Mutated H1N1 flu strain hits US West Coast months early, experts urge immediate vaccination

Flu season has arrived roughly two and a half months ahead of schedule on the US West Coast, driven by a mutated H1N1 strain that is evading immunity and spreading in a pattern that has caught even infectious disease specialists off guard.

In California, flu test positivity hovered just above 1% in early October during each of the past two seasons. State health department data show that number had climbed to 9.65% by October 10. In Washington state, roughly 1 in 10 people tested for flu last week was positive — levels that typically don't appear until January, according to CNN.

We're basically seeing January levels of flu in the last week.— Dr. Alex Greninger, head of the Division of Infectious Disease Diagnostics, University of Washington

Utah recorded 242 influenza A cases as of October 11, up from just 10 cases at the same point last year, according to Germ Watch, an Intermountain Health database. Parts of central Oregon have also seen a surge. According to CNN, the four-state region of Arizona, California, Hawaii, and Nevada saw flu tests come back positive at a rate of at least 10% last week.

The culprit is a mutated version of H1N1 influenza A — the strain behind the 2009 swine flu pandemic. It has accounted for 242 of the 300 influenza viruses reported by public health laboratories to the CDC so far this season, according to The Guardian. At the University of Washington lab, it represents about 98% of samples, Greninger told CNN.

The specific mutation, at a location scientists call G155E, makes the virus better at evading human immunity, according to CNN. Because the mutation emerged after this season's vaccine strains had already been chosen, the timing exposed a structural weakness in how flu shots are developed. Most flu vaccines sold in the United States are still produced by growing virus in chicken eggs — a method too slow to accommodate late-breaking changes, requiring manufacturers to commit to specific strains each spring, well before the season's dominant virus becomes clear.

We're getting closer and closer to the weather report, right? 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.— Dr. Alex Greninger, head of the Division of Infectious Disease Diagnostics, University of Washington

The spread itself is also unusual. Flu seasons in the US typically begin in the South — often Florida, where the virus arrives from the Southern Hemisphere — and move northward as fall and winter progress. This year, the season started on the West Coast and is spreading eastward, according to CNN.

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.— Dr. Caitlin Rivers, epidemiologist, Center for Outbreak Response Innovation, Johns Hopkins Bloomberg School of Public Health

Greninger said the wave hitting the Pacific states came from Asia, and pointed to Japan as a warning sign: the same H1N1 mutation forced school and day-care closures there after cases surged in children, and prompted officials to move up vaccination clinics.

Despite the imperfect match, experts are unanimous that vaccination is still the right move — and urgently so. The FDA approved the 2026-27 flu vaccines in August; they cover H1N1, H3N2, and one influenza B virus. University of Pennsylvania microbiologist Scott Hensley noted that the vaccine appears to be a good match for two of the three circulating strains it targets.

It is probably a good idea to get vaccinated early.— Scott Hensley, microbiologist, University of Pennsylvania

Greninger was more pointed about the timing problem facing the public.

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. Alex Greninger, head of the Division of Infectious Disease Diagnostics, University of Washington

It is very early, and so I worry about vaccine timing. People aren't thinking about getting their flu vaccine so early in the season.— Dr. Caitlin Rivers, epidemiologist, Center for Outbreak Response Innovation, Johns Hopkins Bloomberg School of Public Health

The American Medical Association recommends flu shots for everyone six months and older, noting that vaccines reduce severe illness and hospitalization. CDC figures cited by CNN show that influenza-like illness accounted for about 2.1% of physician visits nationally last week, a level still under the national baseline, which indicates the virus has yet to spread widely across the country.

The push to vaccinate is happening against a complicated political backdrop. According to The Guardian, flu shot promotion has been scaled back under US Health Secretary Robert F. Kennedy Jr., a longtime anti-vaccine activist who has engineered an overhaul of long-standing childhood vaccine schedules.

CNN reported that Walgreens said the current flu season is running approximately four weeks ahead of recent years nationwide, with the pharmacy chain filling more antiviral prescriptions and selling more cough and cold products than it did over the same stretch last year.

Greninger raised the possibility that the early H1N1 peak may not be the end of the story. A second wave — a more typical flu season — could follow once H1N1 subsides, potentially bringing a different dominant strain in January, February, or March.

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.— Dr. Alex Greninger, head of the Division of Infectious Disease Diagnostics, University of Washington

Why it matters — An unusually early, mutation-driven flu season is already producing January-level case counts in October across the West Coast, and experts warn that millions of Americans who haven't yet thought about their flu shot may be running out of time to get protected before the virus reaches them.

Reported by theguardian.com, yahoo.com, reuters.com, cnn.com, rollingout.com