Influenza vaccine effectiveness in preventing hospitalizations among children: a retrospective population-based cohort study in Ribeirão Preto, São Paulo State, Brazil

Abstract

Background: Influenza is a major public health concern. According to the Ribeirão Preto Epidemiological Observatory, 1,384 cases of severe acute respiratory infection (SARI) were reported in 2025, of which 792 had an identified etiology, including 387 cases attributed to influenza. Objective: To assess the effectiveness of the influenza vaccination campaign in preventing influenza-related hospitalizations among children. Methods: A retrospective cohort study was conducted among children aged 6–59 months residing in Ribeirão Preto, Brazil. Data from the Influenza Epidemiological Surveillance Information System (SIVEP-Gripe) were used to identify the diseased group (D), which was subsequently classified into vaccinated (V) and unvaccinated (NV) subgroups, based on data interoperability between SIVEP-Gripe and the National Immunization Program Information System (SI-PNI). Using SI-PNI data on administered vaccine doses and the target population, the non-diseased group (ND) was reconstructed and similarly classified into V and NV subgroups. Results: In this cohort, influenza-related SARI cases decreased by 67% among V-children and increased by 341% among ND-children (p<0.001; qui square test). The odds ratio (OR) and relative risk (RR) were approximately 0.076 (OR≈RR), resulting in an estimated vaccine effectiveness (VE) of 92.4% (95% CI: 87.4-95.4). Conclusion: These findings may provide evidence-based information to strengthen childhood vaccination policies and tackle vaccine hesitancy.