Using real-life data from Spain between October 2023 and March 2024, the number needed to immunise (NNI) with nirsevimab and the cost to prevent one RSV hospitalisation were estimated at 90 infants (95% CI: 77-108) and 19,700 EUR for catch-up immunisation, and 41 infants (95% CI: 35-50) and 9,000 EUR for at-birth immunisation. By month of birth, NNI and cost were lowest in infants born shortly before the RSV epidemic peak, with impact decreasing gradually for earlier or later births.