U.S. flu vaccines were only 25% to 30% effective in preventing adults from seeking medical care this season, the Centers for Disease Control and Prevention reported, with a new H3N2 subclade K strain largely to blame.
The low effectiveness rate, based on CDC data released Friday, reflects one of the weakest performances in recent years. Children who received the vaccine were about 40% less likely to require a doctor’s visit or hospitalization, according to the same report.
“The winter respiratory virus season is slowly coming to a close, and we’re all very grateful for that,” said Dr. William Schaffner, a vaccine expert at Vanderbilt University, noting the intense early flu activity.
The vaccine’s poor showing is attributed to a viral mismatch. The dominant strain this winter belonged to a new version of H3N2 influenza, called subclade K. The vaccine formulation, developed months earlier, was designed for a different H3N2 variant.
“The new strain seemed to spread more easily — though it did not necessarily cause more severe illness,” the CDC explained. Schaffner added that the explosion of subclade K is the likely reason for the reduced effectiveness.
Despite the low overall rate, health officials emphasize that vaccination still provides meaningful protection against severe outcomes. “The flu vaccine may not protect everyone from getting sick, but it can prevent people from becoming severely ill and dying,” Schaffner said. “That’s why getting a flu shot remains worthwhile.”
CDC estimates show at least 27 million flu illnesses, 350,000 hospitalizations, and 22,000 deaths so far this season. For children, the season has been particularly deadly: at least 101 pediatric deaths have been reported. Among cases with known vaccination status, approximately 85% involved children who were not fully vaccinated.
Adult vaccination rates have risen slightly to 46.5%, following a severe 2025 season that set a record for child flu deaths. Child vaccination rates, however, have dipped slightly to an estimated 48% by the end of last month, down from 52% at the same point in 2024.
In January, the Trump administration halted the government’s broad recommendation that all children receive annual flu shots, shifting the decision to parents and family doctors.
Looking ahead to the 2026-27 northern hemisphere flu season, the World Health Organization has already recommended that vaccines be updated to target subclade K. A U.S. Food and Drug Administration advisory committee this week endorsed the WHO’s strain selection.
The CDC noted that doctor’s office and hospital visits for flu symptoms have continued to decline over the past week, with high activity now reported in only 16 states, primarily in a belt from Colorado to Virginia.