Real-world effectiveness of nirsevimab against RSV hospitalisation in infants during a season with low coverage and delayed circulation in Beijing, China: a population-based retrospective cohort study.

Nirsevimab was first deployed in Beijing through a pilot voluntary, self-pay scheme during the 2024-2025 epidemic season, following its approval in China in December 2023. We aimed to evaluate the real-world effectiveness of nirsevimab against RSV-associated lower respiratory tract infection (LRTI) hospitalisation in Beijing, China. We conducted a population-based retrospective cohort study using linked, individual-level data from the Beijing Immunisation Information System and Hospitalisation Information System. The study population comprised all infants born between April 1, 2024, and March 31, 2025, in 24 designated pilot maternity hospitals. The primary endpoint was RSV-associated LRTI hospitalisation. City-wide virological surveillance was used to define the population-level exposure risk period. Nirsevimab effectiveness was estimated using Bayesian Poisson regression in a propensity score-matched cohort, with three additional models (Firth penalized Poisson, Bayesian Cox, and Firth penalized Cox) as sensitivity analyses. Among 44,791 eligible infants, 1166 (2.6%) received nirsevimab. In the propensity score-matched cohort (1129 infants per group), the incidence rates of RSV-associated LRTI hospitalisation were 18.8 vs. 2.3 per 1000 person-years in non-recipients and recipients, respectively. Bayesian Poisson regression demonstrated 83.3% effectiveness (95% CrI: 33.3-97.5%; posterior probability: 0.995). Sensitivity analyses yielded consistent estimates (82.8-84.5%). Analyses in the full cohort produced directionally consistent estimates. This first evidence of nirsevimab's effectiveness in China validates its value in a novel geographical and implementation context, supporting its inclusion as a recommended preventive option. Real-time virological surveillance is essential to optimise immunisation timing amid evolving RSV epidemiology. Capital's Funds for Health Improvement and Research (2026-1G-3012); Beijing Research Center for Respiratory Infectious Diseases (BJRID2024-002); Beijing Municipal Health Commission's Funds for the High-qualified Public Health Professionals Development Project (Discipline Core-03-36); Capital's Funds for Health Improvement and Research (2024-2-30117).