A new ‘super flu’ racing across the US can cause a week-long fever and lingering illness, doctors warn, with experts saying the surge has not yet peaked.
The newly identified subclade K strain, a never-before-seen variant of influenza A H3N2, is spreading fast, with the most recent CDC data showing ‘very high’ illness levels in 32 states.
At least nine children have died from the flu this year, as brutal symptoms leave patients struggling to breathe and hooked up to machines.
Doctors have since warned that the highly contagious variant is bringing harsher symptoms, with more intense fevers being reported nationwide – particularly among children, according to NBC 5 News.
‘More fever with the flu this year than people are accustomed to,’ Dr Mark Loafman, the chair of Family and Community Medicine at Cook County Health, told the outlet.
‘The fever can last up to seven days, so five to seven days,’ he added. ‘And that’s worrisome.’
‘You feel ill, you feel sick and you worry that you’re not getting better.’
New data has revealed a grim reality: one in three flu tests now return positive, a 21 percent weekly jump and a shocking 76 percent surge from last year.
The newly identified subclade K strain, a never-before-seen variant of influenza A H3N2, is spreading fast, with the most recent CDC data showing ‘very high’ illness levels in 32 states
Dr Mark Loafman (pictured), the chair of Family and Community Medicine at Cook County Health, told NBC Chicago that the ‘worrisome’ fevers can last up to a week and that the surge has not yet peaked
At least nine children have died from the flu this year, as brutal symptoms leave patients struggling to breathe and hooked up to machines. Pictured is Sarah Lopez, 2, hospitalized with flu
Since September, 97 percent of US flu samples have been influenza A, with 86 percent of those H3N2 – and 91 percent of the H3N2 samples belonging to subclade K.
‘The question is, where will it peak?’ Loafman told the outlet, warning that even though current levels aren’t extreme, the virus could spread much further.
‘These are contagious viruses and people are indoors and we’ve had a lot of holiday travel and people together,’ he added. ‘So we would expect this spike would continue over the next few weeks.’
Dr Santina Wheat, a family physician with Northwestern Medicine, also sounded the alarm that flu cases could surge next week as students head back to school after the break.
Fevers are among the symptoms showing a change in the virus’s usual pattern, with experts cautioning that in some patients, the fevers resist treatment with Tylenol or Motrin.
According to Dr Juanita Mora, national spokesperson for the American Lung Association, it is ‘one of those high fevers that won’t break.’
‘The other thing is any signs of shortness of breath,’ Mora told NBC.
‘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,’ she added. ‘That’s a sign to go to the ER.’
New data has revealed a grim reality: one in three flu tests now return positive, a 21 percent weekly jump and a shocking 76 percent surge from last year
She explained that patients are suffering from a relentless ‘phlegmy cough,’ vomiting, diarrhea and intense joint and muscle aches.
Vomiting is usually a flu symptom seen in kids, but Loafman said that the new variant is sending more adults to the ER with GI problems.
‘Children with flu do often have GI symptoms – nausea, vomiting – adults less often, but we are seeing anecdotally more signs of some GI illness for the adults who have the subclade K strain of flu,’ the doctor told the outlet.
He cautioned that people with gastrointestinal issues shouldn’t rule out the flu and suggested confirming with an at-home test.
‘The third sign I always teach patients: dehydration,’ Dr Mora told NBC.
‘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,’ she added.
New York recorded over 72,000 flu cases in the week of December 20 – a historic high. Across the Chicago area, the illness has also surged to ‘very high’ levels, the CDC’s most severe category of respiratory activity.
As hospital admissions explode, the Illinois Department of Public Health and other state agencies have issued warnings, noting that most ER visits are tied to severe respiratory illnesses caused by the ‘super flu,’ according to NBC Chicago.
Vomiting is usually a flu symptom seen in kids, but Loafman said that the new variant is sending more adults to the ER with GI problems
Experts explained that patients are suffering from a relentless ‘phlegmy cough,’ vomiting, diarrhea and intense joint and muscle aches
The spike in flu cases across Illinois is taking a serious toll on children, particularly those aged 5 to 17, who are seeing the highest number of hospital visits from the new strain and the acute illnesses it causes.
On Monday, the state confirmed its first flu-related death in a child this season.
‘Vaccinations remain the most effective tool to prevent severe illness from flu, COVID-19 and RSV,’ Dr Sameer Vohra, IDPH director, said in a statement.
The current flu shot wasn’t made for the new variant, but Loafman stressed that receiving it can still help protect against more severe illness.
‘There has been a lot of talk about the fact that the new variant is not well-covered by it. That is true, but you still get protection,’ Loafman told NBC.
‘What we typically see, the folks that are sick, hospitalized, are those who were not vaccinated,’ he added.
‘The vaccine isn’t perfect. It doesn’t protect all aspects of this from folks. It doesn’t keep you necessarily from getting the flu as often as we would like, but it certainly mitigates the severity of illness.’
He explained that people who get the flu shot tend to fare better when battling the illness: their symptoms are less likely to worsen, they are less likely to be hospitalized or die and they are more likely to recover quickly.
The current flu shot wasn’t made for the new variant, but Loafman stressed that receiving it can still help protect against more severe illness
Despite hitting Australia first, the dominant flu strain seemed to have originated in the US, traveled around the globe to drive a harsh southern hemisphere season and has now returned to wreak havoc in America
‘It’s never too late to get it until the flu season’s over – so late March, April, sometime like that,’ he said.
Even if you’ve already had the flu, experts warned that getting the vaccine is important because reinfection is a real risk.
‘There are three strains going around and probably a few more. So you can still be protected from the future,’ Loafman told the outlet.
A study published last month in the journal Eurosurveillance found the H1N1 strain of influenza A was initially dominant in Australia, but in August and September, influenza A H3N2 began to rise and was dominant by October and November.
Australia has seen 457,906 flu cases between January and November 2025, the highest on record since flu became a reportable disease in 2001.
Meanwhile, New Zealand experienced a more moderate flu season but still prolonged, fueled by H3N2 K viruses, which were introduced from Australia, the study found.
In investigating where subclade K came from, researchers discovered that the first K influenza virus sequenced in June 2025 was actually from the US, more specifically, New York.
It was then detected in Wisconsin and Michigan in July 2025. The study noted: ‘This coincided with the first detections of clade K viruses in Australia.’
Despite hitting Australia first, the dominant flu strain seemed to have originated in the US, traveled around the globe to drive a harsh southern hemisphere season and has now returned to wreak havoc in America.
