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.
Vaccine effectiveness for late 2025 and early 2026 against outpatient visits and hospitalization was pegged at 14 percent to 48 percent among children, Dr. Lisa Grohskopf, with the CDC’s Influenza Division, said at a meeting hosted by the Food and Drug Administration (FDA).
The shielding among adults was just 22 percent to 34 percent, she said, based on data from CDC networks in 16 states.
Influenza vaccine effectiveness since 2009 has dropped as low as 19 percent and risen as high as 60 percent. It was 56 percent in late 2024 and early 2025, according to the CDC.
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.
Hellbound and Down 