New mutated flu strain ‘subclade K’ raises concerns as cases spread

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28 October 2020, Berlin: A doctor holds a syringe with an influenza vaccination of the influenza season 2020 / 2021 in his hand. Photo: Jens Kalaene/dpa-Zentralbild/ZB (Photo by Jens Kalaene/picture alliance via Getty Images)

PHILADELPHIA - A new flu variant called subclade K is gaining attention as doctors warn it could lead to a tougher flu season this winter. The strain, which stems from the influenza A (H3N2) virus, has already been identified in several countries and may be spreading more easily than earlier versions of H3N2, according to the Centers for Disease Control and Prevention (CDC),

Flu variant 'subclade K'

What we know:

Subclade K first surfaced overseas and has since risen quickly in circulation, according to reporting from ABC News. Early indications show this strain may be more infectious and is causing notable spikes in flu activity among children.

Health experts told ABC News that subclade K is still considered a form of seasonal flu and not a new virus. However, its mutations may make it harder for the body to recognize if a person has not recently been exposed to similar strains.

Doctors say symptoms remain typical of influenza: fever, cough, sore throat, body aches, fatigue and, in some cases, complications such as ear infections or pneumonia.

Why you should care:

Health officials say the 2025–26 flu vaccine was formulated before subclade K emerged, meaning the match may not be ideal. According to the CDC, even a partial vaccine match can significantly reduce the risk of severe illness, hospitalization and complications.

Experts told Today.com that subclade K’s genetic changes may help it spread faster, especially in schools and child-care settings. Pediatricians have reported rising concern about the strain’s impact on younger patients.

Subclade K is raising alarms because it carries seven new mutations, according to Today.com. This is making it more difficult for the immune system to recognize. 

The strain was first detected in Europe in June after scientists had already selected the components for this year’s flu shot.

How to protect yourself

Doctors and health agencies continue to stress the basics:

• Get a flu shot. Even if the match isn’t perfect, the CDC says it is still the strongest tool to prevent serious illness.
• Seek treatment early. Antiviral medications may shorten the duration or severity of symptoms.
• Stay home when sick. Limiting contact helps curb spread, especially to children, older adults and people with chronic conditions.
• Use everyday precautions. Handwashing, avoiding close contact with sick individuals and masking in high-risk settings can help.

What's next:

The CDC says it is monitoring how widely subclade K spreads in the U.S. and how well the flu vaccine performs against it. Health officials expect to release more data as the season progresses.