(NewsNation) — Cities across the nation are being hit with a new variant of influenza called subclade K, which has been dubbed a “super flu,” and certain states and cities, like New York City, are seeing the worst of it.

As the nation’s largest city preps for the holiday tourist rush, local health officials reported nearly 14,000 flu cases in the first week of December.

That’s a roughly 460 percent increase in infections from the same time last year, when the city recorded about 2,500 cases.

“We see the sudden spike in New York because of their more dense population. They’re a center for international as well as domestic travel,” Dr. Ulysses Wu, chief epidemiologist at Hartford Hospital in Connecticut, told NewsNation.

Dr. Andrew Pekosz, an infectious disease specialist with the Johns Hopkins Bloomberg School of Public Health, says the data shows that the “super flu” strain, or subclade K, is spreading everywhere that influenza is.

“Colorado, Louisiana and New York are [states] that are experiencing really fast increases in influenza,” Pekosz said Tuesday during a public health media briefing. “But even in places like Maryland right now, where cases are not that high, it’s the [sub]clade K virus that seems to be the dominant virus.”

Even with a slower start, 2024 was part of the “most severe” influenza season since 2017, according to the Centers for Disease Prevention and Control (CDC).

This flu season started early and has made its mark around the globe, despite peak season still months away.

Doctors warn the particularly nasty, mutated version of influenza — the so-called “super flu” — is already circulating in most corners of the country.

What is the new subclade K strain of influenza A?

The new variant emerged over the summer and rapidly spread in several countries, including Japan, the United Kingdom and Canada.

It’s a mutation of the H3N2 strain of influenza A, described by the World Health Organization as a seasonal respiratory infection caused by a virus.

Pekosz said in two to three months, by the end of flu season, every state will experience high amounts of influenza activity.

“We just don’t know exactly when each state will start and end in terms of those outbreaks,” Pekosz said.

‘Super flu’ symptoms: What to know about subclade K

Symptoms are similar for both influenza A and its subclade K variant, both of which are more typically more serious than influenza B.

Signs include fever, cough, sore throat, congestion, body aches, headaches and fatigue, according to the CDC.

“Influenza A typically causes worse symptoms compared to influenza B, and patients are more likely to get hospitalized with influenza A compared to influenza B,” Dr. Donald Dumford, an infectious disease specialist at the Cleveland Clinic, told Nexstar.

How to prevent ‘super flu’ subclade K variant

Despite subclade K developing after this year’s flu shot had already been created, doctors are urging people to get vaccinated.

“One hundred thousand Americans died last year from a very robust influenza season,” Wu said. “There are prevention methods like vaccines, immunotherapy that can help prevent complications.”

Wearing a mask, covering your coughs and sneezes, washing your hands and cleaning frequently touched surfaces can also slow the spread. If you get sick, doctors advise avoiding contact with other people and staying home for at least 24 hours.

“It is certainly not too late, especially as we’re thinking about holiday gatherings and travel,” said Pekosz. “Now would be a great time for individuals to go out and get that last influenza vaccine or that COVID vaccine to provide one layer of protection for this holiday season.”

NewsNation’s Steph Whiteside and Anna Kutz contributed to this report.

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