Australia’s 2025 flu season lasted weeks longer than it normally does. Hong Kong’s hit so early that the rollout of seasonal flu shots hadn’t yet started. New York has reported record-breaking flu hospitalizations for the past two weeks.
Welcome to the winter of subclade K.
Even if you weren’t already aware of the name of this year’s flu variant, you may have spent the holidays trying to dodge sick relatives and friends. You’ve likely heard that emergency departments across the country are swamped with patients with flu-like illnesses.
You may have the impression that this is a particularly early or especially virulent flu season, or that something unusual is afoot. We’ve seen headlines referring to this wave of illness as a “super flu.” (Except maybe for pandemics — which this isn’t — that is not a thing.)
We won’t have answers to some of the questions about this flu season for a while, but here is what we do know. Let’s start with an introduction to subclade K.
What exactly is this virus?
Influenza viruses come in many forms, but the ones that cause the most disease in humans are the flu A viruses H1N1 and H3N2, and influenza B.
Subclade K is a variant of the H3N2 family of viruses, which can trigger more severe seasons and which seem to be harder on older people, who are among the most vulnerable to flu.
According to the Centers for Disease Control and Prevention, H3N2 is causing almost all of the illness so far this flu season, and almost all the H3N2 viruses circulating right now are from subclade K. In its most recent flu report, published Monday, the CDC estimated that so far this season roughly 11 million people have been sick with influenza, 120,000 have been hospitalized, and 5,000 people have died, including nine children.
Flu viruses evolve all the time. They have to change to be able to slip past the barriers our immune systems mount. But the timing of the emergence of this new H3N2 variant was highly inconvenient. It was first spotted in New York in June, which was months after a decision was made about which strains to target in the 2025-2026 Northern Hemisphere flu shot. That means this year’s flu shot probably doesn’t protect against H3N2 as well as we might hope. (More on this in a bit.)
Is there anything especially strange about subclade K?
The arrival of subclade K was associated with some very early starts to the flu seasons in a number of places. Japan, Hong Kong, and the United Kingdom all had particularly early flu seasons. But that pattern was not seen everywhere in the Northern Hemisphere.
In the United States, the start of the flu season hasn’t been notably early in most parts of the country, with the exception of New York, said Florian Krammer, a flu expert at Mount Sinai Icahn School of Medicine in Manhattan.
“I would characterize it as unusual, but not because of the U.S. I would characterize it as unusual because of Japan and the U.K.,” he said of subclade K. “I think if you look at the whole U.S. situation, it looks in terms of the timing of the season relatively normal.”
“For me, it’s still hard to understand how these things come about, in terms of why one country is hit so early and so massively, and then just across the channel, in … Germany, or Belgium, there’s not much,” he added.
What is ‘early’? When does flu season normally start?
Though it’s tempting to think about flu season as something with a start and end date, in reality the timing of influenza activity varies year to year.
In the U.S., it’s not uncommon for transmission to pick up steam around Thanksgiving, when cross-country travel and multigenerational gatherings help fuel the virus’ spread. That was what happened this year, as the red line rocketing skyward in the graph below shows.
But sometimes the flu season doesn’t take off until after the Christmas holidays, as it did in the 2007-2008 flu season. See the pink line in the graph. An early season can peak before Christmas, as was seen in the 2022-2023 season. Shown in the orange line, that season topped out at Thanksgiving, and was done by mid-January.
To that point: the U.K.’s early season has already started to subside.
Flu experts have a saying: If you’ve seen one flu season, you’ve seen one flu season. The timing of flu seasons varies. The virus — or combination of viruses — causing the most disease changes from year to year and even across a single season. Some seasons peak once, some have two or even three peaks. Some hit some age groups harder than others; last year, on the other hand, was severe across all ages. Somehow, between severe flu seasons we seem to forget the variability of this mercurial virus and the wallop it can deliver.
“It’s like with snow in New York City. It snows almost every year in New York City and people forget how to deal with it,” Krammer said. “And then it shows up again and they’re like, ‘Oh my God, what is that? How do we deal with that?’”
What advantages do subclade K’s mutations confer?
