Flu activity across the United States is beginning to slow, but health officials say this season’s influenza vaccine offered lower-than-expected protection due to the emergence of a mismatched viral strain.

According to data from the Centers for Disease Control and Prevention (CDC), the vaccine was only about 25% to 30% effective in preventing serious illness among adults—cases that required a doctor’s visit or hospitalization. Among children, the vaccine performed somewhat better, reducing the risk of needing medical care by roughly 40%. Public health experts generally consider flu vaccines successful when they achieve effectiveness between 40% and 60%, placing this season among the weakest in nearly two decades.

Specialists say the reduced protection was largely due to a mismatch between the vaccine and the dominant strain circulating this winter. While the vaccine targeted a predicted version of the influenza A H3N2 virus, a different variant—known as subclade K—spread widely early in the season. This mismatch limited how well the vaccine could prevent infections, although it still helped reduce the severity of illness.

Dr. William Schaffner, a vaccine expert, noted that while the respiratory virus season is now easing, the unexpected strain played a key role in lowering vaccine performance. The newer variant appeared to spread more easily, contributing to a surge in infections in late December. Some areas were hit particularly hard, with New York City experiencing its most intense flu season in 20 years.

Overall, the CDC estimates that the current flu season has led to around 27 million illnesses, 350,000 hospitalizations, and 22,000 deaths across the United States. While last year saw higher numbers of infections and hospitalizations, the number of deaths has remained similar.

Children were significantly affected this season, with at least 101 pediatric deaths reported. In cases where vaccination status was known, the vast majority of those children had not been fully vaccinated. Vaccination rates overall remained relatively modest, with about 46.5% of adults receiving the flu shot and roughly 48% of children vaccinated by late February. Experts believe these lower vaccination rates likely contributed to the severity of the season.

Health officials emphasize that even in years when the vaccine is not a close match to circulating strains, it still provides important protection by reducing the risk of severe illness and complications.

Looking ahead, global health authorities are already preparing for the next flu season. The World Health Organization has recommended that the 2026–27 flu vaccine include protection against the subclade K strain in an effort to improve effectiveness if similar variants continue to circulate.

Experts continue to stress that annual flu vaccination remains the best available defense against influenza, even when its effectiveness varies from year to year.