An early start to the flu season means it might be time to consider a flu shot.

Rebecca Blackwell/AP

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Rebecca Blackwell/AP

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Fall has only just begun, but it’s already time to start thinking about the quintessential winter bug: the flu.

The annual flu season usually starts in October or November. But this year, it looks like the flu started to pick up in early September.

“There are enough signs pointing in the same direction to make me think, ‘OK, yes. This is the start of flu season,'” says Caitlin Rivers, an epidemiologist at the Johns Hopkins Bloomberg School of Public Health.

The percentage of people testing positive for the flu in the West has been rising steadily since around the beginning of September, she says. And the number of people showing up in emergency departments because of the flu has also been rising for weeks, she adds.

“It’s very uncommon to see flu activity rising this early. It’s activity that we might normally see more like November or December,” she says.

The timing of the flu season isn’t the only thing that’s odd. It’s also unusual that flu activity seems to have started in Western states, such as California, Washington, Hawaii and Alaska.

“It typically starts in the South and then expands from there,” Rivers says. “So two uncommon developments there.”

Rivers stresses that the amount of flu activity is still very low in most parts of the country. But that’s starting to change as the flu picks up nationwide.

It’s unclear why flu activity would have started so early, and in such an unusual part of the country. But one clue might be a new variant that evolved a mutation, which appears to make it better at evading existing immunity, especially among teenagers.

“That’s our best argument for what’s going on right now in terms of this early flu season,” says Dr. Alex Greninger, a virologist who heads infectious disease diagnostics at the University of Washington. Doctors there are seeing as much flu right now as they usually would around Christmas, and the mutated variant appears to be common, he says.