The United States is grappling with an unprecedented outbreak of a ‘super flu,’ causing a surge in cases across major metropolitan areas and leaving some children in need of respiratory support.

This alarming situation is attributed to a novel strain known as subclade K, a variant of the influenza A H3N2 subtype, which has not previously affected humans.

According to the latest figures from the Centers for Disease Control and Prevention (CDC) for the week ending December 27, the number of states reporting ‘very high’ flu levels has escalated to 32, a significant jump from 22 states just a week earlier.

Current data indicates a concerning rise in positive flu test results, with one in three tests confirming infection, marking a 21 percent increase from one in four in the prior week. This represents a staggering 76 percent increase compared to the same period last year. Notably, New York experienced a record-breaking week with over 72,000 reported influenza cases as of December 20.

Typically, the US assesses potential flu season severity and emerging strains by observing trends in the southern hemisphere. Countries like Australia and New Zealand experience their flu seasons earlier, providing valuable insight into what might be expected in the Northern Hemisphere.

The Doherty Institute for Infection and Immunity in Australia noted that the recent flu season there deviated from the norm, with heightened flu activity persisting beyond the usual winter months and a shift in the dominant strains circulating.

The institute said toward the end of the season, a ‘fast-spreading H3N2 variant emerged and, in August, was a leading cause of respiratory related deaths in Australia.’ 

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. 

Pictured above is Sarah Lopez, 2, who was hospitalized and had to be placed on a feeding tube and ventilator after she caught the flu

There were 457,906 flu cases in Australia between January and November 2025, the most reported 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. 

Overall, subclade K viruses accounted for roughly half of the Australian viruses and more than two-thirds of the viruses in New Zealand. 

In the US, 97 percent of flu samples since September have been influenza A; of those, 86 percent were H3N2; and of those, 91 percent belonged to subclade K.  

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 the Australian season occurring before America’s, it appears the now-dominant flu strain actually originated in the US, then traveled across the globe and fueled a severe flu season in the southern hemisphere before returning to wreck havoc. 

The study continued: ‘Based on this and the genomic analysis results, the authors hypothesize that A(H3N2) K viruses in Australia and New Zealand may have been imported or originated from the United States.’

Subclade K viruses have now been detected in at least 34 countries. 

The researchers said: ‘Given the speed and size of the outbreaks of K viruses in Australia and New Zealand and the near global spread of these viruses already, it is likely that they will further expand during the northern hemisphere winter season and persist for the remainder of 2025 and into 2026.

‘Countries should be prepared for increased demands on their healthcare systems if this variant predominates, as one might expect it will, based on current global trends.’ 

The study also noted that because of subclade K’s novelty, the flu vaccine will likely be less effective against it. 

Additionally, subclade K ‘could result in increased cases and hospitalizations compared with recent years.’

With the new strain, doctors said infections are commonly starting with an unusually high temperature, before shifting into a cough that is very phlegmy and extreme fatigue.

Dr Mark Loafman, a family doctor at Cook County Health in Illinois, warned there had been ‘more fever with the flu this year than people are accustomed to,’ which can last for 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.’ 

Noah Smothers is pictured here in the center. He died last month from influenza-related complications at just 14 years old

Dr Juanita Mora, national spokesperson for the American Lung Association, told NBC Chicago that the fevers may not always respond to usual over-the-counter treatments such as Tylenol and Ibuprofen.

She said in a statement: ‘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.’

In serious cases, doctors warn that the symptoms can also leave patients struggling to breathe and needing a ventilator.

In one case, two-year-old Sarah Lopez from north Georgia has been hospitalized for weeks after she was infected with the flu.

Her mother Kenia, told WSB-TV 2: ‘When I would say come here, she wouldn’t do it either. She would just throw her upper torso.

‘Overnight, everything got worse. She couldn’t sit up on her own, she couldn’t talk, she couldn’t move pretty much anything, just her head a little bit.’

Lopez was hospitalized with the flu, but then developed transverse myelitis, a rare condition where a patient suffers from inflammation in part of the spinal cord. Symptoms include back pain, difficulty moving and temporary paralysis.

At the hospital, doctors put her on a ventilator to help her breathe and a feeding tube to give her nutrients. 

Speaking on Wednesday, her mother said that Lopez has now recuperated some strength and movement in her legs, but still struggles to stand up. 

She has been taken off the ventilator, and the hospital is now considering transferring her to a rehabilitation unit.

Her parents, who are caring for six other children, have been unable to work during this time. The family is asking for help to raise $11,000 to cover medical expenses. 

It was not clear whether Lopez had received the flu vaccine. She tested positive for influenza A, the same type that includes the subclade K variant.

Of the nine child deaths so far this viral season, four have been reported in Massachusetts and one in Illinois. It is not clear where the others took place.

Overall, officials estimate there have been at least 11million illnesses, 120,000 hospitalizations and 5,000 deaths from flu so far this year.

For comparison, at the same time last year, the CDC estimated there had been 5.3million illnesses, 63,000 hospitalizations and 2,700 deaths. There had also been 11 deaths among children.