Vaccine effectiveness was 34.5% against hospitalization, according to interim estimates from a new article published by the CDC's Morbidity and Mortality Weekly Report, among high-risk groups like young children, people with preexisting conditions and older adults. That means, vaccinated people in those groups were 34.5% less likely than unvaccinated people to get sick enough to go to the hospital. (CBC)  

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.

These data come from a research network coordinated by the Pan American Health Organization, including Argentina, Brazil, Chile, Paraguay and Uruguay. 

The biggest drop in effectiveness this year may be the result of fewer cases from "A(H1N1)pdm09," a strain that has spread since the H1N1 swine flu pandemic in 2009.

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