The effectiveness of vaccines against influenza dropped during the 2025–2026 virus season, officials said on March 12, about two months after the Centers for Disease Control and Prevention (CDC) stopped recommending flu vaccination for all children.
The shielding among adults was just 22 percent to 34 percent, she said, based on data from CDC networks in 16 states.
Grohskopf said the reasons for the decline from the prior season are not yet clear. Factors could include that fewer people received vaccines and a mismatch between strains in the vaccines and the strains that ended up circulating.
Most influenza cases in recent months have been caused by influenza A viruses, particularly an H3N2 subvariant called subclade K.
Grohskopf said the data are preliminary and could end up changing.
William Gruner, representing Department of War scientists, said at the same meeting that vaccine effectiveness among department networks against influenza-like illness from Nov. 9, 2025, through Feb. 21, 2026, was 32 percent among children and 46 percent among adults.
“Still a lot more data to be collected this season, so things can certainly change,” Gruner said.
They presented to the FDA’s Vaccines and Related Biological Products Advisory Committee during the largely virtual meeting.
Dr. Hayley Gans, a committee member, said she was concerned that the estimates were inaccurate.
“I think this data doesn’t support at least for what we see in pediatrics,” she told Grohskopf.
She also expressed a desire to see a wider population included in the CDC networks.
Gans later said to Gruner: “I just think that when people hear these rates of vaccine efficacy … we just have to be careful how that is sort of interpreted. These are largely efficacious to at least severe disease, at least in pediatrics, the ones that we see that are hospitalized largely fall in the undervaccinated group.
“There is some efficacy that we’re not capturing in all this data that we’re presenting.”
Global authorities typically release updated strain recommendations once or twice a year in a bid to improve the effectiveness of flu vaccines by trying to predict which strains will be circulating in the future.
Dr. David Kaslow, director of the FDA’s Office of Vaccines Research and Review, told committee members that the FDA appreciated their recommendation and discussion as officials try to figure out how to develop more effective seasonal influenza vaccines.
“The primary purpose of the childhood influenza vaccine in children is to reduce hospitalizations and mortality in children, as well as transmission to the elderly, who are of higher risk for death, but there are no randomized controlled trials demonstrating these benefits,” the CDC’s acting director at the time, Jim O’Neill, said in a memorandum explaining the decision.
The CDC said on its website in early March that seasonal flu activity remains elevated nationally, causing an estimated 26 million illnesses, 340,000 hospitalizations, and 21,000 deaths. It said that vaccination “has been shown to reduce the risk of flu and its potentially serious complications,” and it noted that several antiviral drugs are available for those who do contract the flu.