How effective is the flu shot this year? New research offers clues

Flu grips much of the United States In the midst of an early and difficult season, a glimmer of promising news is emerging: The flu vaccine may be better than experts predicted.

Early evidence shows that this season’s vaccine can effectively protect against the H3N2 subclade K strain that is spreading across the country. The strain appeared last summer, too late to be included in the 2025-2026 vaccine.

“This is unexpectedly good news,” said Scott Hensley, study author and professor of microbiology at the University of Pennsylvania. “Even though it doesn’t completely match what’s circulating, this vaccine appears to have the ability to produce antibodies that will likely provide protection,” he said.

Visits to the doctor for flu-like symptoms have reached the highest levels for this time of year in almost 30 years. Nearly one in ten outpatient doctor visits were for flu symptoms as of Dec. 27, according to the latest data from the Centers for Disease Control and Prevention.

The new findings were published Tuesday evening on the preprint server medRxiv, a site that publishes research before it undergoes a thorough and rigorous peer review process.

THE The current flu vaccine protects against three strains of flu: two A strains (H1N1 and H3N2) and one B strain. However, last June, after vaccine production began, the situation with the H3N2 virus changed slightly. This raised concerns that the shots might not match well.

The new research comes from the Penn Centers of Excellence for Influenza Research and Response, which annually examines the extent to which the seasonal flu vaccine triggers an antibody response in the immune system. That is, how likely is it that the vaccine will provide measurable protection to most people?

In October and November, researchers analyzed blood samples taken from 76 people aged 24 to 81. A blood test took place just before they got their flu shot. A second sample was taken about a month later, giving the vaccine enough time to defend against the virus.

Before getting vaccinated, 39% of study participants had antibodies against H3N2, likely because they had previously been exposed to the virus. This percentage almost doubled after the vaccine, reaching 71%.

The effect was even more striking when the researchers looked specifically at how the shot worked against the shifted H3N2 strain, subclade K.

Before the shot, only 11% had antibodies against the K subclade. This percentage almost quadrupled after the shot, to 39%.

The results are valid regardless of the age of the study participants.

Although vaccinated people sometimes get infected with the flu, their symptoms tend to be much milder than those of unvaccinated people.

“This confirms that the vaccine may still provide better protection against H3N2 than if you were not vaccinated,” said Angela Rasmussen, a virologist at the University of Saskatchewan in Canada. “The mismatch may not have as much impact as everyone feared. »

Bill Hanage, professor of epidemiology at Harvard TH Chan School of Public Health, said: “This is further evidence that the current vaccine may not be perfect, but it is still more than enough to provide protection, especially when it comes to stopping serious disease.” Neither Hanage nor Rasmussen were involved in the new research.

Flu strains are “master shapeshifters,” Hensley said. That’s part of why it’s so difficult to get the vaccine to match the exact strains every year.

The findings could be especially important given the record number of pediatric flu deaths reported last season, Hanage said.

On Monday, the CDC reported one additional child death last season, bringing the total to 289, eclipsing even the number of pediatric deaths from the 2009 H1N1 pandemic.

Although the Department of Health announced this week that it would no longer recommend the annual flu vaccine for children, the CDC website states that “everyone 6 months and older, with rare exceptions, should get the flu vaccine.”

As of December 27, the percentage of children who have received their flu vaccine this year was 42.5%. This is the lowest level recorded in late 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 consider this news a reminder to get your flu shot if you haven’t already.”