ABSTRACT Background Quantifying the burden of respiratory syncytial virus (RSV) in adults is challenging compared to influenza, and data among older adults remain scarce in Japan. Country‐specific evidence is essential to support RSV vaccination policy. Methods This prospective, multicenter study (APSG‐J2) targeted hospitalized adults with community‐acquired pneumonia (CAP) and other acute respiratory infections (ARI) in seven community hospitals across four catchment areas in Japan between September 2022 and August 2024. Respiratory samples were analyzed using a multiplex polymerase chain reaction (PCR) kit to detect RSV and influenza. Incidence rates of RSV‐ and influenza‐associated hospitalizations were estimated using study data and national statistics, stratified by age and region. Results Among 3047 hospitalized patients with CAP/ARI, 1499 (49.2%) underwent multiplex PCR testing. RSV and influenza were detected in 2.8% and 3.3% of tested patients, respectively. The incidences of RSV‐associated CAP/ARI hospitalizations among adults aged ≥ 65 years were 29 and 36 per 100,000 person‐years in the first and second years, respectively, with higher incidences among those aged ≥ 85 years (150 and 131 per 100,000 person‐years). Influenza incidence increased markedly in the second year (from 11 to 71 per 100,000 person‐years for adults age ≥ 65 years), possibly reflecting post‐COVID‐19 transmission changes. Conclusions In this multicenter study, we estimated the incidence of RSV‐ and influenza‐associated hospitalizations among adults in Japan. The findings indicated that the incidence increased with age, and influenza‐associated hospitalizations increased in the second year. Continued surveillance is essential to accurately assess RSV burden in the adult population.
Maeda et al. (Sun,) studied this question.