This mutated flu strain is triggering fears of a massive outbreak

Axios – Thursday, November 13, 2025
By Herb Scribner

Experts worldwide are warning of a new flu outbreak ahead of winter.

The big picture: The new flu strain, a version of H3N2, emerged over the summer, raising fears that the current flu vaccine might struggle to fight it.

• Health officials in Canada and the U.K. have already warned of an H3N2 wave that’s sending people to the hospital.
• Experts are worried that the new version of H3N2 doesn’t match the strain used to create this year’s flu vaccine, though such mismatches are common with seasonal flu strains.

Here’s what to know about H3N2 and its recent surge.

Experts warn of H3N2 surge

The current version of H3N2 is spreading rapidly across the world.

• “Since it emerged, it’s rapidly spreading and predominating in some countries so far in the Northern Hemisphere,” Dr. Wenqing Zhang, head of the World Health Organization’s Global Respiratory Threats Unit, said at a media briefing on Wednesday.
• “There’s no doubt this winter will be one of the toughest our staff have ever faced,” James Mackey, head of the U.K.’s National Health Service, said in a statement.

Reality check: Experts aren’t considering this to be a pandemic-level threat. However, it could still present significant threats to older people or those with underlying health conditions.

What to know about new H3N2 strain

The new version of H3N2 (subclade K) is a strain of influenza A.

• However, it is different from the H3N2 version used for the flu vaccine because the strain acquired seven different mutations over the summer after the reference strains had been chosen, experts said.
• The current vaccine is based on the “2024-25 subclade J” and “updated 2025-26 subclade J.2” vaccine reference strains, per the University of Minnesota’s Center for Infectious Disease Research and Policy (CIDRAP).

Yes, but: Early data from the UK Health Security Agency suggests there has been some vaccine effectiveness against the H3N2 strain. That’s why health officials there are pushing people, especially the elderly and vulnerable populations, to get vaccinated.
Where H3N2 strain is surging

The United Kingdom is facing an early wave and officials there have issued the most warnings about a potential outbreak.

• The region “looks set to be a cruel winter, with flu cases being triple what they were this time last year,” McKay, of the U.K.’s NHS, said.
• These types of waves “also make it even harder to meet the standard of care patients deserve,” he said.

Canada and Japan have also seen a rise in cases.

• News outlets in Japan reported that flu cases have climbed sixfold over year. Thousands of schools and daycares have been closed there because of the outbreak.
• Canada is predicting a bad flu season given the recent outbreaks.
• Hong Kong has also reportedly seen a jump in cases.

Are there H3N2 cases in the US?

Zoom out: It’s unclear how widespread this version is in the U.S. right now since the Centers for Disease Control and Prevention has delayed any insights, updates and reports due to the government shutdown.

• That said, the U.S. is preparing to watch for COVID and flu symptoms this year. Walgreens launched an expanded tracker to keep an eye on where these illnesses are spreading across the country.
• Consumers will have a chance to see flu and COVID-19 prevalence by state on the tool.

Flashback: The U.S. experienced the worst flu season since 2009 last year, leaving hundreds of thousands of Americans hospitalized.
Common symptoms for H3N2

Because H3N2 is a strain of influenza A, the symptoms are similar to those of the common flu.

• Sore throat.
• Runny nose.
• Fever.
• Cough.
• Headache.
• Muscle aches.
• Tiredness.
• Sweating.
• Chills.

Of note: Common colds tend to begin slowly over the course of several days, per the Mayo Clinic, but the flu tends to begins with two or three days of coming into contact with the virus.

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