Flu viruses have to change to evade the antibodies people have developed, either through vaccination or previous infections. Subclade K has a clutch of mutations that makes it very adept about doing just that. As a result, it’s been able to infect a lot of people. Most of the country is engulfed right now in flu activity. And we’re not done yet.
“I think we’re really at the beginning of this right now,” said Frederick Hayden, an influenza expert who is a professor emeritus at the University of Virginia.
The CDC agrees. In an emailed statement, a CDC spokesperson said the agency expects to see flu activity continue for several more weeks.
Ugh, right?
But here’s a bit of good news: While these viruses are causing chaos on a population-wide basis — clogging emergency rooms, for instance — they don’t appear to be triggering more severe disease on an individual basis. “Right now, there is no evidence to suggest that influenza A(H3N2) subclade K is associated with more severe illness,” the CDC statement said.
So, while subclade K is putting the health care system under stress, on an individual level, it’s regular flu. Which, frankly, is bad enough.
“It’s a lot of cases. That’s, I think, the issue,” said Krammer.
More good news: These viruses haven’t developed resistance to antiviral drugs; the flu drug arsenal remains effective against subclade K. That’s especially important in a year when the vaccine’s protective effects may have been eroded by this new variant.
People who are at high risk of developing severe illness if they contract flu should think about trying to get antiviral drugs if they become sick, Hayden said — or even if someone in their household comes down with flu.
“These drugs work very well for post-exposure prophylaxis in the household setting,” he said. “If there are people who have risk conditions in a household, and someone introduces it, even if they’re immunized … personally, I would use post-exposure prophylaxis.”
So does this year’s vaccine protect against subclade K?
Subclade K viruses weren’t on the radar when influenza experts from around the globe gathered last February to select the strains the 2025-2026 flu shots should target.
Even when the vaccine contains viruses that are well matched to those that are circulating, the H3N2 component often under-performs. It offers protection against severe disease, but may not prevent infection.
That doesn’t mean getting the flu shot was for naught. Studies in past seasons with vaccine mismatches suggest people who received a flu shot were often less likely to become sick enough to require hospital care compared with those who didn’t.
As they do every year at this time, the CDC and other national public health agencies are studying how well the vaccine protects against the circulating flu viruses. Typically preliminary assessments of vaccine effectiveness — VE in flu-speak — are published around mid-February, with final analyses following after the end of the flu season.
But very early data from a couple of places suggests this year’s flu shot confers some protection against subclade K.
This week, scientists at the University of Pennsylvania posted a preprint — a paper that has not yet been peer-reviewed — reporting that in a small cohort of people tested about a month after getting this fall’s shot, 39% had antibodies that recognized subclade K viruses at levels that would suggest they were protected. Before vaccination, only 11% had antibodies recognizing the new viruses.
“Our study highlights the benefits of receiving influenza vaccinations, even in seasons that include circulation of variant viruses,” the researchers wrote.
Earlier, scientists at the U.K. Health Security Agency were able to do a very preliminary assessment of the vaccine’s effectiveness against H3N2 in the fall. Their findings, published in November in the online journal Eurosurveillance, were also reassuring. They reported vaccine effectiveness of between 72% and 75% in children and teens, with effectiveness in adults ranging between 32% and 39%. Those numbers are kind of standard territory for the H3N2 component of flu vaccines. “This is similar to end of season VE against influenza A(H3N2) in recent years in the United Kingdom, Europe and Canada,” they wrote.
It should be noted, though, that the numbers of people — especially adults — in the study were small and the people studied had been very recently vaccinated against flu. Protection from flu vaccine wanes in the months after vaccination, so studies conducted later in the season might show different levels of effectiveness.
Another thing to note: More than half of Americans don’t get vaccinated against flu. In fact, the percentage of people who are vaccinated against the flu has declined since the Covid-19 pandemic, the CDC’s interactive flu vaccination coverage database shows. In 2019-2020, 63.7% of children and teens were vaccinated; so far this year that number is 43%. The decline in people 65 and older has been less dramatic, dropping from nearly 70% in 2019-2020 to just under 66% this year.
The upshot is pretty clear. Whereas people who got a flu shot this year may have somewhat less protection than if subclade K had been included in the shot, the majority are facing this new H3N2 variant without any vaccine help.
This article has been updated with material about a newly released University of Pennsylvania study.
