MMWR Report on South America’s Flu Vaccines in Reducing Flu Hospitalizations

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September 8, 2023

Influenza (Flu)

South America’s Flu Vaccine Success Story Could Mean Similar Benefits for U.S. Flu Season

Evaluations of flu vaccine effectiveness during the April–September Southern Hemisphere flu season can provide valuable information for countries preparing for the October–May Northern Hemisphere flu season. Investigators found that people from five South American countries who had gotten flu vaccines were half as likely to be hospitalized with flu during the 2023 Southern Hemisphere flu season compared to people who did not get vaccines. These preliminary data suggest that if the same viruses continue to circulate during the upcoming flu season in the United States U.S. 2023-2024 flu vaccines could provide similar benefits against serious flu disease. 

Highlights for CDC’s Southern Hemisphere Flu Vaccine Study
  • Scientists collected preliminary flu vaccine effectiveness (VE) data from the REVELAC-i network from Argentina, Brazil, Chile, Paraguay, and Uruguay from March 27-July 9, 2023. 
  • The majority of circulating flu viruses during the 2023 season in the South American countries in the study were flu A (91%) viruses, 99% of which were A(H1N1)pdm09 viruses. 
  • Flu vaccines provided significant protection against flu-related hospitalization associated with circulating flu A(H1N1) and B/Victoria viruses. 
  • VE was 55% against flu-related hospitalization caused by A(H1N1)pdm09 virus.
  • VE was 46% against flu-related hospitalization caused by influenza B viruses. 
  • There were not enough flu A(H3N2) virus infections among people enrolled in the study to assess VE against that subtype.   
  • Flu vaccines also provided significant protection to groups that are at higher risk of severe flu illness. Flu vaccination reduced the risk of flu hospitalization among young children by 70% and among older adults by 38%.
  • Only 40% of people in the study had received a flu vaccine during the 2023 season.
  • In recent weeks, most flu viruses identified in the United States have been A(H1N1) and B/Victoria viruses. This is similar to what was seen among the Southern Hemisphere countries in this study. The composition of the U.S. 2023-2024 flu vaccine is similar to that of flu vaccines used in the Southern Hemisphere in 2023.
  • CDC recommends that everyone 6 months and older get an annual flu vaccine. September and October are generally good times to get a flu vaccine.

Twitter @CDCFlu

CDC recommends a yearly #FluVax as the first and most important action in protecting against flu. Everyone 6 months and older should be vaccinated annually, ideally by the end of October. Find vaccines near you:

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