Scientists are sounding the alarm: this winter’s upcoming flu season in North America could be a real nightmare, thanks to the rapid arrival of a variant that doesn’t match well with the seasonal flu vaccine.

Health officials in Canada issued the warning in a paper late last month. An unexpected variant of the H3N2 flu, dubbed subclade K, quickly emerged during the tail end of the flu season in the Southern Hemisphere this year, they noted. Subclade K is now poised to become one of the most dominant variants of the flu season in the U.S. and Canada, and it’s likely that our vaccines will be less effective overall this winter against the flu than experts had predicted.

“While mismatched vaccines can still provide protection against circulating variants, enhanced surveillance is warranted,” the researchers wrote in their paper, published in the Journal of the Association of Medical Microbiology and Infectious Disease Canada.

A drifting mismatch

During the peak of a flu season, multiple variants of influenza A and influenza B viruses will spread between people.  Because it takes about nine months to ramp up enough vaccines for everyone in a given hemisphere, scientists and health authorities meet twice a year (once for the northern half, once for the southern) to predict which variants are most likely to circulate in the population, and thus, which variants a seasonal flu vaccine should cover. Part of this educated guesswork comes from tracking the flu strains moving across the globe at that time.

Usually, the predictions aren’t too far off, and the vaccines will provide at least moderate protection from illness. But flu viruses are always evolving, and variants can gradually develop mutations (a process called drift) that make them significantly different from what scientists had expected them to look like. That seems to be what’s happened here with the latest versions of H3N2, a type of influenza A.

According to the researchers, an increasing amount of H3N2 variants with concerning mutations emerged during last winter’s flu season in the northern hemisphere. These variants are perhaps one major reason why the U.S. experienced such a hard-hitting flu this past winter.

More recently, a further drifted and poorly matched lineage of H3N2—subclade K—emerged toward the end of the winter in the Southern Hemisphere. This lineage is now “projected to predominate among A(H3N2) viruses for the NH 2025–2026 season,” the researchers wrote.

The possibility remains that subclade K will not spread widely in North America, since there will be other circulating variants of flu around at the same time. But the odds of that aren’t looking too good. In the UK, health officials have already announced an early start to its flu season, with a majority of these cases being caused by subclade K. Hospitals in the region are now preparing for a big surge of flu.

It’s also worth wondering whether the fractured state of public health in the U.S. will further hamper efforts to contain the flu. President Donald Trump and Health Secretary Robert F. Kennedy Jr. have overseen dramatic funding cuts and layoffs throughout the Centers for Disease Control and Prevention and other health agencies this year, while RFK Jr. has fired or pressured senior health officials to leave the CDC.

Why vaccines still matter

Despite the dire outlook, vaccines remain one of the most important tools against the flu this season. There are other strains of flu that the vaccine will cover, and a mismatched vaccine can still provide some protection against the worst outcomes of flu, which can include death. Thankfully, there is good news on that front.

This week, the UK Health Security Agency reported the latest data from its flu surveillance program. Even with a mismatched variant in the picture, the vaccine is currently estimated to be 70% to 75% effective at preventing flu hospitalizations in children and 30% to 40% effective in adults.

“These results provide reassuring evidence that this season’s flu vaccines currently offer important protection to children and adults, despite concerns about the new subclade,” said Jamie Lopez Bernal, consultant epidemiologist for Immunization at UKHSA, in a statement from the agency.

So it’s still worthwhile to get your flu shot as soon as you can. But given the mismatch this winter, it’s all the more important to practice good hygiene, to stay home if you’re sick, and to potentially wear a mask in higher-risk situations for added protection (well-fitting, high-quality masks such as a KN95 or N95 being the most effective).