What to know about the "super flu" and the unprecedented spike in flu cases

• It also arrives as chaos has enveloped the Centers for Disease Control and Prevention, with many questions about vaccination schedules for children.

Driving the news: Cases of the flu remain elevated nationwide, according to data from the CDC.

• New CDC data shows that there's been at least 7.5 million illnesses, 81,000 hospitalizations and 3,100 deaths from the flu so far this season.
• Many of these cases have been tied to "subclade K" — a variant of the H3N2 virus, which is a subtype of influenza A.

Experts and patients say subclade K is an example of the "super flu," referring to a strain that spreads quickly and rapidly, becoming more troublesome.

• "Severity indicators remain low at this time, but influenza activity is expected to continue for several weeks," the CDC says.

Here's what to know...

Where the flu is surging

The flu has been raging coast to coast in the United States.

• New York saw its highest number of positive flu cases ever reported in a single week for the week ending on Dec. 20, per the New York State Department of Health.
• Connecticut has seen cases climb "to highest levels doctors have ever seen," per WFSB-TV, a local news outlet.
• In California, flu cases have increased statewide. And Dawn Terashita, who monitors acute communicable disease at LA County's Department of Public Health, told ABC-7 that "it is scary to hear that this year might be worse" than last year.

Globally, European countries — especially England and Ireland — saw record cases as well, with areas masking up to stay safe.

• Some health experts compared the flu wave in Europe to the COVID pandemic because hospitals, facing mounting pressure from infected patients, asked visitors to mask up and practice hand hygiene to slow the spread.

What to know about the H3N2 strain, "K" variant

The dominant flu strain right now, a version of H3N2, first emerged over the summer. In the fall, health officials in the U.K. and Canada warned that the new strain was causing a rise in hospitalizations in their countries.

• Experts were specifically worried that the current flu vaccine uses the "2024-25 subclade J" and "updated 2025-26 subclade J.2" as reference strains instead of subclade K, which is fueling the H3N2 strain.
• The subclade K strain features seven different mutations compared to the others, experts said.

Yes, but: It's fairly common for vaccines to have mismatches compared to the dominant strain.

• There is data from U.K. health officials suggesting there is some vaccine effectiveness against the H3N2 strain.

What to know about the "super flu"

Reality check: There isn't an official "super flu."

• The term emerges every so often, typically when there's a more severe than usual strain of the flu circulating, experts say.

Zoom out: This time around, subclade K is being associated with the term.

• The most recent CDC tracking data shows that nearly 90% of new flu cases in the country were from subclade K, which appears to be the super flu this time around.

What they're saying: "Super flu" is "a sensationalistic term that doesn't really have any medical meaning," Amesh Adalja, an infectious disease physician at Johns Hopkins University, told Axios.

• Since subclade K "is a variant to which the population has little immunity and [is] not covered completely by the current vaccine, many people are susceptible and are getting infected," he said.
• "There is no indication that this is any more severe than other strains (though H3N2 strains tend to be more severe themselves) and the vaccine protects against what matters, severe disease. Also, the antivirals are completely effective against it."

Context: Andrew Pekosz, a virologist at Johns Hopkins University, said that "we've had influenza viruses like clade K before" and also wouldn't call it a super flu.

• "While we expect more flu cases this year, we aren't sure if this virus causes more severe disease on a per case basis yet," he said. "However, it is spreading so quickly that the increased numbers of influenza cases it causes can swamp a medical center with cases quickly. "

Common symptoms for the "super flu"

Symptoms associated with H3N2 aren't different from those of the common flu, including:

• Sore throat
• Runny nose
• Fever
• Cough
• Headache
• Muscle aches
• Tiredness
• Sweating
• Chills

What to watch for: Common colds often begin very slowly and emerge over several days, per the Mayo Clinic. But a case of the flu ignites within two or three days of coming into contact with the virus.

Flu treatments and vaccines

The bottom line: Pekosz recommends getting a flu shot for those who haven't.

• It takes around 10–14 days from getting the vaccine to build an immune response, which helps fight infection or reduce disease severity.
• It will protect well against H1N1 and H3N2 while giving "some protection against severe disease from clade K."

More from Axios:

• Fewer kids are getting their flu shots
• CDC formally approves RFK Jr. vaccine panel recommendations
• Workplace insurance stymied by shifting vax rules