This year’s flu shot may be more effective than researchers thought – even as cases across the US are rising

This year’s flu vaccine may be more effective than experts previously believed.

People who got the shot had measurable protection from severe illness, researchers at the University of Pennsylvania said in a recent article that has yet to be peer-reviewed.

Looking at blood samples from 76 adults, before and after getting their flu shot, the authors found that the percentage of people with antibodies against the latest strain called H3N2, or proteins that neutralize viruses and bacteria, rose from 39 percent to 71 percent.

This is America’s worst flu season in more than two decades. Cases have reached high or very high levels across the U.S. this winter, resulting in more than 15 million illnesses, 180,000 hospitalizations and 7,400 deaths.

“This is unexpected good news,” Scott Hensley, a professor of microbiology at the university, told NBC News. “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.

Approximately 130 million people have gotten their flu shot this season, less than 40 percent of the population.

This season’s shots

Hensley wrote on social media that while his team had found that the “subclade K” flu variant was able to effectively evade existing immunity from exposure to flu, “this year’s flu vaccine surprisingly elicited antibodies in many individuals that efficiently recognized these viruses.”

The results were the same regardless of age. Participants in the team’s research were between 24 and 81 years old. Older adults are at a greater risk of life-threatening infection from flu.

Flu vaccines work by causing antibodies to develop in the body and protect against flu illness. The current flu shot protects against two strains of influenza A – H1N1 and H3N2 – and one influenza B strain.

But after production started in June, H3N2 mutated slightly, leading to concerns about the vaccine’s effectiveness this flu season. The variant of H3N2 known as “subclade K” has been responsible for a lot of this season’s infections in the U.S. and abroad.

Children at risk again

Flu cases have reached their highest levels in 25 years and symptoms are evolving from last year. There are prolonged fevers with infection, as well as more gastrointestinal issues.

“Nearly every state is experiencing high or very high respiratory illness, coming on the heels of the deadliest flu season on record for children,” the American Academy of Pediatrics said in an update. The 2024-2025 flu season resulted in nearly 290 pediatric flu deaths.

Last week, two-year-old Isaias Castillo IV from Morris County, New Jersey, passed away due to complications from the flu, becoming the tenth pediatric flu death in the U.S. this season. It is unknown if Castillo was vaccinated, but NorthJersey.com reported he had no known underlying health conditions.

Approximately half of children who died from flu last year had underlying medical conditions, and 89 percent were not fully vaccinated, according to research shared by the Centers for Disease Control and Prevention.

Some 42 percent of children have been vaccinated against the flu this year, according to the American Academy of Pediatrics.

Leadership at the CDC recently announced an overhaul of the U.S. childhood vaccine schedule, including removing universal recommendations for flu shots.

“To back off on a flu recommendation in the midst of a pretty severe flu year seems to me to be pretty tone deaf,” Dr. Sean T. O’Leary, chair of the academy’s Committee on Infectious Diseases, said. “And that’s coming off an influenza year where we had the most childhood deaths from influenza in many years. It makes no sense.”

Lingering threat

Experts stressed that it’s not too late to get a flu shot which protects against extreme illness. There is no clear sign that flu season has peaked yet, experts added.

Flu seasons typically peak between December and February, but can run through May.

“The implications are clear: go get this year’s vaccine if you haven’t already!” Hensley said.