There are some unusual symptoms being reported with a new and mutated flu variant quickly spreading in the U.S. ahead of holiday gatherings.
According to Dr. Juanita Mora, the national spokesperson for the American Lung Association, the new “subclade K” variant is hitting kids and adults “hard.”
“We have to be careful,” she said in an interview with Telemundo Chicago. “Just today I’ve got 15 calls with flu positive patients.”
Experts have been warning for weeks of a mutated variant that could make for a particularly challenging flu season. Meanwhile, cases of norovirus, or the “stomach flu,” are also climbing.
Flu symptoms associated with ‘Subclade K’
The new strain is leading to some even bigger complications when it comes to cases, particularly in children.
“This new strain has symptoms of really high fevers, it has a really bad cough that won’t go away, very phlegmy, and also vomiting and diarrhea and lots of joint aches as well as muscle aches,” Mora said.
She noted that in many cases, fevers are being reported between 104 and 105 degrees, and they “don’t break – even when treated with Tylenol, Motrin, etc.” They are also lasting longer.
Flu symptoms typically begin within one to four days of exposure, according to the Cleveland Clinic.
The CDC reports the following symptoms for those with flu:
fever* or feeling feverish/chills cough sore throat runny or stuffy nose muscle or body aches headaches fatigue (tiredness) some people may have vomiting and diarrhea, though this is more common in children than adults*It’s important to note that not everyone with flu will have a fever
Warning signs to watch for
Because the new strain is bringing higher and longer lasting fevers, Mora said it’s important to know the warning signs to seek medical attention.
“One is those high fevers that won’t break,” she said. “Two, the other thing, is any signs of shortness of breath. So when kids or adults are having trouble breathing, they start using their chest muscles, they start wheezing audibly, they start having a cough that won’t go away. That’s a sign to go to the ER. And the third sign I always teach patients: dehydration. So when a kid is hit very hard with the flu or an adult, especially older adults, they don’t want to eat or drink so they’re just laying in bed but that can be very dangerous.”
According to the CDC, the warning signs are different in children and adults.
They include:
In children
Fast breathing or trouble breathing Bluish lips or face Ribs pulling in with each breath Chest pain Severe muscle pain (child refuses to walk) Dehydration (no urine for 8 hours, dry mouth, no tears when crying) Not alert or interacting when awake Seizures Fever above 104 degrees Fahrenheit that is not controlled by fever-reducing medicine In children younger than 12 weeks, any fever Fever or cough that improve but then return or worsen Worsening of chronic medical conditionsIn adults
Difficulty breathing or shortness of breath Persistent pain or pressure in the chest or abdomen Persistent dizziness, confusion, inability to arouse Seizures Not urinating Severe muscle pain Severe weakness or unsteadiness Fever or cough that improve but then return or worsen Worsening of chronic medical conditionsHow long is flu contagious?
Flu can be detected in infected persons at least one day before symptoms begin, and up to seven days after, but those with flu are most contagious around three days into their infection, according to the CDC.
“Some people, including young children and people with weakened immune systems may be contagious for longer periods of time,” the agency reported.
How bad is the season so far?
“I don’t think I ever remember seeing it this severe, this soon,” said Dr. Suchitra Rao, an infectious disease specialist at Children’s Hospital Colorado in Aurora. “Our emergency department is full of kids coming in with the flu.”
Some of the nation’s biggest hot spots for cases are in the Midwest.
While some Midwest states are seeing a “high” number of influenza cases, according to recent data from the Centers for Disease Control and Prevention, Illinois cases remain “minimal.” The Illinois Department of Public Health reported, however, that the latest metrics are showing a rise in cases.
Data from Dec. 7-13 showed Illinois hospital admissions for flu rose to 1%, up from 0.74% the week earlier. That’s still significantly lower from the high of 6.9% reported in February 2025.
Doctors in the Chicago area have also reported seeing a rise in cases, and with at-home testing available, many cases may go unreported.
While testing for the specific variant also isn’t as common, Influenza A is the dominant strain spreading in the U.S. right now. Even more concerning, 89% of the H3N2 infections analyzed earlier this month were the subclade K variant.
What can you do?
Doctors said current vaccines are expected to offer protection against serious illness, particularly when it comes to children, but the vaccines were not formulated with this variant in mind.
“Because we didn’t know about [this mutation] before the influenza vaccine was created for this season, that variant is not in our flu vaccine this year. So it does make that vaccine likely to be less effective, but not ineffective,” Welbel said, adding that “the one thing that would make a difference based on what we’re seeing in the UK is if we have a population that’s well vaccinated and also practicing good respiratory hygiene around others to prevent the spread to those who are most vulnerable.”
Illinois Department of Public Health Chief Medical Officer Dr. Arti Barnes stressed that the vaccine is expected to still offer some protection, though she noted the new strain is still believed to be more contagious.
“This strain is supposed to be more associated with severe flu season, very rapid spread of the infection. So we think it is very likely more transmissible or more infectious. We don’t know yet if it’s more severe,” she said.
Flu seasons often don’t peak until around February, so it’s too early to know how big a problem that mismatch will be.
Flu vaccines are available in the U.S. for anyone 6 months and older and Mora said it “remains your biggest protection” this season.
Her other top recommendations include frequent handwashing and wearing a mask if sick.
“Don’t go kissing that newborn baby or grandma or grandpa at this time,” she said. “Make sure that we protect our most vulnerable.”
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