New flu subvariant 'subclade K' spreads as cases rise

A new variant of the influenza virus, which emerged after this year’s vaccine was formulated, has brought an unexpected twist to flu season, according to infectious disease experts.

Subclade K, a variant of the influenza A strain H3N2, is being detected in the United States. This is the same variant that experts say is responsible for a rough flu season in Japan and the United Kingdom.

Although state Health Department data does not specify whether flu cases on Long Island are subclade K, experts said it's only a matter of time and suspect it might already be here.

Dr. Asif Noor, chief of pediatric infectious diseases at NYU Langone Hospital — Long Island, said it's "highly likely" the subvariant is in the New York region.

"We’re hearing more bad news," said Dr. Bruce Farber, chief public health and epidemiology officer at New Hyde Park-based Northwell Health. "The U.K. is having a bad flu season, and Canada is off to a roaring start. It appears almost all of these cases are H3N2 and those infections tend to be more severe in general."

Although scientists had already said this season could be tough based on the flu season in the Southern Hemisphere, they believe the current vaccine might not be as effective against this variant. But Farber said getting vaccinated is vital because it still helps to dramatically decrease the chance of hospitalization, prolonged illness and death.

"Even though it's not a perfect vaccine, it's the only game in town," he said. "It will protect you."

Statewide, the number of lab-confirmed flu cases is steadily rising from 3,979 on Nov. 1 to 9,858 on Nov. 15, the latest data on the Health Department website.

Cases have also been increasing across Long Island, with 1,172 in Nassau County on Nov. 15, up from 510 on Nov. 1. In Suffolk County, there were 643 lab-confirmed cases of flu on Nov. 15, up from 274 on Nov. 1. The majority of those cases were influenza A but there was no information about whether they were subclade K.

National data on the Centers for Disease Control website show 50% of H3N2 lab-confirmed cases were subclade K.

Lab-confirmed flu cases only represent a portion of overall cases since many people who are sick may not seek treatment or get tested, doctors said. 

"The flu virus has a tendency to change frequently," Noor said. "H3N2 is included in the flu vaccines that we have but there is a small gene mutation which has resulted in subclade K. It’s not a huge mutation that could lead to a pandemic but we believe that’s the reason why the flu is coming early" and there’s high activity.

Adding to the concern is the ongoing decrease in the number of people getting a flu shot.

Between October 2019 and April 2020, about 62% of children in the United States were vaccinated against the flu, according to the CDC. That number dropped to 49% between October 2024 and April 2025.

In New York State, about 19% of all eligible residents, including adults, have been vaccinated this season — 16% in Suffolk and 19% in Nassau.

Both Noor and Farber said the vaccine was important to protect the person who received it as well as the people around them.

"Flu is unpredictable," Noor said. "Those who are vulnerable, are elderly or have medical problems can get very sick, but healthy adults and healthy children can also get sick. No one is protected from the flu just because they are healthy."

If more people are vaccinated, he said, there will be better protection in the community.

Farber suggested that elderly or medically compromised people who were in contact with a person who developed flu take a smaller dose of the medicine Tamiflu, to protect against infection.

By Lisa L. Colangelo

lisa.colangelo@newsday.com@lisalcolangelo

Lisa joined Newsday as a staff writer in 2019. She previously worked at amNewYork, the New York Daily News and the Asbury Park Press covering politics, government and general assignment.