Oct. 4, 2024 – 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.

The seasonal flu vaccine cut the risk of hospitalization for high-risk groups by 35% in Argentina, Brazil, Chile, Paraguay, and Uruguay, the CDC said. Last flu season, the vaccine had a 51.9% effectiveness in those countries.

Vaccine effectiveness “might be similar in the Northern Hemisphere” if similar flu viruses circulate, according to the federal health agency.

How well the vaccine works in South America, where the flu season runs from April to September, provides an idea of how well the vaccine will work in North America, where the flu season usually runs from October to April.

During the past flu season in North America, the CDC estimated the vaccines were 41% to 44% effective in preventing flu-related hospitalization in adults and 52% to 61% effective for children.

This year’s flu vaccines for the United States, all trivalent (protecting against three viruses), will target the three strains expected to circulate – H1N1, H3N2, and influenza B (Victoria). Health officials say that everyone 6 months and older should get a flu vaccination.

An estimated 75 million people in the U.S. got the flu between October 2023 and April 2024, according to the CDC, resulting in 900,000 hospitalizations and between 17,000 and 100,000 deaths.

The data about the South American nations came from a research network coordinated by the Pan American Health Organization.