The analysis used Brazilian national surveillance records of patients hospitalised with severe acute respiratory infection and confirmed influenza from 2023 to October 2025, taking the number of trivalent vaccine doses received within a calendar year as the exposure. Vaccine uptake among these patients fell from around 70% in 2023 to 40-50% in 2024 and 2025; one dose was associated with 10-13% lower adverse outcomes and two doses with 32% lower mortality, 52% fewer ICU admissions and a shorter ICU stay.
Why it is interesting: A severity-reducing effect of influenza vaccination in a large middle-income surveillance system, set against a background of falling uptake, though the dose-response pattern invites scrutiny of confounding by access to care.