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).