A strain of the influenza virus that’s not a precise match for the vaccine has been spreading in the U.K., Canada and Japan, among other countries, and could sicken many in the U.S. this flu season.

“The flu is on our doorstep, and soon it will be knocking on our door,” said Dr. Dora Anne Mills, chief health improvement officer for MaineHealth. “It may be a more severe season than normal.”

Mills said H3N2 tends to be a more potent strain, especially in older adults, which can result in more hospitalizations and illness in long-term care facilities.

Maine reported 17,793 flu cases during the 2024-25 flu season, which runs from October through May, according to the Maine Center for Disease Control and Prevention. So far this year, Maine has recorded 90 cases of influenza.

Scientists each February predict what the predominant circulating strains will be for the following influenza season. Because it takes time to manufacture and distribute the vaccines, sometimes the predictions are not exact.

“Every once in a while we see a shift in the virus that makes the vaccine not a great match,” said Anna Krueger, a Maine CDC epidemiologist.

But the message from public health officials hasn’t varied: Get your flu shot, if you haven’t already.

The dominant circulating strain in the Northern Hemisphere is Influenza A H3N2, according to the World Health Organization. Small variations in the H3N2 strain – called subclades – took scientists by surprise when they emerged at the end of the flu season in the Southern Hemisphere.

But that doesn’t mean the flu vaccine doesn’t work at all. It will still offer protection, especially from severe illness, experts say.

Also, Krueger said, other strains of the flu virus, such as Influenza A, H1N1 or Influenza B, may become the predominant strain at any time during flu season.

“It’s too early to tell if any one strain will dominate,” Krueger said. “There’s no typical flu season, but many times we will see Influenza A in the early season, and Influenza B later.”

Flu cases caused by H3N2 have tripled in the U.K., according to news reports.

“Since it’s emerged, (H3N2) is rapidly spreading and predominating in some countries so far in the northern hemisphere,” Dr. Wenqing Zhang, head of the World Health Organization’s Global Respiratory Threats Unit, said Wednesday during a media briefing.

Krueger said people can get the flu shot and the updated COVID-19 vaccine during the same appointment. Both vaccines are widely available at doctor’s offices and pharmacies.

In addition to getting vaccinated, to avoid flu, “stay home when sick and practice good hand hygiene.”

Vaccinations are available to protect against RSV, another seasonal respiratory illness. RSV vaccines are available for those 50 and older, and those who are between 32 to 36 weeks pregnant.