Nearly 800 Coloradans were hospitalized with the flu last week — the highest number in at least five years — as cases surge across the U.S., according to the latest state data.
Emergency departments and doctors’ offices are also seeing more patients with influenza, the data showed, as a new variant kicked off an early flu season before the holidays.
Medical experts expect flu cases to continue to rise as people have been gathering with their families and friends in recent weeks.
“We’re not out of the worst of it yet,” said Dr. Mark Montano, medical director for HCA HealthOne‘s CareNow Urgent Care. “…We are still going to see a couple more weeks of rising flu rates.”
At least 782 people were hospitalized with the flu during the week that ended Dec. 27, which is up by 247 patients from the previous week — and more than twice the number of people hospitalized with the flu during the same period last year, according to data from the Colorado Department of Public Health and Environment.
Influenza cases are rising across the U.S., driven by a new variant known as “subclade K” and fewer people getting their flu shots, medical experts said.
There are two subtypes of Type A flu, and subclade K is a mutated version of one of them, named H3N2. That H3N2 strain is always harsh, especially for older adults.
Flu symptoms are the same with the new strain, including cough, congestion, and fever.
But patients are showing more significant signs of fatigue and body aches with subclade K, which can lead to complications, such as dehydration, that result in a person being hospitalized for the flu, said Dr. Jennifer Corliss, medical director of UCHealth‘s Lone Tree Primary Care clinic.
Subclade K’s mutations aren’t enough of a change to be considered an entirely new kind of flu. But they’re different enough to evade some of the protection from this year’s vaccine, said Andrew Pekosz, a virus expert at Johns Hopkins Bloomberg School of Public Health.
The vaccine still lowers the risk that a person will be hospitalized with the virus, Montano said.
“It’s still worthwhile to get (the shot),” he said. “It will give you some protection.”
In the U.S., flu typically begins its winter march in December. On Tuesday, the Centers for Disease Control and Prevention reported high or very high levels of illness in more than half the states.
The CDC estimated there have been at least 7.5 million illnesses, 81,000 hospitalizations and 3,100 deaths from the flu so far this season. That includes at least eight child deaths, and is based on data as of Dec. 20, before major holiday gatherings.
Some states are particularly hard-hit. New York’s health department said the week ending Dec. 20 marked the most flu cases the state had recorded in a single week since 2004: 71,000.
In Colorado, at least 5,959 people tested positive for the flu during the week of Dec. 27. One child has also died from the virus in Colorado since the 2025-26 flu season began, according to the state health department.
About 27% of Coloradans have received the flu vaccine this year, which is down from 28.5% last year, according to the state health department.
“There have been a lot of changes in the government and CDC that may have led to patients questioning the vaccine,” said UCHealth’s Corliss.
The CDC recommends that everyone 6 months and older receive the flu vaccine. It takes two weeks for the shot to become fully effective.
People should also wash their hands, disinfect surfaces and wear a mask if they want to lower their risk of getting the flu, medical experts said.
The Associated Press contributed to this report.
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