Impact and effectiveness of RSV maternal immunization on infant hospitalizations in Buenos Aires: a hospital-based, multicentre, retrospective surveillance cohort study

Respiratory syncytial virus (RSV) is a major cause of hospitalizations and mortality in young infants worldwide. The RSVpreF maternal immunization (MI) was recently introduced in Argentina.

This study assessed the impact of RSVpreF MI on RSV-related acute lower respiratory tract infections (ALRTI) hospitalizations through a hospital-based, multicentre, retrospective surveillance cohort study, and measured vaccine effectiveness (VE) using a nested test-negative case–control study. Data of hospitalized infants under 18 months of age was collected and analysed within seven years from three Argentine tertiary hospitals. VE analysis included ALRTI-hospitalized infants who were born between March 1 and November 9, 2024, were under 6 months of age when tested for RSV, and whose mothers were eligible for prenatal RSV immunization. Expected RSV-ALRTI hospitalizations were compared with observed cases using a Poisson model. We estimated the VE of RSVpreF MI against RSV-ALRTI hospitalizations, paediatric intensive care unit (PICU) admissions, and extended hospital stays by comparing these rates in vaccinated and unvaccinated under 3 and 6 months.

A total of 3373 participants were included in the impact analysis, fromof whom 323 were born during the vaccination period and were eligible for the VE analysis. The VE of RSVpreF MI was 80·8% (95% CI: 62·8–90·5%), and 66·1% (95% CI 30·1–83·8) for infants under 3 and 6 months, respectively, adjusted for age, sex, comorbidities, and epidemiological weeks. VE for PICU admission was 87·2% (95% CI 52·6–97·0) and 88·6% (95% CI 62·3–97·1) for extended hospital stays in infants under 6 months. The vaccine reduced RSV-ALRTI hospitalizations in infants under 6 months by 33·6% (95% CI 29·5–37·2) in 2024 compared to expected cases from previous years. The number needed to immunize to prevent one RSV-related hospitalization was 83·9 (95% CI 65·9–185·4).

RSVpreF MI significantly reduced RSV-ALRTI hospitalizations, averting one-third of such hospitalizations in infants under 6 months. These findings provide valuable evidence for policymakers and health authorities.

Gates Foundation and Thrasher Research Fund.