The flu is growing rapidly, driven by new versions. Here’s what to know

WASHINGTON (AP) — The flu is on the rise across the U.S., driven by a new strain of the virus — and cases are expected to increase as the holiday season approaches.

That variant, known as “subclade K,” caused initial outbreaks in the United Kingdom, Japan, and Canada. In the US, the flu usually starts in December through March. On Tuesday, the Centers for Disease Control and Prevention reported high or very high levels of the disease in more than half of the states.

The CDC estimates 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 through Dec. 20, before major holiday gatherings.

Some states are particularly hard hit. The New York Department of Health said the week ending Dec. 20 was the highest number of flu cases recorded in the state since 2004, at 71,000 in a single week.

It’s too early to know if this flu season will be as severe as last winter.

But it’s not too late to get a flu shot, which health experts say can prevent serious illness even if someone is infected. Although this year’s vaccine is not an exact match to the subclade K strain, preliminary UK analysis has found it offers at least partial protection, reducing people’s risk of being hospitalized.

According to the CDC, about 42% of adults and children have received the flu vaccine so far this year.

What is subclade K flu?

The flu virus is shape-shifting, constantly mutating, and it comes in many forms. There are two subtypes of type A flu, and subclade K is a mutated version of one of them, called H3N2. That H3N2 strain is always tough, especially for older adults.

Mutations of subclade K are not sufficient for the change to be considered a completely new type of flu.

But they’re different enough to negate some protection from this year’s vaccine, said Andrew Pekoz, a virus expert at the Johns Hopkins Bloomberg School of Public Health.

Does subclade K make people sick?

The CDC said it’s too early to know how severe this season will be.

Flu seasons dominated by any version of H3N2 tend to be worse, with more infections overall and more people getting seriously ill. But Hopkins Pecos cautioned that it will take time to tease out whether the subclade K variant is spreading more easily or is more dangerous.

That question aside, the CDC notes that there are some prescription drugs to treat the flu — usually recommended for people at high risk of complications. But they usually need to be started a day or two after the onset of symptoms.

Who needs the flu vaccine?

The CDC and major medical societies recommend the flu vaccine for everyone 6 months of age and older. Despite a lot of recent misinformation and confusion about vaccinations, flu recommendations have not changed.

The flu is especially dangerous for people age 65 and older, pregnant women, young children, and people of any age who have chronic health problems, including asthma, diabetes, heart disease, and a weakened immune system.

Vaccines are made to protect against three influenza strains. Despite concerns over those new H3N2 variants, they appear to be a good match against H1N1 and type B flu that may still be circulating this year, Pekoz said.

There are shots for all ages, as well as Flumist, a nasal spray for ages 2 to 49. For the first time this year, some people may be eligible to be vaccinated with flumist at home.

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The Associated Press Department of Health and Science receives support from the Department of Science Education at the Howard Hughes Medical Institute and the Robert Wood Johnson Foundation. AP is solely responsible for all content.