Effectiveness of nirsevimab against respiratory syncytial virus- related hospitalization in Madeira, Portugal: 2023-24

Introduction and Objectives: Respiratory syncytial virus (RSV) is a leading cause of hospitalizations for acute lower respiratory tract infection (LRTI) in infants. Madeira was the first Portuguese region to recommend immunoprophylaxis with nirsevimab, an anti-RSV monoclonal antibody, for all infants in their first RSV season. The immunization campaign, which ran from November 2023 to March 2024, targeted infants born during the campaign (seasonal group), and those born between April and October of 2023 (catch-up group). This study aimed to assess the effectiveness of nirsevimab in preventing RSV-related LRTI hospitalizations. Methods: A population-based longitudinal observational study was conducted. Follow-up lasted until 150 days post-immunization, the end campaign (for non-recipients), hospitalization or death. The effectiveness of nirsevimab effectiveness was estimated using a Cox proportional hazards model adjusted for age and sex. The number needed to immunize (NNI) was calculated from absolute risk reduction. Averted cases were estimated using historical data, excluding the period of the COVID-19 pandemic. Results: The overall immunization rate was 97.4% (1,710/1,756), and the result was similar in the seasonal (97.7%, 728/745) and catch-up (97.1%, 982/1,011) groups. There were 0.4% (6/1,710) RSV-related LRTI hospitalizations among nirsevimab recipients versus 4.3% (2/46) among non-recipients, with an estimated effectiveness of 94.6% (95% CI 72.6-98.9). The NNI was 25. Supplemental oxygen therapy duration decreased significantly compared to previous seasons (P = .039). The number of averted cases was 45 (IQR 15-83), with a 79.6% reduction in hospitalizations (IQR 62.6-90.9). Conclusion: Nirsevimab was effective in reducing hospitalizations for RSV-related LRTI in a real-world setting, offering useful evidence for future RSV immunization strategies.