The 2018-2019 influenza season is one such case. The vaccine’s effectiveness during that year was an abysmal 29%, according to the CDC. But a 2020 study found the shot still prevented an estimated 4.4 million infections, 58,000 hospitalizations and 3,500 deaths in the U.S.

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.

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.

The CDC says the influenza vaccine being used this flu season might be less effective than the one used last flu season, based on reports of how well the vaccine is working in five South American countries.

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%.