Nirsevimab and Hospitalization for Lower Respiratory Tract Infection During the Second Season

OBJECTIVES: To assess the effect of nirsevimab in preventing hospitalizations for lower respiratory tract infection (LRTI) during the second respiratory season of life among infants who were administered nirsevimab in their first months of life. METHODS: Community-based case-control study conducted in 9 tertiary pediatric hospitals in Andalusia, Southern Spain. Infants born between April 1, 2023, and March 31, 2024, were included. Cases were infants hospitalized for LRTI during the 2024 to 2025 season. For each case, 4 community controls were matched by birthdate and province. Exposure was nirsevimab administration during the 2023 to 2024 respiratory syncytial virus (RSV) season. The main outcome was hospitalization for LRTI during the 2024 to 2025 season. Inverse probability weighting (IPW) and doubly robust augmented IPW (AIPW) models were used to estimate the average treatment effect, expressed as absolute risk reduction (ARR). RESULTS: The study included 1375 infants (275 cases and 1100 controls). Nirsevimab administration during the 2023 to 2024 season was associated with a 17.0% ARR (95% CI, 3.2%-30.8%) in LRTI hospitalizations during the 2024 to 2025 season in the AIPW model. Stronger associations were observed among RSV-positive cases (ARR = 25.4%; 95% CI, 4.9%-45.8%) and in infants with a history of inhaler use (ARR = 27.7%; 95% CI, 6.4%-48.9%). A negative control exposure (rotavirus vaccination) showed no association, supporting no evidence of relevant residual confounding regarding health-seeking behaviors. CONCLUSIONS: Nirsevimab administration was associated with a reduction in second-season LRTI hospitalizations, suggesting meaningful benefits beyond the season of administration and potential implications for future cost-effectiveness evaluations.