Flu is surging across most of the U.S. But, amid an early, harsh season, there is a glimmer of promising news: The flu shot may be more effective than experts predicted.
Early evidence shows that this season’s vaccine can indeed protect against the H3N2 subclade K strain that’s spreading nationwide. The strain emerged last summer, too late for it to be included in the 2025-26 vaccine.
“This is unexpected good news,” said Scott Hensley, professor of microbiology at the University of Pennsylvania. “Although it’s not a complete match with what’s circulating, this vaccine appears to have the capacity to produce antibodies that will likely provide protection,” he said.
The Penn Centers of Excellence for Influenza Research and Response takes an early look every year at how well the seasonal flu shot prompts an antibody response in the immune system. That is, how likely the shot is to offer measurable protection for most people.
This year's findings were released Tuesday night on the preprint server medRxiv, a site that publishes research before it’s gone through an extensive and rigorous peer-review process.
The current flu shot protects against three flu strains: two A strains (H1N1 and H3N2) and one B strain. Last June, however, after vaccine production had begun, H3N2 changed slightly. This raised concerns that the shots would be a poor match.
In October and November, the researchers analyzed blood samples taken from 76 people, ages 24 to 81. One blood draw occurred just before they got the flu shot. A second sample was taken about a month later, giving the vaccine ample time to mount a defense against the virus.
Before getting the shot, 39% of the study participants had antibodies against H3N2, likely because they’d already been exposed to the virus. That percentage nearly doubled after the vaccine, to 71%.
The effect was even more striking when researchers looked specifically at how the shot worked against the shifted H3N2 strain, subclade K.
Before the shot, just 11% had antibodies against subclade K. That percentage nearly quadrupled after the shot, to 39%.
The results held regardless of the study participants’ ages.
Last season’s flu shot reduced the risk of testing positive for flu during an outpatient visit by 56%. Although people who are vaccinated sometimes still get infected with flu, their symptoms tend to be much milder compared with those of people who aren’t vaccinated.
FOR SUBSCRIBERSA harrowing ordeal aboard an Israel-bound plane
FOR SUBSCRIBERSThe debate over AI’s future rages on both coasts
“This confirms the vaccine still can offer better protection against H3N2 than if you were unvaccinated,” said Angela Rasmussen, a virologist at the University of Saskatchewan in Canada. “The mismatch may not be as impactful as everyone feared.”
FOR SUBSCRIBERSA harrowing ordeal aboard an Israel-bound plane
FOR SUBSCRIBERSThe debate over AI’s future rages on both coasts
Bill Hanage, a professor of epidemiology at the Harvard T.H. Chan School of Public Health, said, “It’s more evidence that the current vaccine might not be perfect, but it’s still more than good enough to provide protection, especially when it comes to stopping serious illness.” Neither Hanage nor Rasmussen were involved with the new research.
Flu strains are “master shapeshifters,” Hensley said. That’s partly why it’s so difficult to get the vaccine to match the exact strains each year.
The findings may be of particular importance given the record high number of pediatric flu deaths reported last season, Hanage said.
Doctor visits for flu-like symptoms have hit the highest levels for this time of year in nearly three decades. According to the latest data from the Centers for Disease Control and Prevention, nearly 1 in 10 outpatient doctors’ visits were for flu symptoms as of Dec. 27.
On Monday, the CDC reported an additional death of a child last season, bringing the total to 289, eclipsing even the number of pediatric deaths from the 2009 H1N1 pandemic.
Although the Department of Health this week announced it would no longer recommend annual flu shots for children, the CDC website states that “everyone 6 months and older, with rare exceptions, should get a flu vaccine.”
As of Dec. 27, the percentage of kids who’ve gotten their flu shot this year was 42.5%. That’s the lowest recorded by the end of December since at least 2019, according to CDC records.
“Vaccines won’t do any good if people don’t get them. It’s not too late,” Rasmussen said. “Everyone reading should take this news as a reminder to get a flu shot if you haven’t already.”
