Temporal Changes in Bronchiolitis Hospitalization Across Seven Seasons in a French University Hospital

Background/Objectives: We examined seasonal changes in bronchiolitis-related hospital use following the introduction of respiratory syncytial virus (RSV) prevention in France. Methods: This single-center observational study included 7083 emergency department presentations over five pre-implementation (2018–2023) and two post-implementation (2023–2025) seasons. Conventional ward hospitalization proportions were compared between infants aged <1 year and older children using difference-in-differences regression. Immunization histories were unavailable; virological information was limited to selectively tested subsets. Results: Infant hospitalization proportions decreased from 35.2% to 31.6%, while proportions among older children increased from 24.9% to 29.9%. The older-versus-infant interaction incidence rate ratio was 1.339 (95% confidence interval 1.038–1.727; p = 0.025) with Poisson regression but was not significant with quasi-Poisson regression (95% confidence interval 0.950–1.886; p = 0.087). Excluding 2023–2024 removed statistical significance. Conclusions: An age-specific temporal association was observed, with uncertainty across models and seasons. Two post-implementation seasons, maternal vaccination introduced only in the second, incomplete virological ascertainment, absent immunization histories, and changing care organization prevent causal attribution or separation of the contributions of nirsevimab and maternal vaccination.