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.

Related: Flu shots have changed this year — here's why

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.

Crunching the numbers, the researchers found that the vaccine likely lowered the risk of flu hospitalization by 34.5%. Breaking it down further, the shot lowered the risk of hospitalization by nearly 59% among adults with medical conditions, 39% for young children and 31% for older adults.

"While only one in five [severe acute respiratory infection] patients had received the 2024 influenza vaccine, those who were vaccinated were at significantly lower risk for hospitalization from any influenza virus infection," the study's authors concluded. These data suggest the flu shot was "effective in preventing approximately one third of influenza-related hospitalizations among groups prioritized for vaccination."

Trends in the Southern Hemisphere's flu season don't always accurately predict what will happen in the Northern Hemisphere. However, assuming the same flu virus strains circulate in the upcoming months, "health authorities might anticipate similar levels of protection from the 2024-25 vaccine," the authors wrote.

With a few exceptions, the CDC recommends that everyone 6 months and older get the annual flu vaccine. Additional measures, such as regular handwashing, can help reduce the spread of flu. For those who do catch the flu, getting antiviral treatments, such as Tamiflu, as soon as possible can significantly reduce the risk of flu-related complications.

This article is for informational purposes only and is not meant to offer medical advice.

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