Protective effect of influenza vaccine in Brazilian patients hospitalised for severe acute respiratory infection, 2023-2025
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Introduction
Despite a significant increase in influenza burden in Brazil in 2025, influenza vaccine effectiveness has not included the key end-of-winter period so far. Methods: The key exposure measure was the number of trivalent influenza vaccine doses received within a calendar year. In-hospital mortality and length of stay, use of the intensive care unit (ICU), and ICU stay were outcome measures. Secondary data from January 2003 to October 2025 were retrieved and analysed by Poisson regression.
Results
Influenza vaccine uptake among SARI-hospitalised patients dropped from about 70% in 2023 to a 40-50% level in 2024 and 2025. A single vaccine dose reduced outcomes by 10-13% compared to unvaccinated patients. Two doses per year significantly reduced mortality (32%), ICU admissions (52%), and ICU stay (28%).
Conclusion
Two doses of influenza vaccine were moderately effective in decreasing in-hospital mortality and stay among SARI-hospitalised influenza patients. Including 50-59-year-olds in a priority vaccination group may significantly reduce influenza-related adverse outcomes.
