Aggressive new flu variant (subclade K) drives global surge, doctors warn of severe symptoms

A mutated flu variant has sparked a global rise in illness, with health authorities warning that this season could be unusually severe. Health researchers have traced a mutation of influenza A H3N2, known as subclade K, as the driver of rising cases in multiple countries, including the United States. The World Health Organization described subclade K as representing "a notable evolution in influenza A (H3N2) viruses," a development that has prompted questions about how well this season's vaccine will protect against circulating strains.

Across many regions, doctors say patients are presenting with typical influenza symptoms, but clinicians note the illness can be more severe than in earlier seasons. Fever, chills, headaches, fatigue, cough, sore throat and runny nose are common; however, clinicians say the disease can advance more quickly in some patients. Public health experts say subclade K differs enough from prior variants that community immunity is lower and transmission may be more efficient, contributing to higher counts. In the United States, officials have reported rising activity and hospitalizations in several regions as winter approaches.

Dr. Neil Maniar, professor of public health practice at Northeastern University, described the situation to Fox News Digital as evolving and potentially more severe. "It�s becoming evident that this is a pretty severe variant of the flu," he said. "Certainly in other parts of the world where this variant has been prevalent, it�s caused some severe illness, and we�re seeing an aggressive flu season already." He noted that subclade K is "quite different" from prior variants and that lower vaccination rates this year leave "less natural immunity at the community level," increasing both spread and the potential for severe disease. "The vaccine is very important to get, but because it�s not perfectly aligned with this variant, I think that�s also contributing to some degree to the severity of cases we�re seeing," he added. "We�re going in [to this flu season] with lower vaccination rates and a variant that in itself seems to be more aggressive."

Maniar also emphasized broader preventive steps: proper hand hygiene, staying home when ill, and seeking testing if symptoms arise. "It�s important that everyone stays vigilant and tries to take care of themselves and their families," he said. "If you�re not feeling well, stay home. That�s a great way to recover faster and to ensure that you�re not going to get others around you sick."

Health officials say the flu vaccine remains a critical tool to reduce hospitalizations and severe outcomes, even if it may offer less direct protection against subclade K. In addition to vaccination, people are advised to wash hands frequently, avoid close contact with sick individuals, and stay home during contagious periods, especially when winter gatherings and travel heighten exposure risk during the holiday season. Physicians stress that antiviral medications can help reduce duration and severity if started promptly, so those who are at high risk or who develop symptoms should consult a health care provider to determine testing and treatment options. Some providers may be able to prescribe medicines to reduce the severity and duration of the illness.

With the holiday season driving gatherings and travel, the risk of transmission remains elevated. Officials urge vaccination as the best available defense and remind people that influenza can be serious for older adults, young children, pregnant people and those with underlying medical conditions. Health officials say vigilance is essential as the season progresses.