Flu shot lowers hospitalization risk by 35% in vulnerable groups, data hint

This year's flu shot lowers the chance of being hospitalized for flu by 35% among high-risk groups, data released by the Centers for Disease Control and Prevention (CDC) suggests.

The new data, published Oct. 3 in the CDC's Morbidity and Mortality Weekly Report (MMWR), was pulled from five South American countries.

The Southern Hemisphere's flu season typically runs between April and September, while the Northern Hemisphere's runs from about October to May. Because of this alternating timing, the Northern Hemisphere looks to Southern Hemisphere data to see which strains of flu virus are circulating, to plan the design of flu shots for the upcoming season, and to determine how well those vaccines will likely work.

In the new MMWR, researchers looked at the flu shot's effectiveness in Argentina, Brazil, Chile, Paraguay and Uruguay. The report focuses on people at high risk of severe flu infections, including young children, older adults and people with conditions such as chronic lung disease, liver disorders, heart disease or immune-system problems.

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Between mid-March and mid-July, these five countries reported that more than 11,700 people in these groups were hospitalized with severe respiratory illnesses. Of these, about 3,850 tested positive for the flu, with the vast majority testing positive for an iNFLuenza A virus. (INFLuenza A and iNFLuenza B are the two broad groups of flu viruses that circulate seasonally, and in the study, two subtypes of iNFLuenza A stood out as dominant: H3N2, followed by H1N1.)

The remaining 7,850 or so people in the study tested negative for both flu and COVID-19, and they acted as a point of comparison for the analysis. About 23% of these patients had been vaccinated for the flu, compared with only 18% of the group that ended up hospitalized for the viral infection.