Nirsevimab and Acute Bronchiolitis Episodes in Pediatric Emergency Departments.

Background And Objectives: In the 2023-2024 respiratory syncytial virus (RSV) season, Spain became one of the first countries to introduce universal RSV prophylaxis, during which all infants born at this time were eligible to receive nirsevimab. Locally, most Spanish regions also immunized infants younger than age 6 months at the start of the season (extended catch-up). The aim of this study was to assess how RSV prophylaxis affected the number of infants presenting to pediatric emergency departments with acute respiratory infections.

Methods: A retrospective study was conducted in 15 Spanish pediatric emergency departments from 9 different regions between the 2018 and 2024 epidemic seasons (November-January). We compared the seasons occurring in 2018-2023 and the 2023-2024 season regarding the number of episodes of lower respiratory tract infection and acute bronchiolitis, acute bronchiolitis-related hospital admissions, and PICU admissions.

Results: A comparison with the average rates for the previous epidemic seasons revealed a 57.7% decrease in episodes of lower respiratory tract infection in 2023-2024 (95% CI, 56.5-58.8; P < .001; range among hospitals, 4.8-82.8), a 59.2% decrease in episodes of acute bronchiolitis (95% CI, 57.9-60.4; P < .001; range, 6.9-84.1), a 63.1% reduction in acute bronchiolitis-related hospital admissions (95% CI, 60.9-65.2; P < .001; range, 31.4-86.8), and a 63.1% reduction in PICU admissions (95% CI, 58.1-67.9; P < .001; range, 18.2-81.8). Hospitals in regions applying extended catch-up showed better results.

Conclusions: Nirsevimab can protect a broad infant population against RSV infection with high effectiveness. Approaches including extended catch-up are the most effective, although cost- effectiveness must be considered.