Flu Season Started Early in Parts of the U.S., and Experts Say You Should Consider Getting the Seasonal Jab ASAP. Here’s What to Know

The season usually starts in October or November. But some western states, including California, Washington, Alaska and Hawaii, began seeing a rise in flu cases as early as August

Sara Hashemi | Daily Correspondent

October 8, 2026 11:24 a.m.

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Every year, autumn brings gorgeous foliage, crisp air, shorter days—and a slew of respiratory illnesses. Flu season usually starts in October or November, but infection data suggests it has kicked off early in parts of the Northern Hemisphere.

Flu rates began rising in August or September in many western states, including California, Washington, Alaska and Hawaii. That’s unusual because the respiratory illness typically starts in the Southeast and spreads toward the West Coast. Infection rates have also been unusually early in places like Japan and the United Kingdom.

We’re “looking at an early flu season this year,” said James Watt, deputy director of infectious diseases at the California Department of Public Health, at a briefing on October 6, reports Catherine Ho at the San Francisco Chronicle.

This early rise is concerning because people haven’t started getting their vaccines yet, says Danielle Zerr, an infectious-diseases specialist at Seattle Children’s, to Lena H. Sun at the Washington Post. “The way flu is circulating now, you wanted that vaccine in you two weeks ago, if not even a little before that.”

Here’s what you need to know about this year’s early flu season.

What is the flu?

The flu is caused by an infection with an influenza virus. There are two subtypes of Influenza A—H3N2 and H1N1—and one type of Influenza B, according to the Centers for Disease Control and Prevention (CDC). The severity of each season depends on which version of the virus is spreading. H3N2 influenza strains are linked to more severe flu seasons and more hospitalizations. This year, the other one, H1N1, is dominating.

Flu spreads via droplets spewed when an infected person coughs, sneezes or talks. Symptoms generally appear one to four days after infection. Signs of illness can include fever or feverish chills, cough, sore throat, runny nose, muscle aches and fatigue. A sick individual is most contagious during their first three days of symptoms.

Why is this year’s flu season early?

Experts don’t know for sure, but one possibility is that a new variant of H1N1 evolved a mutation that is better at evading some people’s immune systems. “That’s our best argument for what’s going on right now in terms of this early flu season,” says Alex Greninger, a virologist at the University of Washington, to Rob Stein at NPR.

Another reason might be that because H3N2 dominated last year’s flu season, people have less recent immunity against the version that’s currently running rampant, Helen Chu, an infectious-diseases specialist at the University of Washington, tells the Washington Post.

It’s still too soon to tell what this year’s season will look like. Looking at the Southern Hemisphere can offer clues, since our seasons are flipped, writes Katelyn Jetelina for Your Local Epidemiologist. “Flu season was severe in many places this summer,” she explains, with Argentina recording its highest number of flu-like illness cases in a decade. That doesn’t necessarily mean things will look the same in the Northern Hemisphere, but it’s something epidemiologists are keeping an eye on.

What’s more, “an early start doesn’t necessarily mean a more severe season,” says James Hay, an epidemiologist at the University of Oxford in England, to Nature’s Mariana Lenharo.

How can I protect myself?

Flu vaccines for 2026-27 are already available, and the best thing you can do is get your shot as early as possible—even if you got one last year. The vaccine is adjusted every year to target the strains that researchers predict will be circulating that season. While it’s hard for them to get it exactly right, vaccines still offer an important level of protection that can reduce the severity of the illness.

“This is one of those seasons where the key message is to go get vaccinated now,” Greninger tells Nature.

Sara Hashemi is a science writer and fact-checker currently based in New York City. Her work has appeared in Sierra, The Body, Maisonneuve magazine and more.