Effectiveness of nirsevimab against RSV-bronchiolitis in paediatric ambulatory care: a test-negative case-control study.

Background: Respiratory syncytial virus (RSV) is the leading cause of lower-respiratory-tract infection in children. Nirsevimab, a monoclonal antibody against RSV, was implemented in a few countries in September 2023. However, its post-license effectiveness in ambulatory care settings is unknown. We aimed to assess the effectiveness of nirsevimab against RSV-bronchiolitis in outpatients aged <12 months. Methods: We conducted a test-negative case-control study based on a national ambulatory surveillance system. We included all infants aged <12 months who had bronchiolitis and results of an RSV rapid antigen test performed, visiting a network of 107 ambulatory paediatricians from September 15, 2023, to February 1, 2024. Case patients were infants with bronchiolitis and a rapid antigen test positive for RSV. Control patients were infants with bronchiolitis and a rapid antigen test negative for RSV. Effectiveness was assessed by a logistic regression model adjusted for potential confounders. A range of sensitivity analyses were conducted to assess the robustness of the findings. Findings: We included 883 outpatients who had bronchiolitis and results of an RSV rapid antigen test (453 were case patients, and 430 were control patients). Overall, 62/453 (13.7%) case patients and 177/430 (41.2%) control patients had been previously immunised for nirsevimab. The adjusted effectiveness of nirsevimab against RSV-bronchiolitis was 79.7% (95% CI 67.7-87.3). Sensitivity analyses gave similar results. Interpretation: This post-license study indicates that nirsevimab was effective in preventing RSV-bronchiolitis in ambulatory care settings. Funding: The study was supported by Association Clinique et Thérapeutique Infantile du Val de Marne (ACTIV), French Pediatrician Ambulatory Association (AFPA) and unrestricted grants from GSK, MSD, Pfizer and Sanofi.