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

OBJECTIVES

In 2024, two respiratory syncytial virus (RSV) immunization 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 hospitalized 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 hospitalization was estimated using multivariable logistic regression adjusting for age <3 months, sex, and prematurity. Infants born after 1 September 2024, hospitalized with ALRI, and tested for RSV were eligible. Time series modeling 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 hospitalization was 90.7% (95% CI: 80.7-95.8) for nirsevimab and 83.9% (95% CI: 41.5-96.6) for RSVpreF. Modeling showed significant reductions in ED bronchiolitis attendances; greater in ROI (52%) than NI (37%).

CONCLUSIONS

Both interventions were highly effective against RSV-ALRI hospitalization. However, coverage differed markedly, with important implications for population-level impact.