A mutated influenza virus is pushing the United States into flu season months earlier than usual, catching people before many have considered getting their annual shots. The unusual surge struck the West Coast first and is now spreading eastward as infections rise across every region of the country.

This season began 2 1/2 months earlier than is typical, according to Dr. Alex Greninger, head of the Division of Infectious Disease Diagnostics at the University of Washington’s Department of Laboratory Medicine. He said the early concentration of cases in West Coast states was also unusual.

The American flu season typically begins in the South, often in Florida after the virus arrives from the Southern Hemisphere. It then tends to move north as fall and winter continue, but this year’s opening pattern has been dramatically different.

Seattle, by contrast, normally experiences its strongest flu activity in January, with most cases appearing from late December through February. “In Seattle, we’re pretty far north. We see a pretty strong peak in January,” Greninger said.

Washington state is already confronting activity that resembles the middle of winter. Roughly 1 in 10 people tested for influenza in the state last week received a positive result, according to data from the state Department of Public Health.

“We’re basically seeing January levels of flu in the last week,” Greninger said. Walgreens reported that the national season is running about four weeks ahead of recent seasons, while the company is dispensing more antiviral prescriptions and cough and cold medications than it did during the same period last year.

Across the country, about 2.1% of doctor visits last week involved illnesses resembling influenza, a measure reflecting symptoms such as fever and cough. That figure remained below the national baseline reported by the US Centers for Disease Control and Prevention, indicating that the virus had not yet taken off everywhere.

Dr. Caitlin Rivers, an epidemiologist who directs the Center for Outbreak Response Innovation at Johns Hopkins Bloomberg School of Public Health, said Pacific states started their seasons exceptionally early. She said the season is now moving eastward, with every region of the country recording increases.

Greninger said the wave striking Pacific states came from Asia and is dominated by the H1N1 virus, which first emerged in 2009 and sparked a pandemic. H1N1 currently represents about 98% of the samples reaching his laboratory.

The circulating H1N1 viruses have acquired a mutation that makes them better able to evade existing immunity. The version has already caused disruption in Japan, where surging infections among children forced schools and day care facilities to close and prompted officials to move vaccination clinics forward.

The mutation, identified at a location called G155E, emerged after the strains for this year’s flu shots had already been selected. The vaccines target a version of H1N1 and should continue to provide some protection, Greninger said, although the changed virus can still break through.

Tools developed during the Covid-19 pandemic now allow scientists to follow the spread of viral variants almost in real time. “We can get the sequence quickly, so we can say exactly what the mutations are. What this really puts the focus on is, we’ve got to be able to make these vaccines faster,” Greninger said.

The strains included in this year’s shots were chosen during the spring, before the new variant became a threat. Selection happens so early because most flu vaccines used in the United States are still grown in chicken eggs, a practice Greninger said persists because it is inexpensive.

A newly approved flu shot using mRNA technology could potentially make improved vaccines available more quickly, according to Greninger. Whether that technology eventually replaces vaccines grown in eggs remains a separate economic question.

Doctors often recommend receiving a flu shot before Halloween to maintain protection through spring, but Greninger now advises vaccination as soon as possible. “But really, the time to get vaccinated was actually two to three weeks ago.”

Rivers expressed concern that people may not be considering vaccination because the season arrived so unexpectedly early. Whether the timing will produce a more severe season is not yet clear, and Greninger said another wave could follow after H1N1 reaches its peak.

“I wouldn’t be surprised to see a second flu season, like a normal flu season. You’ll have H1N1 cases that will peak in 2026, and then who knows what comes in January or February or March,” Greninger said.