A mutated virus is pushing the United States into an early flu season, especially on the West Coast.

Flu season is starting months earlier than expected with a mutated virus spreading in parts of the United States, surprising some people who may not even be thinking about their annual flu shot yet.

This year the season started 2 and a half months earlier than usual and hit the West Coast states first, which is unusual, said Dr. Alex Greninger, chief of the infectious disease diagnostics division at the University of Washington’s department of laboratory medicine.

Usually, flu season in the United States begins in the South – often in Florida, where it arrives from the Southern Hemisphere. Then it tends to spread northward as fall and winter progress.

“In Seattle, we’re pretty far north. We see a pretty big spike in January,” Greninger said, noting that the bulk of their cases usually occur between late December and February.

That’s not how it’s going this year.

About 1 in 10 people tested for the flu in Washington state last week tested positive, according to data from the state Department of Public Health.

“We basically saw January flu levels last week,” Greninger said.

Walgreens says that nationally, this flu season is about four weeks ahead of recent seasons and that it is dispensing more prescriptions for antivirals as well as cough and cold medications than during the same period last year.

Nationally, about 2.1% of doctor visits last week were for flu-like illness, a rough measure of the misery that flu symptoms like fever and cough cause across the country. However, according to data from the U.S. Centers for Disease Control and Prevention, that figure remains below the national benchmark, meaning the virus hasn’t really taken off everywhere yet.

“All the Pacific states, I would say, started their season very early, but we are now seeing, as the season progresses, an eastward spread, and so all regions of the country are seeing increases at this point,” said Dr. Caitlin Rivers, an epidemiologist who directs the Center for Epidemic Response Innovation at the Johns Hopkins Bloomberg School of Public Health.

In the region that includes Arizona, California, Hawaii and Nevada, the proportion of positive flu tests reached at least 10% last week, indicating significant levels of flu. The South is quickly catching up to those numbers, said Rivers, who tracks the flu closely through his Force of Infection newsletter.

The flu wave hitting Pacific states came from Asia, Greninger said, and is dominated by the H1N1 virus, which sparked a pandemic when it emerged in 2009.

That represents about 98% of the samples his lab currently receives, he said.

These H1N1 viruses have also detected a genetic mutation that makes them better able to evade our immunity.

This version of the virus has already wreaked havoc in Japan, where it forced the closure of schools and daycares after a surge in cases among children. This has also prompted authorities to increase vaccination clinics to help protect the population.

The mutation, at a place called G155E, which emerged after strains had already been selected for this year’s flu vaccines, Greninger said. So even though vaccines target one version of H1N1 and should still provide some protection, the virus can still spread.

Greninger notes that the Covid-19 pandemic has contributed to the development of new tools that allow us to track the spread of virus variants in almost real time.

“We’re getting closer and closer to the weather, aren’t we? he said. “We can get the sequence quickly, so we can tell exactly what the mutations are. What this really emphasizes is that we need to be able to make these vaccines faster.”

The strains used in this year’s flu vaccines were chosen in the spring, before the new virus variant became a threat. They are selected so early because the majority of flu vaccines in the United States are still grown in chicken eggs, a practice that persists, Greninger added, because it is very cheap.

There is a recently approved flu vaccine that uses mRNA technology, and he said it has the potential to make better vaccines available more quickly. Whether it will ever replace egg-based vaccines is another question: an economic question.

Doctors often advise getting a flu shot before Halloween to provide the best possible protection until spring. But Greninger’s best advice now is to get vaccinated as soon as possible. “A lot of people, like myself, would wait until the end of October, which is when we’re kind of already there. But in reality, the time to get vaccinated was actually two to three weeks ago.”

Other experts agree.

“It’s very early and so I’m concerned about the vaccination schedule,” Rivers said. “People don’t think about getting a flu shot this early in the season.”

Whether this will make the overall season more severe remains to be seen.

Greninger said there could also be a second wave of flu cases after the H1N1 peak.

“I wouldn’t be surprised to see a second flu season, like a normal flu season. You’ll have cases of H1N1 flu peaking in 2026, and then who knows what happens in January, February or March,” he said. “So maybe interesting to watch.”