Hospital and primary care outcomes following universal nirsevimab immunization in infants in Puglia, Italy: a population-based observational study

Abstract Background Respiratory syncytial virus (RSV) is a leading cause of lower respiratory tract infections and hospitalization in infants. In 2024, the Puglia Region introduced universal infant immunization with nirsevimab, providing an opportunity to assess its real-world impact on RSV-related outcomes across both hospital and primary care settings. Aims To evaluate the effectiveness of nirsevimab in reducing RSV-related hospitalizations and disease severity during the 2024–2025 RSV season compared with the 2023–2024 season, and describe the burden and clinical characteristics of bronchiolitis reported in primary care. Results Following the introduction of nirsevimab, RSV-related hospitalizations declined from 931 to 259 cases (3.71% vs. 1.08%). RSV-related hospitalization rates were 71% lower in 2024–2025 than in 2023–2024; equivalently, infants in the 2023–2024 seasonal population had a 3.45-fold higher hospitalization risk (RR 3.45, 95% CI 3.01–3.96; p < 0.001). Within the 2024–2025 cohort, infants who received nirsevimab had an 84% lower risk of hospitalization compared with non-prophylaxed infants (0.39% vs. 2.41%; RR 0.16, p < 0.00001). In the primary care survey, 49/229 pediatricians (21.4%) reported no clinically diagnosed bronchiolitis cases during 2024–2025, compared with 7/229 (3.1%) for 2023–2024. Regarding nirsevimab status, 64.3% of respondents reported that none of their bronchiolitis cases had received nirsevimab, whereas 28.6% reported that fewer than 30% of their cases had been immunized. These categorical responses did not permit estimation of patient-level effectiveness. Pediatrician-reported adverse events were predominantly mild; however, the survey was not designed as an active pharmacovigilance study. Conclusions In this regional real-world setting, universal nirsevimab implementation was associated with substantially lower RSV-related hospitalization rates. The primary care survey also indicated a lower reported burden of clinically diagnosed bronchiolitis, although patient-level effectiveness could not be estimated from aggregate survey data.