DR. MONIQUE YOHANAN: Good News For This Season’s Flu Vaccine

The 2025 flu season has begun, and the new flu vaccine was released in the United States in September. Influenza viruses are unique in their ability to mutate or change, so every year the flu vaccine needs to be updated in response. This year the early news is good: the vaccine appears to be an unusually close match to this year’s flu strains.

This optimistic take is drawn from results in Southern Hemisphere countries that experience winter months before the U.S. and give us an early signal about what to expect. This year, the match between the flu strains that were the basis for the vaccine and the flu viruses that actually circulated was ~98%. That’s about as close a match as we can get.

A strong match is one of many components that determine how well the flu vaccine will work, and unfortunately there are many real-world factors that work against it. In a well-matched year, overall vaccine effectiveness usually ranges from 50% to 60% for preventing medical visits and hospitalizations, and this is what we are seeing so far.

Because it’s so early in the season, it’s helpful to look back to past U.S. flu seasons that began with similar conditions to gauge what to expect. The 2022-23 and 2023-24 seasons had a similar match, and a vaccine effectiveness rate of roughly 50%. In contrast, the 2017-18 season had much lower vaccine effectiveness  – about 10% – and clinical outcomes were devastating. Deaths and hospitalizations reached 50,000 and 1 million, respectively, nearly double the rates seen in years with a good match.

Even in a year with a great match, vaccines don’t completely prevent flu, but they do blunt its severity. The evidence so far suggests that with good uptake of this year’s flu shot we should expect fewer hospitalizations  and fewer deaths – especially among those at highest risk – older adults, infants and children younger than 5 years of age, pregnant women, and people with chronic illnesses. That could be tens of thousands of lives saved thanks to this vaccine.

A question likely to surface is whether the vaccines offered this year contain thimerosal, a mercury-based additive used in some forms of the flu shot. In July 2025, Health and Human Services Secretary Robert F. Kennedy, Jr. announced that beginning with the 2026-27 flu season that all vaccines were expected to be thimerosal-free.