Respiratory syncytial virus outcomes and duration of protection following nirsevimab administration in high-risk infants: a real-world study under year-round epidemic circulation in Japan.

BACKGROUND

Recent respiratory syncytial virus (RSV) epidemic patterns in Japan have become year-round rather than seasonal, with peaks occurring two or more times per year. Nirsevimab, a long-acting human monoclonal RSV antibody, prevents RSV-induced lower respiratory tract disease for up to 150 days after administration; however, these data were based on seasonal epidemic patterns. Furthermore, data demonstrating real-world outcomes and effect duration following drug administration in high-risk infants, such as those with preterm birth or congenital heart disease, are limited.

METHODS

This retrospective, observational cohort study included infants who received nirsevimab at our hospital between June 2024 and June 2025. Subsequently, follow-up assessments regarding RSV infection status, RSV-related hospitalizations, and treatments received were conducted at each outpatient visit through January 2026. We investigated the timing of RSV infection after nirsevimab administration in conjunction with RSV epidemic patterns.

RESULTS

In total, 183 infants received nirsevimab, and 178 were followed up. Among the 17 infants with RSV, five were hospitalized, one of whom received respiratory therapy via a high-flow nasal cannula. No differences in patient characteristics were observed based on RSV infection status. The event-free rates at 250 days after nirsevimab administration were 0.91 for RSV infection, 0.97 for RSV-induced hospitalization, and 0.99 for RSV-induced severe lower respiratory tract infection.

CONCLUSIONS

Nirsevimab showed a high event-free rate even among high-risk infants, suggesting that its protection extends beyond 250 days after administration, even in regions with atypical RSV epidemic patterns, such as Japan.