• Posted March 16, 2026

New Flu Strain Weakened This Year’s Vaccine Protection, CDC Says

Flu activity in the United States is finally slowing down, but health experts say this year’s flu vaccine didn't offer as much protection as officials hoped.

New data from the U.S. Centers for Disease Control and Prevention (CDC) shows that the vaccine was only about 25% to 30% effective in preventing illness serious enough to send adults to the doctor’s office or hospital.

Among kids, the vaccine reduced the risk of needing medical treatment by about 40%.

While health officials generally consider flu vaccines successful when they are 40% to 60% effective, this season’s protection ranked among the lowest in two decades.

“The winter respiratory virus season is slowly coming to a close, and we’re all very grateful for that,” Dr. William Schaffner, a vaccine expert at Vanderbilt University in Nashville, Tennessee, told The Associated Press.

Experts say the lower protection happened mostly because the virus that spread the most this winter was not the same one that scientists expected.

The dominant strain belonged to a flu type known as A H3N2. This new version, also known as subclade K, spread widely early in the winter.

The vaccine used this winter targeted a different version of the H3N2 virus.

Because of that mismatch, the shot did not protect as well as hoped, Schaffner said.

The newer strain appeared to spread more easily, although it did not seem to cause more severe illness.

Flu infections spiked late in December and hit some regions especially hard. In New York City, officials said it was the most intense flu season in 20 years.

So far this season, CDC scientists estimate there have been at least:

• 27 million illnesses

• 350,000 hospitalizations

• 22,000 deaths from flu

At the same point last year, the U.S. had 40 million illnesses and 520,000 hospitalizations, and a similar number of flu deaths.

This season, at least 101 kids have died from flu. Among cases where the vaccination status was known, about 85% were not fully vaccinated.

What's more, about 46.5% of adults received a flu shot this season, slightly more than last year.

Among kids, about 48% were vaccinated by late February. That’s about the same as last year but lower than the 52% vaccination rate in 2024.

Low vaccination rates likely made the season worse, experts said.

Even when the shot does not match the virus, Schaffner said vaccination still matters, because it can reduce the risk of severe illness.

Scientists are already planning for next flu season's vaccines.

Last month, the World Health Organization recommended that vaccines for the 2026-27 flu season include protection against the subclade K strain.

More information

Yale Medicine has more on subclade K.

SOURCE: The Associated Press, March 13, 2026