Mutated H1N1 Virus Drives Early Flu Season Across US

A genetically mutated H1N1 influenza virus is accelerating the 2026 flu season in the United States, with West Coast states experiencing unusually early increases in infections. Health experts say the strain is spreading eastward after emerging from Asia, raising concerns that the virus can partially evade immunity despite this year's vaccines targeting H1N1.

Dr. Alex Greninger, head of the Division of Infectious Disease Diagnostics at the University of Washington's Department of Laboratory Medicine, said the season began approximately 2 1/2 months earlier than usual. The virus has also become the dominant strain in samples tested by his laboratory.

Washington state data show that approximately one in 10 people tested for influenza last week received a positive result, according to the state Department of Public Health.

“In Seattle, we're pretty far north. We see a pretty strong peak in January,” said Greninger. He noted that cases typically concentrate between late December and February.

National indicators also show earlier activity. Walgreens reported that the flu season is progressing about four weeks ahead of recent seasons, alongside increased dispensing of antiviral prescriptions and cough and cold medications compared with the same period last year.

According to the US Centers for Disease Control and Prevention, approximately 2.1% of doctor visits last week involved influenza-like illness. That figure remains below the national baseline, indicating that widespread increases have not yet reached every part of the country.

Greninger described the latest local activity as unusually intense for this point in the calendar.

“We're basically seeing January levels of flu in the last week,” he said.

The dominant H1N1 strain accounts for approximately 98% of the influenza samples Greninger's laboratory is currently receiving. H1N1 first emerged as a pandemic virus in 2009, and the latest version has acquired a genetic change identified as G155E.

Greninger said the mutation appeared after scientists had already selected the strains for this year's flu vaccines in the spring. Although the vaccines target an H1N1 version and are still expected to provide some protection, the mutation may help the circulating virus evade existing immunity.

Japan has already experienced substantial disruption from the variant, with rising infections among children prompting school and day-care closures. Officials there also moved vaccination clinics to earlier dates to help protect the population.

Greninger said advances in viral surveillance since the COVID-19 pandemic have made it possible to track genetic changes more quickly.

“We're getting closer and closer to the weather report, right?” he said.

He explained that rapid sequencing can identify specific mutations, highlighting the need to accelerate vaccine development.

“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.”

Most US flu vaccines are still produced using chicken eggs, a relatively inexpensive method that requires manufacturers to select vaccine strains well before the flu season begins. A newly approved mRNA-based flu vaccine could potentially allow updated formulations to be developed more quickly, although whether it will replace egg-based vaccines remains an economic question.

Greninger recommended vaccination sooner than the usual late-October target because infections have already increased in several states.

“A lot of people, like even myself, would wait until late October, which we're sort of in already. But really, the time to get vaccinated was actually two to three weeks ago.”

Dr. Caitlin Rivers, an epidemiologist who directs the Center for Outbreak Response Innovation at Johns Hopkins Bloomberg School of Public Health, said the early activity is spreading beyond the Pacific states.

“All of the Pacific states, I would say, started their season very early, but we are now seeing as the season progresses a spread eastward, and so all regions of the country are seeing increases at this point,” Rivers said.

In the region encompassing Arizona, California, Hawaii and Nevada, at least 10% of influenza tests came back positive last week. Rivers said the South was rapidly approaching comparable levels of activity.

She also warned that the early start complicates decisions about when to receive seasonal vaccination.

“It is very early, and so I worry about vaccine timing,” Rivers said.

Rivers said public awareness of vaccination may lag because many people do not expect flu activity to increase so early in the year.

“People aren't thinking about getting their flu vaccine so early in the season.”

The eventual severity of the season remains uncertain. Greninger said another wave could follow the current H1N1 surge, potentially bringing additional influenza activity during the winter and early spring.

“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,” he said.

Greninger said the possibility of another wave remains uncertain as the current strain continues to spread.

“So maybe interesting to watch.”