotype (71.7%) throughout the study period. Notably, GII.7 emerged in 2023-2024 and became the second most prevalent genotype. Analysis of the ORF1/ORF2 junction identified multiple RdRp-capsid combinations, with recombinant strains accounting for 92.5% of detections. The predominant recombinant lineages were GII.4[P16] and GII.4[P31], followed by GII.2[P16]. Phylogenetic analysis indicated close relatedness to strains circulating internationally. These findings demonstrate the sustained predominance of GII.4, the emergence of GII.7, and the high prevalence of recombinant strains among Japanese children, highlighting the importance of continuous molecular surveillance.