South America’s 2023 Flu Vaccines Cut Risk of Flu Hospitalizations by About Half

September 8, 2023—A new study conducted by PAHO, CDC, and five South American countries found that people who had gotten a flu vaccine were half as likely to be hospitalized with flu during the 2023 Southern Hemisphere flu season. The U.S. 2023-2024 flu vaccines have a similar vaccine virus composition as the 2023 Southern Hemisphere flu vaccines. If the same viruses continue to circulate during the upcoming flu season in the United States, these preliminary data suggest U.S. 2023-2024 flu vaccines could provide similar benefit against serious flu disease.

Scientists collected preliminary flu  vaccine effectiveness (VE) data from the REVELAC-i network from five South American countries (Argentina, Brazil, Chile, Paraguay, and Uruguay) during March 27-July 9, 2023. VE was estimated by flu virus subtype and by selected populations (young children, older adults, and people with underlying medical conditions).

Flu vaccines provided significant protection against the flu viruses that were circulating in South America during the 2023 Southern Hemisphere flu season. Vaccines reduced flu hospitalizations against the predominant flu viruses, influenza A(H1N1), by 55% and against influenza B viruses by 46%. There were too few influenza A(H3N2) virus infections among people enrolled in the study to produce a VE estimate 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%.

Most flu viruses identified in the U.S. in recent weeks have been influenza A(H1N1) and influenza B viruses and the same as those identified in South America during their 2023 season. If these same viruses continue to circulate in the Northern Hemisphere, U.S. 2023-2024 flu vaccines have the potential to provide similar protection during the upcoming U.S. flu season since the composition of U.S. 2023-2024 influenza vaccines is similar to the 2023 Southern Hemisphere vaccines. It is important to note; however, that flu activity is unpredictable and what happens in the Southern Hemisphere is not always predictive of what will happen in the Northern Hemisphere.

This study underscores the protective benefits provided by flu vaccination and provides insight into what we might expect in the Northern Hemisphere this upcoming flu season. As we approach the 2023-2024 flu season in the United States, it is important to remember that CDC recommends everyone 6 months and older in the United States get a yearly flu vaccine with rare exception. September and October are generally good times for most people to be vaccinated against flu.

This study was a collaboration between CDC and the WHO Pan American Health Organization.