Incidence of RSV- and Influenza-Associated Hospitalizations With Community-Acquired Pneumonia and Other Acute Respiratory Infection Among Adults in Japan in 2022-2024: APSG-J2 Study.

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.