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%.

In 2024, the Center for Disease Control and Prevention has noted 17 cases of avian influenza in humans.Avian influenza is typically found in dairy cows, poultry and other animals.The CDC recommends everyone six months or older get their flu vaccine.

Although the effectiveness of the vaccine in preventing hospitalizations appears to be lower in South America, the agency said vaccines still “significantly lowered the risk for hospitalization."

This year's flu vaccine significantly reduced the risk of hospitalization in the Southern Hemisphere but was not as effective as the vaccine used in the prior season, according to an early study done by the US Centers for Disease Control and Prevention.

This flu season, the United States is using trivalent (three-strain) vaccines with the same components as those in Southern Hemisphere vaccines, Zeno said.

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.