12/17/2025
The ominous new ‘K’ flu variant is spreading as flu shot rates fall
Health experts are warning that this winter’s flu season could be severe. Flu cases are rising now, but it’s not too late to get your shot and protect loved ones before the holidays.
By: Katie Kerwin McCrimmon, UCHealth
Dec. 10, 2025
Fewer people are getting their annual flu shots this year, just as flu infections are rising and the ominous new “K” flu variant is spreading.
In Colorado, only about 25% of people have gotten their flu shots so far this fall, according to data from the Colorado Department of Public Health and Environment.
That’s down from about 27% of people who had gotten their flu shots by this time last year. And it’s a significant drop from the 32% of Coloradans who got flu shots during the entire winter respiratory virus season in 2024 and 2025.
While misinformation about vaccines has driven down the number of people who are getting all types of vaccines — including those that protect people from the flu — the drop in the percentage of Coloradans who are getting annual flu shots comes at a particularly dangerous time.
That’s because a new flu variant has been surging around the world and has caused bad, early flu outbreaks in Japan, the United Kingdom and Canada. This variant, known as the Type A H3N2 “subclade K” is now spreading in Colorado and across the U.S., prompting warnings from infectious disease experts that we could face a tough flu season.
“In the U.S., 20,000 to 30,000 people still die every year from the flu,” said Dr. Michelle Barron, UCHealth’s senior medical director for infection control and prevention.
“That’s not insignificant,” said Barron, who is also a professor of medicine and infectious diseases at the University of Colorado School of Medicine on the Anschutz Campus.
At the same time, the “K” variant, which soon could become dominant in the U.S., appears to be especially good at evading vaccines and bypassing our bodies’ immune systems.
This year’s flu shot isn’t perfect, but you should still get it
Even though this year’s flu shot is a “mismatch” for the “K” variant, that doesn’t mean you should skip getting flu vaccines for yourself and your family members, Barron said.
Getting a flu shot still protects people from getting severely ill or dying if they get the flu.
That’s why Barron’s advice is simple and clear. If you haven’t gotten your flu shot yet or made an appointment to get your kids vaccinated, do it now.
“It’s never too late. Now that we’re seeing an uptick in flu cases, it’s a great time to go get your vaccine because the numbers will continue to rise,” Barron said. “You’re going to need that extra level of protection as more people get sick and as more people will be traveling or having visitors.”
Barron encouraged people to enjoy fun times with family and friends rather than picking up a lousy respiratory virus.
“The worst part of the holiday season is if you’re sick and stuck in bed. You want to be able to go out and enjoy the holidays,” Barron said.
To learn more about the flu, we asked Barron for answers to your top questions.
Why do you expect the 2025-2026 respiratory virus season to be bad for the flu?
Confusion about vaccines is driving down vaccination rates. When fewer people are protected, viruses can spread more easily and make more people sick, Barron said.
Vaccination rates are lower across the board, and some people who don’t want COVID-19 vaccines are also skipping flu shots.
Barron encourages people to talk with their providers and get both their flu and COVID-19 vaccines.
“Flu is a separate virus (from the one that causes COVID-19),” Barron said. “People are going to get really sick from the flu this year. The very young, the very old and people with underlying medical conditions are at the greatest risk.”
Last year’s flu season also was very severe.
An estimated 27,000 to 130,000 people died in the U.S. due to the flu. And a record number of children — at least 279 — died of confirmed flu cases during the 2024 and 2025 flu season, according to data from the U.S. Centers for Disease Control and Prevention (CDC). Nearly all of these children previously were healthy.
Getting vaccines can help prevent the tragic deaths of young and old people alike. That’s why Barron said it’s so critical for people to get their vaccines now.
What new flu virus is going around? How did the ‘K’ variant develop, and should I worry about it?
It’s normal for viruses to change. As they spread, they mutate.
There are multiple strains of the influenza A and B viruses; however, in general, there are specific strains that emerge and cause yearly outbreaks that give rise to our traditional flu season, which can stretch from September through April.
The most common influenza A strains that are circulating these days are H1N1 and H3N2. This year, the H3N2 strain changed more than it typically does, and multiple mutations have given birth to the “K” variant
“They’re like copying errors,” Barron said. “It has a number of mutations. This is common. Flu mutates all the time.”
Unfortunately, the timing of the “K” strain’s emergence has been challenging. Vaccine makers already had created flu shots. This year’s version includes components that will allow your immune system to fight H3N2 strains, but they don’t specifically fight the “K” variant because it’s too new.
Barron says the “K” variant is complicating the ability of this year’s flu shot to fight the flu. She said it’s like mailing a package to someone but using the wrong ZIP code.
“You change one number on the ZIP code, and it could send your mail next door, down the street or 50 miles away.”
With a vaccine, the “ZIP code” tells your body exactly where to hunt down intruders that can make you sick. If the vaccine has bad directions and is hunting for an older version of the H3N2 strain, you’ll get some protection, but the vaccine may not fully protect you.
“The vaccines still work. Your body will know that the virus is present. But these mutations can evade your immune system. When the strains in the vaccine don’t match what is circulating in the community, you have a vaccine mismatch,” Barron said. “The flu shot still protects you. When it’s a mismatch, you might get sick, but the vaccine decreases the likelihood that you will be hospitalized and decreases the likelihood that you will die from complications of the flu.”
The challenges related to the “K” variant and the way that it has behaved in other countries have sparked concern for infectious disease experts like Barron. She and many other pros are keeping a close eye on how the “K” variant will affect flu cases here in the U.S.
In Colorado, when doctors test hospital patients for the flu (and other respiratory viruses), they send the lab samples to experts at the Colorado Department of Public Health and Environment. Workers at the state lab then test the samples to determine exactly which strains of the flu are spreading.
Currently, the majority of flu samples that lab workers are testing have been the subclade “K” strain. (View data about influenza and other respiratory viruses from the Colorado Department of Public Health and Environment.)
How concerned are you about the drop in flu shots and decreased rates for other vaccines?
Barron is very concerned that vaccination rates are declining, and more people will get sick and will need to be hospitalized. Some will die.
“We’re seeing a lot less uptake of the flu vaccine than we’ve seen historically, and that’s not a good trend, especially because we know these vaccines (flu and COVID-19) are very good and reduce the severity of illness,” she said.
Barron strongly encourages anyone who has doubts about vaccines to speak directly with their medical providers.
“The vaccine misinformation is not based on sound science, and it’s causing a lot of distrust,” Barron said. “My hope is that people will still consult with their providers. They know you and the nuances about your health. When they make a recommendation for you — whether it’s to get a flu vaccine or COVID-19 or a pneumonia shot, it’s because they know you and they want to keep you healthy.”
What is the greatest concern as we head into what could be a tough flu season?
Barron said it’s particularly heartbreaking to see children die of the flu.
“These were generally healthy kids. Parents may think a flu shot is not a big deal. But even healthy kids have immune systems that are not as mature. And that just makes them more vulnerable,” she said. “It distresses me greatly when parents don’t fully understand the risks and don’t get their children vaccinated.”
Katie Kerwin McCrimmon, UCHealth, Author at UCHealth Today..
