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Data from the southern hemisphere show that this year's flu shot was against hospitalization in high-risk groups during that half of the globe's winter respiratory season.

It's not possible to say exactly how that will translate to the northern hemisphere's upcoming flu season, but the figure is in the ballpark of recent vaccine effectiveness for this half of the world.

The CDC conducts studies each flu season to determine how well vaccines are working, and on its website -- and it's clear that the 34.5% effectiveness rate is comparable to recent figures:

• 2021-2022: 36%
• 2022-2023: 30%
• 2023-2024: 42%

Overall, however, influenza vaccine effectiveness seems to be on the decline from earlier years, peaking at 60% effectiveness in 2010-2011, followed by 56% in 2009-2010 and 52% in 2013-2014.

Following an abysmal 19% effectiveness rate in 2014-2015, effectiveness remained in the 40% range for another 2 years, but fell into the 30% range thereafter and never pushed out of it until last winter season. (Vaccine effectiveness for 2020-2021 was never estimated because of low viral circulation due to COVID pandemic precautions.)

Kawsar Talaat, MD, of Johns Hopkins University in Baltimore, that the 34.5% effectiveness from the southern hemisphere is "a little bit disappointing," adding that physicians would like it to be closer to 50%.

Indeed, if the trend tracks last year, the U.S. could be looking at a far less successful vaccine effectiveness this winter season. For winter 2023 in the southern hemisphere, vaccine effectiveness was about 52% against hospitalization, according to CDC data. However, last year's U.S. winter respiratory season saw an influenza vaccine effectiveness that was 10 percentage points lower.

This year's southern hemisphere data came from more than 2,500 hospitals in five countries -- Argentina, Brazil, Chile, Paraguay, and Uruguay -- and was reported earlier this month in CDC's . Most of those countries used trivalent vaccines, but Paraguay used the quadrivalent shot that also targeted the B/Yamagata lineage.

When analyzed by influenza subtype, effectiveness against the predominant circulating strain A(H3N2) was 36.5%, while against A(H1N1)pdm09 it was 37.1%.

The paper also found that vaccine effectiveness was 58.7% among people with comorbidities, 39% among young children, and 31.2% among older adults.

The authors concluded that the findings suggest "influenza vaccines are effective in preventing approximately one third of influenza-related hospitalizations among groups prioritized for vaccination" and that they "support CDC and [World Health Organization's] recommendation that all eligible persons aged ≥6 months should receive influenza vaccination."

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