New Flu Strain and Low Vaccination Rates Could Mean Aggressive Flu Season Ahead

Flu season shows up every year, but late 2025 is shaping up to be a bit more unpredictable. After watching how outbreaks have unfolded in Europe and Australia, which usually are good previews of what’s coming to North America, experts are raising their eyebrows. A newly mutated influenza strain, known as subclade K, is now circulating widely, and people are understandably asking whether this year’s flu shot still has them covered.

Fortunately, early data says the vaccines are still doing their job. However, the season is starting early, vaccination rates are lagging behind previous years, and health-care workers are bracing for a busier winter.

A New Flu Strain: Subclade K

According to the Center for Disease Control and Prevention (CDC), the majority of current flu cases are from the A-type strain H3N2. About half of those belong to subclade K — the same variant behind a particularly rough 2025 flu season in Australia.

When scientists pick which strains go into the annual flu shot, they rely on what’s circulating around the month of February. At that point, subclade K wasn’t a major player, so this year’s vaccine targets a slightly different member of the H3N2 family. And while they’re related, subclade K has built up enough genetic changes to grab scientists’ attention.

Like all influenza viruses, H3N2 is constantly evolving. The World Health Organization tracks these shifts closely because even small tweaks can change how well the vaccine matches the circulating strains. The most important changes happen on hemagglutinin (H) — the protein vaccines train your immune system to recognize. That’s the “H” in names like H3N2 or H5N1.

Over the summer, subclade K picked up seven new mutations, making it noticeably different from the strain chosen for this year’s shot, as laid out in a preliminary report by the UK Health Security Agency (UKHSA). While it’s not clear that this variant makes individual infections more severe, H3N2 in general tends to hit older adults harder than H1N1 does. If subclade K spreads quickly more infections alone could drive hospitalizations up, experts told CNN.

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An Earlier Flu Season Matters

Flu activity is still considered low nationally, but numbers are rising, especially in kids. The timing looks similar to other years with early starts, including last season, according to the CDC.

Subclade K appears to have emerged late in the Southern Hemisphere season, and early reports from the U.K. and Japan show its dominance in most flu samples. Adam Finn from the University of Bristol told the Vaccine Alliance Gavi that an early uptick like this usually means the circulating strains are spreading more easily than usual, even before winter weather gives them a boost.

Additionally, when flu season gets a head start, many high-risk people haven’t been vaccinated yet, meaning more vulnerability right as cases begin climbing, he said.

Does the Current Vaccine Still Work?

Although subclade K differs from the strain included in the current vaccine, health professionals expect protection to remain solid.

Real-world data from the U.K. shows this year’s flu shot is 70 to 75 percent effective at keeping vaccinated kids (ages 2–17) out of the hospital and 30 to 40 percent effective in adults. These numbers, however, come early in the season, before immunity has had time to develop and are from the preprint study by the UKHSA.

The bigger concern is that fewer people are getting vaccinated. Between August 2025 and the end of October 2025, U.S. pharmacies administered 2 million fewer flu shots than the same period last year, according to the prescription data company IQVIA. Australia saw similar declines, and with subclade K dominating there, the Australian Department of Health reported a record 443,000 flu cases.

Taken together, the combination of a new variant, an early season, and lower vaccination rates suggests this winter could be bumpier than usual, but the flu shot still offers meaningful, real protection.

This article is not offering medical advice and should be used for informational purposes only.

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