H3N2 "super flu" drives record flu cases in Mass., nationwide

• Massachusetts is in the midst of a winter flu surge, and state health officials are reporting rising influenza activity statewide as the season peaks.

Driving the news: The Massachusetts Department of Public Health reported thousands of residents have become ill and hospitalizations are exceeding levels typical for this time of year.

• That strains emergency departments and urgent care centers.
• Pediatric flu deaths have been reported in the state, along with dozens of adult deaths.

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.

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 the "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.

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.

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.

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.