Initial data from the Southern Hemisphere suggests that this year's flu shot may be less effective than past seasons if similar viruses are dominant in the U.S., the CDC said.
The Oct. 3 study, "Morbidity and Mortality Weekly Report," used data from a research network coordinated by the Pan American Health Organization that includes Argentina, Brazile, Chile, Paraguay and Uruguay. Officials often look at Southern Hemisphere countries to get a preview of how this year's flu season might go since their winter occurs during the U.S. summer.
Here are five things to know:
- Vaccine effectiveness was 34.5% against hospitalization among high-risk groups.
- 2023's vaccine effectiveness was 51.9%.
- The drop in effectiveness could be due to fewer cases of the strain "A(H1N1)pdm09."
- In 2023, H1N1 dominated cases in South America, but this year there are more H3N2 cases, WHO said.
- If the U.S. sees similar patterns, vaccine effectiveness can be expected to be similar. However CDC officials still encourage the public to get the shot.