Last year, the CDC's report had estimated vaccine effectiveness in South America was 51.9% against hospitalization among at-risk groups. A study by the same group looking at data from 2013 to 2017 estimated effectiveness was around 43% for fully vaccinated young children and 41% for older adults.

Last year, the U.S. Centers of Disease Control and Prevention said that data indicated vaccines would reduce the risk of hospitalization by 50%. This year’s data indicates the 2025-25 season vaccine will reduce the risk by just 35%.

Early estimates on the effectiveness of this year’s flu vaccines indicate they may be less effective at preventing hospitalization than in years past, based on data from South America, where the annual flu season occurs earlier.