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Background: To better establish the value of vaccination against influenza viruses, we estimated vaccine-averted influenza illnesses among young children and older adults in Chile, Guyana, and Paraguay., Methods: We gathered country- and target population-specific data on monthly influenza hospitalizations, vaccine coverage, and vaccine effectiveness from surveillance records and immunization registries during 2013-2018. We applied a static compartmental model to estimate differences in the number influenza-associated respiratory disease events (symptomatic non-hospitalized illnesses, medically attended illnesses, hospitalizations) in the presence and absence of influenza vaccination programs., Results: Between 2013 and 2018, vaccinating 68% of children aged 6-23 months in Chile averted an annual mean of 14,617 non-hospitalized, 9,426 medically attended, and 328 hospitalized influenza illnesses; vaccinating 28% of children aged 6-23 months in Paraguay averted 1,115 non-hospitalized, 719 medically attended, and 25 hospitalized influenza illnesses. Vaccinating 59% of older adults in Chile averted an annual mean of 83,429 non-hospitalized, 37,079 medically attended, and 1,390 hospitalized influenza illnesses; vaccinating 36% of older adults in Paraguay averted an annual mean of 3,932 non-hospitalized, 1,748 medically attended, and 66 hospitalized influenza illnesses. In Guyana, a hypothetical campaign vaccinating 30% of children <5 years could have prevented an annual 1,496 non-hospitalized, 971 medically attended, and 10 hospitalized influenza illnesses. Vaccinating 30% of adults ≥65 years could have prevented 568 non-hospitalized, 257 medically attended, and 10 hospitalized influenza illnesses., Conclusions: Influenza vaccination averted tens of thousands of illnesses and thousands of hospitalizations in Chile and Paraguay; influenza vaccination could have had a proportional benefit in Guyana., (© The Author(s) 2025. Published by Oxford University Press on behalf of Infectious Diseases Society of America.)

Background: We estimate annual viral influenza-associated mild-to-moderate illness, hospitalizations, and deaths in six South American countries (Argentina, Brazil, Chile, Ecuador, Paraguay, and Uruguay) during the 2015-2019 influenza seasons as a first step in evaluating the full value of influenza vaccination in the subregion., Methods: We applied a multiplier methodology using monthly hospital discharge and vital statistics death records, influenza surveillance data, and population projections to estimate mild-to-moderate influenza-associated illness, hospitalizations, and deaths. We estimated the uncertainty bounds based on the 2.5th and 97.5th of the Monte Carlo simulated distributions for the number of cases and obtained the ranges from the minimum value of the 2.5th percentile and the maximum value of the 97.5th percentile., Results: In selected countries with a total population of 307 million people, the yearly influenza-associated burden of disease ranged between 51 and 78 million mild-to-moderate influenza illnesses, between 323,379 and 490,049 hospitalizations, and between 22,662 and 46,971 deaths during the 2015-2019 influenza seasons., Conclusions: Each year, influenza is associated with millions of illnesses, hundreds of thousands of hospitalizations and tens of thousands of deaths in six South American countries, affecting a significant portion of the population. Such findings can be used to estimate the number of illnesses averted through vaccination programs and the cost-benefit of influenza vaccines., (© The Author(s) 2025. Published by Oxford University Press on behalf of Infectious Diseases Society of America.)