A new flu variant known as subclade K is on the rise across the country. Although a spike in this particular strain of influenza, referred to by some as a “super flu,” has been recorded, symptoms remain similar to the most common form of flu called influenza A. Doctors say vaccination is the best way to avoid hospitalization or complications for those at risk.
What is subclade K?
Subclade K or J.2.4.1 is a subgroup of influenza A (H3N2) viruses. In other words, it is a new strain of the most commonly detected type of flu.
Symptoms are similar to those caused by other seasonal influenza viruses and include fever, cough, sore throat, runny or stuffy nose, muscle or body aches, headache and fatigue. Severity may vary depending on the age, immune status and underlying health conditions of the person infected, according to the World Health Organization and as reported by The Seattle Times.
Where has subclade K spread so far?
This new strain of the flu has been detected in most of the U.S., as well as in other countries. Most cases were first identified in August 2025 in Australia and New Zealand. The virus has since been detected in over 34 countries.
In New York City, 14,000 cases of the flu in the first week of December. That’s about a 460% increase from the same time last year, according to New York State and as reported by KTLA.
“We see the sudden spike in New York because of their more dense population. They’re a center for international as well as domestic travel,” Dr. Ulysses Wu, chief epidemiologist at Hartford Hospital in Connecticut, told KTLA.
Dr. Andrew Pekosz, an infectious disease specialist with the Johns Hopkins Bloomberg School of Public Health, added that subclade K has been identified anywhere the flu has been recorded.
“Colorado, Louisiana and New York are [states] that are experiencing really fast increases in influenza,” he said during a public health media briefing on Tuesday, according to KTLA. “But even in places like Maryland right now, where cases are not that high, it’s the [sub]clade K virus that seems to be the dominant virus.”
How to prevent infection from subclade K?
Doctors recommend wearing a mask, covering coughs and sneezes, hand washing and cleaning frequently touched surfaces in order to slow the spread of the virus. They advise staying home and avoiding contact with other people for at least 24 hours after getting sick, according to KTLA.
Getting vaccinated against the flu is also an effective way to prevent complications from the virus and potential hospitalization for children and adults at risk.
“One hundred thousand Americans died last year from a very robust influenza season,” Wu told KTLA. “There are prevention methods like vaccines, immunotherapy that can help prevent complications.”
Doctors also recommend getting vaccinated ahead of the holiday season.
“It is certainly not too late, especially as we’re thinking about holiday gatherings and travel,” Pekosz said. “Now would be a great time for individuals to go out and get that last influenza vaccine or that COVID vaccine to provide one layer of protection for this holiday season.”