Real-World Effectiveness of Nirsevimab in Preventing RSV Hospitalizations: Evidence of Protection in Southern Italian Infants, 2024-2025.

Respiratory syncytial virus (RSV) is a leading cause of hospitalization in infants, resulting in millions of cases and hundreds of thousands of deaths worldwide every year. Nirsevimab, a long-acting monoclonal antibody approved in 2023, has 75%-90% protection against RSV-related admissions. This study evaluated the effectiveness of nirsevimab in preventing RSV-hospitalization among infants in southern Italy during 2024-2025 season. Active surveillance was conducted in all hospitals in the Foggia district. Infants born between January 2024 and March 2025 who were hospitalized with lower respiratory tract infections from October 2024 to April 2025 were tested for RSV. Information on immunoprophylaxis was provided by the Regional Immunization Information System. The effectiveness of nirsevimab was estimated using the screening method, a test-negative case-control design and a cohort study approach. Of the 4820 eligible infants, 2635 (54.7%) were immunized. Of the 256 infants admitted with lower respiratory tract infections, 82 tested positives for RSV; of these, 13 had received immunoprophylaxis. The effectiveness was estimated to be 84.4% (95% CI: 71.7-91.4%) using the screening method, 72.5% (42.1%-87.0%) using the test-negative design, and 81.6% (66.4-90.0) using the cohort approach. As in previous study, these findings showed than nirsevimab provide substantial protection against RSV-related hospitalization.