Residual burden of severe RSV disease in infants during the first season of maternal RSVpreF immunization in Argentina: A hospital-based, multicentre, retrospective cohort study

Background Argentina pioneered the implementation of maternal immunization (MI) with RSVpreF to prevent RSV hospitalizations in infants. This study aims to assess the residual burden of RSV-related hospitalizations in infants under 12 months during Argentina's first season of MI. Methods This multicentre, retrospective cohort study utilized the RIMA network across five tertiary hospitals in the Metropolitan Area of Buenos Aires. We analysed RSV-related hospitalizations in infants <12 months old during the first post-MI season (2024) and compared them with historical data (2018–2023). The primary objective was to characterize the “protection gap” among hospitalized infants. Findings Overall, 10,860 infants aged <12 months were hospitalized with ALRTI during the study; 3967 (36.29%) were RSV-positive. During the first vaccination season, 91.73% (477/520) of RSV-hospitalized infants were born to unvaccinated mothers. The primary protection gap was temporal: 43.18% (133/308) of infants under 6 months hospitalized during 2024 were born before the vaccination campaign started. Severe RSV disease remained prevalent, with 131/148 infants (88.52%) requiring PICU admission. Interpretations While maternal immunization is a landmark intervention, a substantial residual burden of severe RSV remains in Argentina. This burden is driven by infants born outside the vaccination window and high-risk preterm cohorts. To achieve comprehensive protection, coordinated strategies, including the integration of long-acting monoclonal antibodies alongside maternal vaccines, are necessary to close existing equity and protection gaps. Funding This work was supported by SANOFI and AstraZeneca (RSV00105).