Clinical Observations: Complete Genome Analysis, and Evolutionary Insights into Rare Sapovirus GII.8 in Japanese Children

The increasing incidence of Sapovirus (SaV) infection, particularly following the introduction of rotavirus vaccines, has raised a growing global public health concern. This study investigated the clinical, molecular, and evolutionary characteristics of two rare SaV GII.8 strains detected in Japan in 2022 using full-genome sequencing and comprehensive phylogenetic analyses. The infections occurred in two siblings aged 9 and 1.5 years, with mild disease in the older child and moderate severity in the younger; vomiting was more frequent than diarrhea. Bayesian analyses under both relaxed-clock models yielded similar evolutionary rates (4.2–4.5 × 10−3 substitutions/site/year). The emergence of the GII.8 lineage was estimated between 2004 and 2006 (combined 95% HPD: 1994–2008). Multiple amino acid substitutions were identified across both open reading frames; however, all major motifs associated with viral functions remained conserved, similar to those in reference SaV GII.8 strains. Phylogenetic analysis further revealed two temporally distinct lineages, with our strains clustering within a recent global lineage (2019–2022) containing strains from multiple countries, consistent with international circulation rather than geographic restriction. Overall, the findings provide detailed insights into the evolution of the rare GII.8 genotype, indicating gradual genetic diversification despite relatively stable population dynamics.