Flu vaccine lowered risk of hospitalization in Southern Hemisphere by 35%: CDC study

This year's flu vaccine significantly reduced the risk of hospitalization in the Southern Hemisphere but wasn't as effective as the vaccine used in the prior season, according to an early study from the Centers for Disease Control and Prevention published Thursday.

Countries in the Southern Hemisphere experience their flu season before countries in the Northern Hemisphere. This often provides a glimpse as to what the upcoming flu season may potentially look like for the Northern Hemisphere, though it is not fully predictive of what may occur in each individual country, health officials said.

Researchers found the seasonal flu vaccine reduced the risk of hospitalization from flu by 35% across five Southern Hemisphere countries: Argentina, Brazil, Chile, Paraguay and Uruguay. The shot was nearly 59% effective at reducing hospitalization among those with underlying medical conditions.

"Northern Hemisphere countries may experience similar levels of protection if similar influenza viruses predominate during the upcoming 2024-2025 flu season," the authors wrote.

While the vaccine did offer protection, it was lower than the 2023 flu vaccine, which cut flu-related hospitalizations in the Southern Hemisphere by half, according to the CDC.

Each year, scientists update the flu vaccine based on their best estimate about what formula will be a good match against a constantly evolving virus.

In seasons where the flu vaccine matches circulating strains, the flu shot has historically been shown to reduce the risk of going to the doctor by 40% to 60% in seasons, according to the CDC.

Recent CDC estimates indicate the upcoming respiratory disease season will likely have a similar or lower number of combined peak hospitalizations due to COVID-19, influenza and RSV compared to last season.

During the 2023-2024 season in the U.S., there were between 380,000 and 900,000 flu hospitalizations. Estimates from the CDC indicate flu vaccination prevented 105,000 to 240,000 hospitalizations.

Everyone over the age of 6 months is recommended to get a flu shot each season - ideally before the end of October, according to the CDC.

Some children, including those under 8 years old receiving their first flu shot, may need two doses four weeks apart. Older adults over age 65 may opt to get a higher dose version.

The CDC study said that although South American countries prioritized vaccinating young children, people with comorbidities, and older adults, influenza vaccination coverage levels were below pre-COVID-19 norms.

"This finding is consistent with post pandemic declines in vaccination coverage across the America during the COVID-19 pandemic," the authors wrote. "Vaccination remains one of the most effective measures to prevent influenza-associated complications, including death."