The emergence of the H3N2 variant has led experts to warn that this flu season could be particularly severe.
A new and potentially more severe variant of the flu, known as H3N2 subclade K, has reached the United States, prompting heightened vigilance from public health officials as the holiday season and winter travel approach.
According to the Centers for Disease Control and Prevention’s flu report, there have already been at least 650,000 illnesses, 7,400 hospitalizations, and 300 deaths from flu this season, ending the week of November 15. Among the cases reported nationwide last week, 71.9 percent, 115 cases, were H3N2 while 26.9 percent were H1N1. The other 1.2 percent were the H5 sample specimens from the same person. “There is basically a new variant of influenza circulating that has mutated a little bit,” Caitlin River, epidemiologist at Johns Hopkins told NPR. “And that means that it’s just different enough from what your body or the vaccine may recognize that it can kind of get around those protections.” Newsweek has reached out to the CDC for further comment by email Sunday. Why It Matters The emergence of the H3N2 subclade K variant has led experts to warn that this flu season could be particularly severe. The strain, which has dominated outbreaks in Japan, the United Kingdom, and Canada, is showing signs of spreading rapidly in the U.S. where vaccine uptake is also lagging. Experts stress that although the seasonal flu shot provides some protection, the new variant’s genetic mutations have raised worries about reduced vaccine effectiveness, especially for vulnerable populations such as the elderly. What To Know The H3N2 subclade K strain first appeared in Japan and the U.K. before spreading rapidly throughout Canada, where it is now the dominant flu subtype and responsible for a rise in hospitalizations, particularly among the elderly and children. According to the European Centre for Disease Prevention and Control, the strain has been detected on all continents since May 2025 and accounted for nearly half of H3N2 cases in the European Union by November. Flu activity in the U.S. has increased quickly in recent weeks, with most current cases attributed to Influenza A H3N2, and about half linked to subclade K. The CDC recently resumed full operations after a government shutdown, which lasted for more than a month, paused some surveillance activities, making precise assessment difficult for several weeks. Recent lab data suggest a brisk uptick in positivity rates, and surveillance systems such as WastewaterSCAN confirm rising flu signals in several states, CNN reported. The symptoms of H3N2, including its mutated subclade K, are similar to typical flu infections—fever, cough, runny nose, fatigue, muscle aches, and chills. However, reports from international outbreaks show this variant tends to cause more severe illness, particularly among the elderly and young children, sometimes resulting in higher fevers and more frequent complications. While the current 2025–26 flu vaccine protects against H3N2, it was developed before subclade K emerged. Data from the U.K. shows that, for children, the vaccine reduces the risk of hospitalization by about 75 percent while the protection for adults, especially those over 65, is lower—around 30 percent to 40 percent, according to a report from NPR. Despite the mismatch, experts continue to recommend vaccination, emphasizing its ability to blunt severe illness and complications. Flu vaccination rates in the U.S. have fallen compared to previous seasons. The CDC reported 147.6 million doses distributed for the 2024–2025 season—down from 157.7 million in the season prior. In addition to vaccination, experts urge Americans to practice preventive behaviors: wear masks in crowded indoor spaces, wash hands for at least 20 seconds, and avoid crowded gatherings if possible—especially for those who are immunocompromised or otherwise at high risk of severe disease. A patient is given a flu vaccine October 28, 2022, in Lynwood, California. What People Are Saying Dr. William Schaffner, infectious disease expert at Vanderbilt University Medical Center, told Local 10 News Friday: “We’re looking forward to the CDC’s laboratories and epidemiologists getting back in the swing after they’ve recovered from being away from the office for so long …My fingers are crossed that they have enough personnel … to put together this good information on a regular basis.” Dr. Richard Webby, who studies the flu at St. Jude Children’s Research Hospital in Memphis, told NPR: “The flu season might have a little bit of a punch to it this year…H3N2 viruses tend to be a little bit more problematic…With this new variant, the flu shot this year may not be as good a match as if we hadn’t seen this new variant.” Dr. Adam Lauring, chief of infectious diseases at the University of Mich...
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