As the flu season in the United States draws to a close, health officials have reported that this year’s flu vaccine demonstrated one of its lowest effectiveness rates in over ten years.

A newly dominant strain of the flu, which appeared early in the winter, was not well aligned with the vaccine, leading to a particularly severe start to the season.

Recent data released by the Centers for Disease Control and Prevention (CDC) on Friday indicates a continued decrease in flu-related doctor and hospital visits up to last week. The number of states experiencing high flu activity has reduced to 16, particularly in a region extending from Colorado to Virginia.

“We’re all very thankful that the winter respiratory virus season is gradually wrapping up,” stated Dr. William Schaffner, a vaccine specialist from Vanderbilt University.

According to a CDC report released this week, the flu vaccines this season were only 25% to 30% effective in preventing adults from becoming sick enough to require medical attention. For children who were vaccinated, the likelihood of needing treatment at a medical facility was reduced by about 40%.

Typically, officials are satisfied if a flu vaccine achieves an effectiveness rate of 40% to 60%. Compared to previous CDC research, this season’s vaccine performance ranks among the lowest in the past two decades.

Flu infections surged in late December and were especially intense in some parts of the country. New York City health officials called it the most intense season in 20 years.

Relatively low flu vaccination rates did not help, but experts also blamed the new flu strain that was causing most infections.

The new strain belonged to a category of flu virus, called A H3N2. This new version, subclade K, seemed to spread more easily – though it did not necessarily cause more severe illness.

The vaccine available for this season was built to address a different version of H3N2, and the new strain’s explosion is a likely explanation for why the vaccine was less effective, Schaffner said.

CDC scientists estimate there have been at least 27 million illnesses, 350,000 hospitalizations and 22,000 deaths from flu so far this season. At the same point last year, the estimates were at least 40 million illnesses, 520,000 hospitalizations, but about the same number of deaths.

At least 101 children have died so far this season. For those whose vaccination status is known, about 85% were not fully vaccinated against flu.

The flu vaccine may not protect everyone from getting sick, but it can prevent people from becoming severely ill and dying. That’s why getting a flu shot remains worthwhile, Schaffner said.

CDC data suggests adult vaccination rates are up slightly this season, to 46.5%, following an unusually bad season last year that set a record for the most child deaths this century.

An estimated 48% of U.S. kids were vaccinated against flu around the end of last month. That’s about the same as last year, but down from the 52% vaccinated at this point in 2024, according to CDC data.

Starting in 2010, the government recommended annual flu vaccinations for Americans 6 months and older. In January, however, the Trump administration stopped broadly recommending flu shots for all children, saying instead that it’s up to parents and family doctors to decide.

Meanwhile, work is already underway for next winter’s flu season. Last month, the World Health Organization announced its recommendations for which virus strains to address in the vaccines for the 2026-27 northern hemisphere flu season. The vaccines should be built to handle subclade K, the organization said. This week, a U.S. Food and Drug Administration advisory committee endorsed the WHO recommendations.