A cohort study in France assessed the monoclonal antibody nirsevimab against respiratory syncytial virus (RSV) in infants at 1 year and 2 years after immunisation. Outcomes were examined in infants born before the first RSVpreF vaccination campaign for pregnant women started in September 2024. Nirsevimab was associated with lower rates of RSV-lower respiratory tract-related hospitalisations during the first year after immunisation: 299 (0·8%) of 37 366 immunised versus 899 (2·4%) of 37 366 non-immunised infants in 2023, and 456 (0·5%) of 87 763 immunised versus 1711 (1.9%) of 87 763 non-immunised infants in 2024 (weighted hazard ratios 0·34, 95% CI 0·30–0·39 for the 2023 cohort and 0·28, 0.25–0·31 for the 2024 cohort, corresponding to nirsevimab effectiveness estimates of 66% and 72%, respectively).
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