As the end of the year approaches, health authorities are keeping a close watch on a new dominant flu strain circulating across the country. Identified as subclade K, this variant has quickly overtaken other strains, raising questions about the effectiveness of the current vaccine and prompting public health messaging around continued prevention.
The Centers for Disease Control and Prevention (CDC) estimates that this flu season has already resulted in at least 4.6 million infections, 49,000 hospitalizations, and 1,900 deaths nationwide. According to data collected since late September, approximately 89 percent of H3N2 samples tested have been identified as subclade K, making it the most common flu virus circulating in the U.S.
Genetic Changes Distinguish Subclade K from the Current Vaccine Strain
Subclade K is a branch of influenza A belonging to the H3N2 category, which has historically been associated with more severe flu seasons. According to the CDC, this particular strain was first identified in August and has since become the leading cause of flu cases in the U.S.
This version of the virus carries minor genetic mutations that set it apart from the H3N2 strain used to formulate the 2025–2026 U.S. flu vaccine. These changes are the result of what scientists refer to as antigenic drift, a common process in which flu viruses evolve over time. While this genetic drift does not make the virus inherently more dangerous, it can influence how well existing vaccines work against it.
Dr. Parth Bhavsar, a family medicine physician and founder of TeleDirectMD, described subclade K as “a genetically drifted subgroup of seasonal influenza.” According to Newsweek, he noted that the difference lies mainly in the virus’s outer surface proteins, which are typically the primary target of vaccines. As a result, the immune response triggered by vaccination may not fully match this new variant.
Despite its genetic differences, current reports suggest that subclade K infections do not cause more severe illness than other strains of H3N2. “There isn’t an apparent indication that K subclade infections are inherently more severe than H3N2 infections,” Bhavsar said, emphasizing that symptoms remain consistent with the usual seasonal flu, including fever, body aches, fatigue, and dry cough.
Vaccine Continues to Offer Protection despite Strain Mismatch
Though subclade K is genetically different from the vaccine strain, health experts maintain that flu vaccination remains a critical tool for reducing hospitalizations and severe illness. According to Dr. Tyler B. Evans, an infectious disease specialist and former chief medical officer for New York City, there is still benefit in getting vaccinated even at this stage of the season.
“It is expected that vaccination can provide substantial protection, especially against hospitalization, with or without variant drift,” said Bhavsar. While the vaccine may not completely prevent infection, it can still lessen the severity of illness, helping patients recover faster and avoid serious complications.
Evans reinforced this point, explaining to Newsweek that even a partially effective vaccine plays a vital role. “If you get your flu shot, it can turn a serious illness into a much milder one—even if it doesn’t completely stop you from catching the flu,” he said. “Think of the vaccine as taking the flu from ‘wild to mild.’”
According to early findings, the current vaccine continues to offer some level of protection against subclade K, though it may be reduced compared to seasons with a closer strain match. Evans emphasized that vaccination is still worthwhile: “That’s better than not getting the vaccine.”
