Background Respiratory Syncytial Virus (RSV) is a major cause of hospitalisations and deaths among infants worldwide. France was one of the first countries to administer nirsevimab, a single-dose long acting monoclonal antibody treatment to prevent lower respiratory tract infection caused by RSV. Effectiveness in real-world settings and impact in terms of hospitalisations averted are not well known. Methods We developed an age-structured deterministic model characterising RSV transmission as well as plausible scenarios for the administration of nirsevimab doses based on delivery data to maternity wards and pharmacies. We estimated nirsevimab effectiveness and the number of hospitalisations for RSV bronchiolitis following emergency room visits (HBER-RSV) averted, by calibrating the model to hospital and virological surveillance data from mid-2017 to February 4, 2024, alongside serological data. Findings We estimated that nirsevimab administration prevented 5,800 (95% CI: 3,700-7,800) HBER-RSV, including 4,200 (2,900-5,600) among those aged 0-2 months, between September 15, 2023 and February 4, 2024. This corresponds to a 23% (16%-30%) reduction in the total number of HBER-RSV (35%, 25%-44%, among 0-2 months) compared to the scenario without administration. In our baseline scenario with 215,000 doses administered by January 31, 2024, the effectiveness of nirsevimab against HBER-RSV was estimated at 73% (61%-84%), which corresponds to 1 HBER-RSV case prevented for every 39 (26-54) doses administered. The estimated number of HBER-RSV averted was insensitive to the assumed administration scenario; but effectiveness varied slightly. Interpretation With 1 HBER-RSV averted for about every 40 children treated, our study highlights the relevance of nirsevimab administration to reduce RSV hospitalisations