What is ‘Subclade K’? The new flu variant may already be in Spokane

Much of the country is facing record flu cases thanks to a new strain of the virus. And the “subclade K” variant is likely already here in Spokane.

The new strain formed last year as influenza A’s H2N3 virus mutated. According to the Centers for Disease Control and Prevention, the mutated form of the virus accounts for 91.2% of flu present in the United States. The surprise variant was not factored into this year’s flu vaccine, so the vaccine may be less effective because of it.

Each year, the flu vaccine is developed to fight against the strains experts believe will be most prevalent in the fall and winter. subclade K could not be targeted because it did not exist when this season’s vaccine was developed.

It is unclear what strain is circulating in Eastern Washington. Such subtyping is done at the federal level and may not yet be available down to the county level. But Spokane Regional Health District epidemiologist Mark Springer suspects the Spokane flu activity is subclade K.

“There’s a strong likelihood the majority of what we’ll be seeing is subclade K,” Springer said. “Our numbers are not dramatic compared to other parts of the country. But those national numbers show what we will see when flu escalates in Spokane.”

Flu across the United States has spiked across the holiday break. According to the CDC, positive cases of flu increased by 32.9% in the week ending Dec. 27 compared to the week prior. The last full week of 2025 was the first time the CDC has categorized this flu season as “moderately severe.”

This sharp increase has not yet been seen in Spokane. While flu accounted for 8.3% of all emergency room visits across the nation, only 2.7% of emergency room visits in Spokane were caused by flu in the week ending Dec. 27.

Harborview Medical Center physician John Lynch said Washington as a whole was “starting a little bit late” compared to other flu seasons.

“We are behind the curve, but we’re headed in the same direction as everyone else. It’s shaping up to be a very serious flu season,” Lynch said.

While Spokane remains below the national rate of flu cases, hospitalizations have spiked in recent weeks. According to SRHD, early December saw fewer than five flu hospitalizations each week, and that has more than quadrupled by the beginning of January. The week ending on Jan. 3 saw 25 flu hospitalizations in Spokane out of 75 hospitalizations here across the flu season that started in mid-October.

That spike coincided with the detection of high levels of influenza A in Spokane wastewater on December 26. Springer attributes a more viral flu season with the emergence of subclade K, which has created a “greater mismatch” with the annual vaccine.

“Flu has a lot of variability, which is why we have to remake the flu each year. The genetic changes seen in subclade K are not dramatic, but the vaccine was developed with other strains in mind,” he said.

Still, Brown University epidemiology professor Jennifer Nuzzo said the effectiveness of this year’s flu vaccine may be higher than expected.

“That the vaccine is less effective is more of a worry at this point than shown through clear data of reduced protection,” she said.

Nuzzo also noted there is no evidence subclade K is inherently more infectious or creates more extreme symptoms than other flu strains.

“I don’t want people to believe this is a completely abnormal occurrence. Mismatches between flu strains and vaccines happen. But we see over and over that vaccines do a great job of keeping people out of the hospital,” she said.

On Monday, the CDC removed longstanding vaccine guidance for many childhood vaccines, including the flu vaccine. Under the new guidance, children not in high-risk groups should only receive certain vaccines based on “shared clinical decision-making” between physicians and parents. Vaccines in this guidance include COVID-19, influenza, meningococcal disease, hepatitis A and hepatitis B.

Washington’s Department of Health no longer follows CDC vaccine recommendations, having joined the West Coast Health Alliance, which provides its own vaccine guidance and still recommends the flu shot for all children.

Nuzzo worries the dual guidance of rival agencies and overall vaccine skepticism coming from Robert F. Kennedy Jr.’s CDC may have deterred flu vaccinations ahead of this flu season.

“From the provider’s perspective, nothing’s changed,” Nuzzo said.

“The flu vaccine is just as important. You can definitely talk to your healthcare provider, but I don’t want people to kind of see those guidelines and think anything’s changed about either the importance of the vaccine, or how harmful flu is,” she said.

It is generally recommended to be vaccinated for the flu at the beginning of flu season in October. But according to Springer, those who get vaccinated now will be fully protected by the time the full flu wave hits Spokane.

“We’re starting to see this uptick in early January, so there is still time to get vaccinated and be fully protected in a few weeks. Because between mixing during the holidays and kids getting back to school, it’s the perfect recipe for us to see transmission in the next few weeks in Spokane.”