Scientists are warning: This winter’s upcoming flu season in North America could be a real nightmare, due to the rapid arrival of a variant that is not matched by the seasonal flu vaccine.
Health officials in Canada issued the warning in a paper late last month. They reported that an unexpected strain of H3N2 flu, called subclade K, emerged rapidly at the end of flu season in the Southern Hemisphere this year. Subclade K is now poised to become one of the most dominant strains of the flu season in the US and Canada, and it is likely that our vaccines will be less effective against the flu this winter than experts have predicted.
“Although mismatched vaccines may still provide protection against circulating variants, increased surveillance is warranted,” the researchers write in their paper. Journal of the Canadian Association of Medical Microbiology and Infectious Diseases,
a flowing mismatch
During the peak of flu season, several strains of influenza A and influenza B viruses will spread among people. Because it takes about nine months to produce enough vaccines for everyone in a given hemisphere, scientists and health officials meet twice a year (once for the northern half, once for the southern half) to estimate which variants are most likely to spread in the population, and thus, which variants the seasonal flu vaccine should cover. Part of this educated guess comes from tracking the types of flu circulating around the world at the time.
Typically, predictions are not too far off, and vaccines will provide at least moderate protection against the disease. But flu viruses are always evolving, and variants can gradually develop mutations (a process called drift) that make them significantly different from what scientists expected them to look like. This appears to be what has happened with the latest versions of H3N2, a strain of influenza A.
According to the researchers, an increasing amount of H3N2 variants with related mutations emerged during last winter’s flu season in the Northern Hemisphere. These variants are probably a major reason why the US experienced such a severe flu season last winter.
More recently, another aberrant and poorly matched lineage of H3N2—subclade K—emerged in the late 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 subtype K will not spread widely in North America, as there will be other circulating forms of flu nearby at the same time. But its chances are not looking very good. In the UK, health officials have already announced an early start to the flu season, with the majority of cases being caused by subtype K. Area hospitals are now preparing for a major surge of flu.
It’s also worth wondering whether the fragmented state of public health in the US will further hinder efforts to stop the flu. President Donald Trump and Health Secretary Robert F. Kennedy Jr. have overseen dramatic funding cuts and layoffs at 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 do vaccines still matter?
Despite the grim scenario, vaccines remain one of the most important tools against flu this season. There are other types of flu that the vaccine will cover, and a mismatched vaccine may 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 Protection Agency reported the latest data from its flu surveillance programme. Even with the mismatched variant in the picture, the vaccine is currently estimated to be 70% to 75% effective in preventing flu hospitalization in children and 30% to 40% effective in adults.
“These results provide convincing evidence that this season’s flu vaccines currently provide significant protection to children and adults, despite concerns about the new subtype,” Jamie López Bernal, an epidemiologist on vaccination at the UKHSA, said in a statement from the agency.
So it’s still worthwhile to get your flu shot as soon as possible. But given the discrepancy this winter, it’s even more important to practice good hygiene, stay home if you’re sick, and potentially wear a mask in high-risk situations for extra protection (well-fitting, high-quality masks such as a KN95 or N95 are most effective).