Effectiveness of catch-up and at-birth nirsevimab immunisation against RSV hospital admission in the first year of life: a population-based case-control study, Spain, 2023/24 season.

Summary

Nationwide matched case-control analyses showed nirsevimab reduced RSV hospitalizations by 71–80% (catch-up) and 78–83% (at-birth), with similar protection against ICU admission and mechanical ventilation. Effectiveness was modestly lower in preterm and low-birthweight infants but remained substantial.

Key Findings

• Catch-up nirsevimab effectiveness: ITT 71% (95% CI 65–76), PP 80% (95% CI 75–84).
• At-birth nirsevimab effectiveness: ITT 78% (95% CI 73–82), PP 83% (95% CI 79–87).
• Protection extended to ICU admission and mechanical ventilation; effectiveness slightly lower (≈60–70%) in preterm/low-birthweight infants.

Clinical Implications

Health systems can expect substantial reductions in RSV hospitalizations and critical care demand with broad nirsevimab coverage. Prioritize outreach to preterm/low-birthweight infants given slightly reduced effectiveness.

Limitations

• Observational design susceptible to residual confounding and misclassification.
• Evaluation limited to a single RSV season; durability and inter-season variability not assessed.

Future Directions

Assess multi-season durability, cost-effectiveness, and implementation strategies; evaluate effectiveness in high-risk subgroups and interaction with maternal RSV vaccination programs.