Effectiveness of Nirsevimab Immunoprophylaxis Against Respiratory Syncytial Virus-related Outcomes in Hospital Care Settings: A Seasonal Cohort Study of Infants in Catalonia, Spain.

BACKGROUND

In Catalonia, infants <6 months old were eligible to receive nirsevimab, a novel monoclonal antibody against respiratory syncytial virus (RSV). We aimed to analyze nirsevimab's effectiveness in hospital-related outcomes of the seasonal cohort (born during the RSV epidemic from October to January 2024) and compared them with the catch-up cohort (born from April to September 2023).

METHODS

Retrospective cohort study of all infants born between October 1, 2023, and January 21, 2024, according to their immunization with nirsevimab (immunized and nonimmunized). We followed individuals until the earliest of an outcome-hospital emergency visits, hospital admission or pediatric intensive care unit (PICU) admission due to RSV-associated or all-causes bronchiolitis-death or the end of the study. We used the Kaplan-Meier estimator and fitted Cox regression models using a calendar time scale to estimate hazard ratios (HRs) and their 95% confidence interval (CI). Sensitivity analysis was performed through matching.

RESULTS

Among 15,341 infants, a dose of nirsevimab led to an adjusted HR for hospital admission, PICU admission and emergency visits due to RSV bronchiolitis of 0.26 (95% CI: 0.17-0.39), 0.15 (95% CI: 0.07-0.28) and 0.46 (95% CI: 0.23-0.90), respectively. For all-causes bronchiolitis, the former adjusted HRs were 0.45 (95% CI: 0.31-0.63), 0.23 (95% CI: 0.13-0.41) and 0.49 (95% CI: 0.35-0.68), respectively.

CONCLUSIONS

Nirsevimab was associated with reductions of 74% and 85% hospitalizations and PICU admissions regarding RSV-associated bronchiolitis, respectively. These percentages are slightly lower than those for the catch-up cohort. This information may help the implementation of RSV-immunization campaigns by public health authorities.