Abstract
We measured effectiveness of nirsevimab against laboratory-confirmed respiratory syncytial virus (RSV) infection in a test-negative case-control study among children aged < 24 months hospitalised for severe acute respiratory infection in three European countries. The overall effectiveness in the 2024/25 season among 2,201 children was 79% (95% CI: 58 to 89) and 85%, 78% and 69% at < 30, 30-89 and 90-215 days since immunisation. Immunisation was effective for preventing RSV-related hospitalisation in children, but effectiveness by time since immunisation needs monitoring in future seasons.