Last year, the U.S. Centers of Disease Control and Prevention said that data from South America’s flu season 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%.
“[Vaccine effectiveness] might be similar in the Northern Hemisphere if similar A(H3N2) viruses predominate during the 2024–25 influenza season,” said the CDC.
Data cited by the CDC came from Argentina, Brazil, Chile, Paraguay, and Uruguay. In the Southern Hemisphere, flu season comes earlier and can offer a preview of what to expect in the Northern Hemisphere.
From March 13 through July 19, 11,751 cases of flu-associated severe acute respiratory illness were reported throughout the five South American countries. A little more than 21% were vaccinated against influenza, and the adjusted effectiveness against hospitalization was 34.5%.
“The adjusted VE against the predominating subtype A(H3N2) was 36.5% and against A(H1N1) was 37.1%,” the CDC added. Per the centers, this data indicates that vaccines still offered “significantly lowered risk” of hospitalization, even if it isn’t as strong as last year.
Going into flu season, the CDC said the data suggests a need for robust vaccination campaigns in the Northern Hemisphere due to the expected modest protection of vaccines against the viral illness. Data from the 2023-24 flu season in the U.S. shows that coverage decreased by 2% for children ages 6 months old to 17 years old compared to the previous year and 8.3% compared to pre-COVID-19 pandemic season, hitting a 12-season low of 55.4%. In adults, vaccination coverage was 44.9%, also down 2% compared to the prior season.
“CDC recommends that all eligible persons aged ≥6 months receive seasonal influenza vaccine,” said the centers. “Early antiviral treatment can complement vaccination to protect against severe influenza-related morbidity.”
In 2022, influenza caused nearly 6,000 deaths in the U.S., per the CDC.