Flu season is 'early' and 'aggressive' in New York City. Here's what that means for the rest of the country.

MW Flu season is 'early' and 'aggressive' in New York City. Here's what that means for the rest of the country.

By Jaimy Lee

A new variant called H3N2 subclade K is causing most of the illness this season

A flu shot is prepared for a patient at a doctor's office in Coral Gables, Fla., in September.

A new flu variant is causing an early uptick in cases across the country, particularly in New York, Louisiana and Colorado.

For the week ending Dec. 13, the number of people testing positive for the flu increased 8.1%, and 3.2% of visits to a healthcare provider were for a respiratory illness, up from the baseline of 3.1%, according to the Centers for Disease Control and Prevention.

"This is an aggressive early season," said Michael Phillips, chief epidemiologist of the NYU Langone Health hospital system. "It's unusual to see two severe seasons back to back."

In New York City, flu tends to pick up speed around mid-December, but this year's cases came early. At NYU Langone's hospitals and clinics, 1,200 patients tested positive for the flu the week of Dec. 7, doubling the number of cases from the prior week. In Brooklyn, 30% of adults who came in with a respiratory illness tested positive for the flu that week, while 65% of the children and adults younger than 24 years old ended up having the flu, according to Phillips.

That's a trend that's reflected in national data, as well. The CDC's data show that flu cases are mostly rising in children and teens. One child has died from the flu during this year's season.

"That's common," Phillips said. "You'll start to see it first in kids before you start seeing it in adults."

The dominant variant in circulation this year is new. It's called H3N2 subclade K, which was first identified in August in Australia and is responsible for most of the flu cases in the U.S. this season. The World Health Organization has said the variant doesn't appear to increase the disease's severity, but some experts are already worried about how the flu season is shaping up.

Flu cases are up 29% in New York, 18.2% in Colorado and 16.3% in Louisiana. In New Jersey, the number of flu cases was up 17.9% for the week of Dec. 3. Hospitalizations have also spiked in all four states.

Getting a flu shot can limit how sick you get, but the number of people getting vaccinated against the flu has been declining. As of Nov. 29, 127.75 million doses of the flu vaccine had been distributed, according to CDC data. That's 24% lower than the same week in 2019, when 168.52 million doses had been distributed.

"We are seeing a fair amount of COVID, but it's just not resulting in hospitalization in young, otherwise healthy people," Philipps said. "But we are seeing people that are relatively young getting severe flu and getting hospitalized."

He encouraged people with underlying health conditions who develop symptoms to get tested as soon as possible so that they can begin treatment with an antiviral sooner.

-Jaimy Lee

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December 17, 2025 16:02 ET (21:02 GMT)

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