BACKGROUND: Invasive pneumococcal diseases (IPDs) remain a leading cause of morbidity and mortality in children worldwide, despite the widespread use of pneumococcal conjugate vaccines. Respiratory syncytial virus (RSV) disease is suspected to be associated with an increased risk of IPD. We assessed whether immunisation with nirsevimab, a monoclonal antibody targeting RSV, reduced the risk of IPD in children. METHODS: This population-based, retrospective cohort study used data from the French National Health Data System. All liveborn children in metropolitan France between Feb 6, 2023, and Jan 31, 2024, for whom nirsevimab immunisation status was available, were included. The date of nirsevimab immunisation defined the inclusion date of immunised children and their birth-month-matched non-immunised children. Children immunised with nirsevimab during the first year of life constituted the immunised group, whereas children non-immunised with nirsevimab constituted the non-immunised group. The primary outcome was hospitalisation for IPD within 6 months after inclusion. Propensity score analysis using inverse probability of treatment weighting (IPTW) was conducted to adjust for differences in baseline characteristics between the two groups. Immunisation effectiveness was calculated with the use of the following equation: effectiveness=100% × (1 - odds ratio [OR]), with the OR reflecting the association between nirsevimab and IPD. FINDINGS: Of the 608 641 children born in France during the study period, 527 971 were included, of whom 119 435 (22·6%) received nirsevimab. During the 6-month follow-up period, 112 cases of IPD occurred: 17 (14·2 per 100 000) in the immunised group versus 95 (23·3 per 100 000) in the non-immunised group. After IPTW, nirsevimab was associated with a 36% reduction in the odds of hospitalisation for IPD 6 months after immunisation (OR 0·64, 95% CI 0·46-0·82). 9 months after immunisation, effectiveness remained consistent with a 34% reduction (0·66, 0·51-0·85). INTERPRETATION: Nirsevimab immunisation in children younger than 12 months was associated with a lower risk of IPD for at least 6 months following immunisation. These findings suggest an additional benefit of nirsevimab implementation beyond RSV disease prevention, but further studies are needed to confirm these findings. FUNDING: AP-HP Research fellowship, ATIP-Avenir partnership, and AP-HP Foundation. TRANSLATIONS: For the French translation of the abstract see Supplementary Materials section.