Impact of the first nirsevimab immunization campaign on infant RSV-related emergency department visits and hospitalizations in Québec, Canada.

OBJECTIVES: During the 2024-2025 respiratory syncytial virus (RSV) season, a universal nirsevimab infant immunization program was implemented in the province of Québec. We evaluated its population-level impact on RSV-related emergency department (ED) visits and hospitalizations in 0-5-month-old infants, using both active surveillance and administrative health data. METHODS: Study population included all Québec children <18 years old. We calculated incidence rates of RSV-confirmed hospitalizations, RSV-associated hospitalizations and ED visits linked to a positive RSV test, and acute bronchiolitis ED visits for nirsevimab-eligible (0-5 months old) and non-eligible (6-11 months and 1-17 years old) age groups during the 2024-2025 season. We also compared these 2024-2025 figures to previous seasons using varying lookback periods. Using difference-in-differences and constant proportion of admissions counterfactual approaches, pre-/post-intervention variations in 0-5-month-olds were adjusted for any time trends occurring in non-targeted age groups. RESULTS: Decreases of 59% (95% CI: 49-70) and 66% (95% CI: 58-71) were observed in RSV-confirmed and RSV-associated hospitalizations, respectively. Acute bronchiolitis ED visits decreased by 35% (95% CI: 28-41), and RSV-associated ED visits by 60% (95% CI: 56-65). In the 2024-25 season, ED visits and hospitalizations were less frequent than expected (i.e. without nirsevimab) in 0-5-month-old infants, suggesting that between 323 and 746 hospitalizations may have been prevented. CONCLUSION: The 2024-2025 nirsevimab campaign was associated with a two-thirds reduction in RSV-related hospitalizations and RSV-associated ED visits in Québec among 0-5-month-old infants. ED visits for acute bronchiolitis were also reduced by a third.