The emergence of unusual G9P[8]-E2 human rotaviruses in the Tokyo metropolis, Japan in 2018 has been reported. During rotavirus strain surveillance in different regions of Japan (Mie, Okayama, and Chiba prefectures), G9P[8]-E2 strains were detected in diarrheic children in all three prefectures. Here, we characterized the whole genomes of seven representative G9P[8]-E2 strains. On full-genome-based analysis, the seven study strains exhibited a unique genotype configuration having the NSP4 gene of genogroup 2 in a genogroup 1 genomic backbone: G9-P[8]-I1-R1-C1-M1-A1-N1-T1-E2-H1. This genotype constellation is shared by Tokyo G9P[8]-E2 strains. Phylogenetic analysis showed that all the 11 genes, except the NSP4 one, of the seven study strains appeared to have originated from co-circulating Wa-like G9P[8]-E1 strains. On the other hand, the NSP4 gene appeared to have originated from co-circulating DS-1-like G2P[4]-E2 strains. Thus, these study G9P[8]-E2 strains appeared to be derived through reassortment between G9P[8]-E1 and G2P[4]-E2 strains in Japan. Notably, the seven study G9P[8]-E2 strains and Tokyo G9P[8]-E2 strains were revealed to have 11-segment genomes almost indistinguishable from one another in their sequences (99.3-100%), indicating all these G9P[8]-E2 strains had a common origin. To our knowledge, this is the first description of the rapid spread of G9P[8]-E2 strains across a country.

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