This nationwide matched cohort study evaluated nirsevimab effectiveness against RSV-LRTI-related hospitalizations across two consecutive RSV seasons (2023–2024 and 2024–2025) in French infants, and assessed whether protection persisted into a second year and whether nirsevimab was associated with hospitalizations for non-RSV infections and asthma.
Nirsevimab reduced RSV-LRTI-related hospitalizations by 66% (95% CI, 61%–70%) in year 1 of the 2023 cohort and 72% (95% CI, 69%–75%) in year 1 of the 2024 cohort (0.8% vs 2.4% and 0.5% vs 1.9% immunized vs unimmunized, respectively). No protection carried into the second year (wHR, 1.03; 95% CI, 0.73–1.43). Notably, nirsevimab was associated with higher rates of ENT bacterial infection hospitalizations in year 1 (wHR 1.36–1.43) and in year 2 (wHR, 1.56), and with higher asthma hospitalization rates in year 2 (wHR, 1.24; 95% CI, 1.05–1.46).
- Observational design cannot exclude residual confounding despite 1:1 matching and weighted Cox models. - The ENT and asthma signals in year 2 may reflect surveillance or healthcare-seeking bias rather than a true causal effect. - Full text was not available; analysis is based on the abstract and reported results only.
Nirsevimab provides strong, consistent protection against RSV hospitalization in the first season across consecutive years of use, but clinicians should be aware that protection does not extend into a second RSV season. The unexpected associations with ENT bacterial infections and asthma hospitalizations in year 2 warrant further investigation before drawing causal conclusions.
