Nirsevimab Effectiveness at Preventing RSV-Related Hospitalization in Infants

BACKGROUND: In pivotal trials, nirsevimab showed promising efficacy in reducing hospitalizations for respiratory syncytial virus-associated lower respiratory tract infections (RSV-LRTIs). Nirsevimab's real-world effectiveness needs to be assessed. METHODS: This population-based study used the French National Health Data System. All infants born between February 6 and September 15, 2023 were eligible. Each day during the study period (September 15, 2023, to January 31, 2024), all infants newly passively immunized with nirsevimab were matched to unimmunized controls in a 1:1 ratio according to sex, birth month, gestational age, department of residence, and the French Social Deprivation Index (Fdep). Study outcomes included RSV-LRTI-related hospitalization, RSV-LRTI necessitating admission to a pediatric intensive care unit (PICU) or high dependency unit (HDU), and RSV-LRTI requiring ventilation support or oxygen therapy. We estimated nirsevimab effectiveness using propensity score-weighted conditional Cox models. RESULTS: The study included 82,474 infants (41,237 in each group) with a median follow-up of 118 days (interquartile range, 76 to 125). The population included predominantly male infants (52.5%) born at term (94.6%), mostly between April and July 2023 (64.0%), and from more advantaged municipalities (FDep first quintile: 29.8%). In total, 342 infants (0.8%) in the nirsevimab group and 992 (2.4%) in the unimmunized group were hospitalized for RSV-LRTI. Nirsevimab's effectiveness was 65% (95% confidence interval CI, 61 to 69) for RSV-LRTI hospitalizations; 74% (95% CI, 56 to 85) for RSV-LRTI PICU admissions; 64% (95% CI, 55 to 71) for RSV-LRTI HDU admissions; 66% (95% CI, 51 to 76) for RSV-LRTI hospitalization requiring ventilation support; and 67% (95% CI, 57 to 75) for RSV-LRTI hospitalization requiring oxygen therapy. Subgroup and sensitivity analyses yielded consistent effectiveness estimates. CONCLUSIONS: This study in a nationwide monoclonal antibody infusion setting suggests that a single injection of nirsevimab was associated with substantial protection of infants against hospitalization for RSV-LRTI.