Southern Hemisphere Flu Season Foreshadows Vaccine Effectiveness

According to the U.S. CDC's Morbidity and Mortality Weekly Report (MMWR), only 21.3% of patients hospitalized for flu during the Southern Hemisphere flu season had received an influenza vaccine in Argentina, Brazil, Chile, Paraguay, and Uruguay.

Published on October 3, 2024, this MMWR reported that the effectiveness of the seasonal flu vaccine was about 35%, which is a decrease from the 50% reported in 2023.

Among 3,848 influenza case patients, 18.3% had received a 2024 seasonal influenza vaccine.

The CDC wrote flu shot effectiveness might be similar in the Northern Hemisphere, including the United States, if similar A(H3N2) viruses predominate during the 2024–25 influenza season.

The CDC expects the upcoming fall and winter respiratory disease seasons to have a similar or lower number of combined peak hospitalizations due to COVID-19, influenza, and RSV than last season.

To protect yourself, the CDC recommends that all eligible persons receive seasonal influenza and RSV vaccines, generally available at clinics and pharmacies in the U.S. 

Furthermore, to enhance this year’s modest influenza vaccine protection against hospitalization, providers should treat patients with suspected or confirmed influenza with antivirals as soon as possible.