Protective effect of influenza vaccine in Brazilian patients hospitalised for severe acute respiratory infection, 2023-2025
TL;DR
Two doses of influenza vaccine were moderately effective in decreasing in-hospital mortality and stay among SARI-hospitalised influenza patients, and including 50-59-year-olds in a priority vaccination group may significantly reduce influenza-related adverse outcomes.
Abstract
Introduction: Despite a significant increase in influenza burden in Brazil in 2025, the associated vaccine effectiveness has not included the key end-of-winter period. Methods: The number of trivalent influenza vaccine doses received within a calendar year was the key exposure. 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.