As the flu season in the United States draws to a close, health officials are reflecting on an underwhelming performance by this year’s flu vaccine, marking one of its least effective years in more than a decade. A predominant strain that emerged during the early winter months didn’t align well with the vaccine, resulting in a vigorous and early surge of flu cases.

The Centers for Disease Control and Prevention (CDC) recently released data indicating a steady decline in visits to doctors’ offices and hospitals for flu-like symptoms. The number of states reporting high flu activity has reduced to 16, primarily forming a band from Colorado to Virginia.

Dr. William Schaffner, a vaccine expert from Vanderbilt University, remarked, “The winter respiratory virus season is slowly coming to a close, and we’re all very grateful for that.”

According to a report from the CDC this week, the flu vaccines administered this season were only about 25% to 30% effective in preventing adults from reaching a level of illness that required medical attention. For children, the vaccine offered slightly better protection, reducing the likelihood of needing treatment at a doctor’s office or hospital by approximately 40%.

Typically, health officials consider a flu vaccine successful if its effectiveness ranges between 40% and 60%. In light of past CDC research, this season’s performance ranks among the lowest in effectiveness over the past two decades.

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.

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