Effectiveness of nirsevimab and maternal RSVpreF vaccination against infant hospitalisation across the island of Ireland.

OBJECTIVES: In 2024, two respiratory syncytial virus (RSV) immunisation strategies were introduced on the island of Ireland: nirsevimab in Republic of Ireland (ROI) and RSVpreF in Northern Ireland (NI). The primary objective was to estimate effectiveness against hospitalised RSV-associated acute lower respiratory infection (ALRI) in infants <6 months. The secondary objective was to compare impact on emergency department (ED) bronchiolitis attendances. METHODS: We conducted a multicentre, test-negative, case-control study across five sites during the 2024/25 season. Effectiveness against hospitalisation was estimated using multivariable logistic regression adjusting for age <3 months, sex, and prematurity. Infants born after 1 September 2024, hospitalised with ALRI, and tested for RSV were eligible. Time series modelling compared ED bronchiolitis attendances during the study season with five preceding seasons. RESULTS: 254 infants were included (ROI:165, NI: 89). In ROI, 34% of cases and 83% of controls received nirsevimab; in NI, 6% and 30% received RSVpreF respectively. Adjusted effectiveness against hospitalisation was 90.7% (95% CI 80.7-95.8) for nirsevimab and 83.9% (95% CI 41.5-96.6) for RSVpreF. Modelling showed significant reductions in ED bronchiolitis attendances; greater in ROI (52%) than NI (37%). CONCLUSIONS: Both interventions were highly effective against RSV-ALRI hospitalisation. However, coverage differed markedly, with important implications for population-level impact.