U.S. Flu Vaccine Effectiveness Hits Decade Low Due to New Strain

As the U.S. flu season winds down, health officials say the flu vaccine didn’t work very well, with one of its worst effectiveness rates in more than a decade. A new strain that dominated the early winter was not well matched to the vaccine, leading to an intense early onslaught of flu. The Centers for Disease Control and Prevention on Friday posted data that showed a continued decline in doctor’s office and hospital visits for flu symptoms through last week. The number of states reporting high flu activity dropped to 16, many of them in a belt stretching from Colorado to Virginia. “The winter respiratory virus season is slowly coming to a close, and we’re all very grateful for that,” said Dr. William Schaffner, a Vanderbilt University vaccine expert. This season’s vaccines were around 25% to 30% effective in preventing adults from getting sick enough from the flu that they had to go to a doctor’s office, clinic or hospital, according to a CDC report this week. Children who were vaccinated were about 40% less likely to get treatment at a doctor’s office or hospital. Officials generally are pleased if a flu vaccine is 40% to 60% effective. Judging from past CDC research, this season saw one of the lowest effectiveness rates in the last 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.