In Japan, nationwide surveillance of acute respiratory infections (ARI) was launched in April 2025, enabling monitoring of multiple respiratory pathogens. Human parainfluenza virus type 3 (HPIV3) is an important cause of respiratory infections. In 2025, HPIV3 increased from spring to summer, and Japanese media raised concerns about the emergence of a novel pathogen because public surveillance data on HPIV3 trends had not previously been available. In this study, phylogenetic analysis of HPIV3 isolates obtained in eastern Japan in 2025 was performed using nearly complete genome sequences. The isolates were grouped within previously circulating clusters (C1 and C2), and several sequences were closely related to viruses detected in Fukushima between 2018 and 2024, suggesting continued circulation of HPIV3 within the same geographic region (Fukushima Prefecture and surrounding areas), although broader circulation cannot be excluded. Some sequences were related to strains reported outside Japan, suggesting possible introduction from abroad. Overall, the sequences were highly similar and no distinct lineage forming a new cluster was observed. These findings indicate that the HPIV3 strains detected in 2025 belong to previously circulating lineages and do not represent the emergence of a new lineage.
Shirato et al. (Thu,) studied this question.