Protecting infants from respiratory syncytial virus (RSV) in Ireland: Impact of a national nirsevimab immunisation programme, 2024/2025

Vaccine. 2026 Mar 19:76:128344. doi: 10.1016/j.vaccine.2026.128344. Epub 2026 Feb 13.

Abstract

Background: To reduce the burden of respiratory syncytial virus (RSV) in infants, Ireland implemented a national immunisation programme in the 2024/2025 season. Infants born between 1 September 2024 and 28 February 2025 were offered nirsevimab, a long-acting monoclonal antibody.

Aim: To estimate the impact of the nirsevimab immunisation programme on RSV-related morbidity among eligible infants.

Methods: We conducted a retrospective, population-based ecological study including infants born during the programme period. RSV case distribution and clinical outcomes in 2024/2025 were described and compared with historical data. Immunisation uptake was monitored weekly. Using immunisation effectiveness estimates, uptake and notification data of cases, we estimated averted cases, disease prevented fractions, and the number needed to immunise (NNI) to prevent one RSV-notified case, emergency department (ED) presentation, hospitalisation, and intensive care unit (ICU) admission applying adapted Machado et al. formulas.

Results: In 2024/2025, 83% (22,444) eligible infants received nirsevimab. In total 360 laboratory confirmed RSV cases were notified in infants born between September and February, compared to 1142 during the same period of 2023/2024 (severe season) and 997 during 2022/2023 (mild season), representing a 68% and 64% reduction, respectively. 1055 (95%CI:1038-1071) RSV notified cases, 459 (95%CI: 452-467) ED presentations, 437 (95%CI:430-443) hospitalisations, and 76 (95%CI:74-78) ICU admissions were averted in the 2024/2025 birth cohort. Disease prevented fraction was 74.5% (95%CI:73.6-75.4). NNI to prevent one RSV-notified case, was 22 (95%CI:21-24), 51 (95%CI:47-55) for ED presentations, 54 (95%CI:50-58) for hospitalisations, and 309 (95%CI:249-369) for ICU admissions.

Conclusion: Immunisation with nirsevimab significantly reduced RSV-related illness in Irish infants in 2024/2025, supporting its continued use and potential expansion to older infant groups.

Keywords: Immunisation impact; Infant immunisation; Nirsevimab; Number needed to immunise (NNI); Respiratory syncytial virus (RSV).

MeSH terms

  • Antibodies, Monoclonal, Humanized* / administration & dosage
  • Antibodies, Monoclonal, Humanized* / therapeutic use
  • Female
  • Hospitalization / statistics & numerical data
  • Humans
  • Immunization Programs*
  • Infant
  • Infant, Newborn
  • Ireland / epidemiology
  • Male
  • Respiratory Syncytial Virus Infections* / epidemiology
  • Respiratory Syncytial Virus Infections* / prevention & control
  • Respiratory Syncytial Virus Vaccines
  • Respiratory Syncytial Virus, Human / immunology
  • Retrospective Studies

Substances

  • Antibodies, Monoclonal, Humanized
  • Respiratory Syncytial Virus Vaccines