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                            Ad Feedback       (https://www.cnn.com/health) Health  4 min read    A mutated virus is driving the US into an early flu season, especially on the West Coast   By Brenda Goodman    Updated Oct 22, 2026, 6:00 AM ET Published Oct 22, 2026, 6:00 AM ET     PUBLISHED Oct 22, 2026, 6:00 AM ET            (Even though this year's flu vaccines target a strain of H1N1, a new gene mutation means the virus can still break through.)    Even though this year's flu vaccines target a strain of H1N1, a new gene mutation means the virus can still break through.  Hannah Beier/File/Reuters            Respiratory viruses     Vaccines       (https://www.cnn.com/follow) See all topics      (Share with Facebook)   Facebook  (Share with X)   Tweet  (mailto:?subject=CNN%20content%20share&body=Check%20out%20this%20article%3A%0Ahttps%3A%2F%2Fwww.cnn.com%2F2026%2F10%2F22%2Fhealth%2Fflu-2026-mutation) (Share with email)   Email  (Copy link to clipboard)     Link  (Share with Threads)    Threads    Link Copied!   (Open social share)    (Close social share)         Follow       Flu season is picking up months ahead of schedule with a mutated virus spreading in some parts of the United States, surprising some people who might not even be thinking about their annual flu shots yet.  This year, the season started 2 1/2 months earlier than what’s typical and hit states on the West Coast first, which is unusual, said Dr. Alex Greninger, head of the Division of Infectious Disease Diagnostics at the University of Washington’s Department of Laboratory Medicine.  Usually, the US flu season starts in the South – often in Florida, where it arrives from the Southern Hemisphere. Then it tends to spread north as fall and winter progress.  “In Seattle, we’re pretty far north. We see a pretty strong peak in January,” Greninger said, noting that the bulk of their cases typically come in late December through February.  That’s not how it’s playing out this year.  ‘January levels of flu’  Roughly 1 in 10 people tested for the flu in Washington state last week was positive, 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 says that nationally, this flu season is (https://www.walgreens.com/seasonal/respiratory-illness-index) tracking about four weeks ahead of recent seasons, and it’s dispensing more antiviral prescriptions 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 influenza-like illness, a rough measure of the misery that flu symptoms like fever and cough are causing across the country. That’s still below the national baseline, however, according to (https://www.cdc.gov/fluview/surveillance/2026-week-40.html) data from the US Centers for Disease Control and Prevention, meaning the virus hasn’t really taken off everywhere yet.    “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,” said Dr. Caitlin Rivers, an epidemiologist who directs the (https://cori.centerforhealthsecurity.org/outbreak/respiratory-illness-risk-assessment/) Center for Outbreak Response Innovation at Johns Hopkins Bloomberg School of Public Health.  In the region that includes Arizona, California, Hawaii and Nevada, the proportion of flu tests that come back positive reached at least 10% last week, indicating significant levels of flu. The South is catching up to those numbers quickly, said Rivers, who closely tracks the flu through her (https://www.forceofinfection.com/outbreak-outlook-national-october-18/) Force of Infection newsletter.  A dominant strain  The wave of flu hitting the Pacific states came from Asia, Greninger said, and is dominated by the H1N1 virus, which sparked a (https://www.cdc.gov/orr/responses/h1n1-influenza-pandemic.html) pandemic when it first emerged in 2009.  It represents about 98% of the samples his lab is getting now, he said.  (https://www.cnn.com/2026/09/02/health/cdc-vaccine-guidance-gap) Related article   (Pharmacy manager Aylen Amestoy draws a dose of respiratory syncytial virus (RSV) vaccine into a syringe as she prepares a shot for a patient at a CVS Pharmacy in Miami, in 2025.)    Pharmacy manager Aylen Amestoy draws a dose of respiratory syncytial virus (RSV) vaccine into a syringe as she prepares a shot for a patient at a CVS Pharmacy in Miami, in 2025.  Rebecca Blackwell/AP      Respiratory virus season has arrived. Here’s what to know about flu, Covid-19 and RSV vaccines this fall  8  min read     These H1N1 viruses have also picked up a gene mutation that makes them better at evading our immunity.  This version of the virus has already wreaked havoc in Japan, where it forced school and day-care closures after cases surged in kids. It also prompted officials to move up vaccination clinics to help protect people.  The mutation, at a location called G155E, cropped up after strains were already selected for this year’s flu shots, Greninger said. So even though the vaccines target a version of H1N1 and should still offer some protection, the virus can still break through.  Get vaccinated now, experts say  Greninger notes that the Covid-19 pandemic helped develop new tools that allow us to track the spread of viral variants almost in real time.  “We’re getting closer and closer to the weather report, right?” he said. “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.”  The strains used in this year’s flu shots were chosen in the spring, before the new viral variant had become a threat. They’re selected so early because the majority of US flu shots are still grown in chicken eggs, a practice that persists, Greninger added, because it is so cheap.  (https://www.cnn.com/health/first-mrna-flu-vaccine) Related article   (An assistant conducts cancer vaccine research in the lab at the Moderna Inc. headquarters in Cambridge, Massachusetts, US, on Tuesday, March 26, 2024. The forthcoming RSV shot, expected to get US regulatory approval this month, is Moderna's first chance to show the versatility of mRNA technology to more effectively treat and prevent a range of illnesses from the flu to cancer. It will be the first mRNA product for something other than Covid. Photographer: Adam Glanzman/Bloomberg via Getty Images)    An assistant conducts cancer vaccine research in the lab at the Moderna Inc. headquarters in Cambridge, Massachusetts, US, on Tuesday, March 26, 2024. The forthcoming RSV shot, expected to get US regulatory approval this month, is Moderna's first chance to show the versatility of mRNA technology to more effectively treat and prevent a range of illnesses from the flu to cancer. It will be the first mRNA product for something other than Covid. Photographer: Adam Glanzman/Bloomberg via Getty Images  Adam Glanzman/Bloomberg/Getty Images/File      The first mRNA flu vaccine is now available for older adults. Here’s what to know  10  min read     There’s a newly approved flu shot that uses mRNA technology, and he said that has the potential to make better shots available more quickly. Whether it will someday replace egg-based vaccines is a different issue – an economic one.  Doctors often advise getting a flu shot before Halloween, in order to provide the best protection through spring. But Greninger’s best advice now is to get vaccinated as soon as possible. “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.”  (Doctors often advise getting a flu shot before Halloween, in order to provide the best protection through spring.)    Doctors often advise getting a flu shot before Halloween, in order to provide the best protection through spring.  Witthaya Prasongsin/Moment RF/Getty Images     Other experts agree.  “It is very early, and so I worry about vaccine timing,” Rivers said. “People aren’t thinking about getting their flu vaccine so early in the season.”  Whether that will make the overall season more severe remains to be seen.  Greninger said there could also be a second wave of flu cases after H1N1 peaks.  “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. “So maybe interesting to watch.”         Respiratory viruses     Vaccines       (https://www.cnn.com/follow) See all topics      (Share with Facebook)   Facebook  (Share with X)   Tweet  (mailto:?subject=CNN%20content%20share&body=Check%20out%20this%20article%3A%0Ahttps%3A%2F%2Fwww.cnn.com%2F2026%2F10%2F22%2Fhealth%2Fflu-2026-mutation) (Share with email)   Email  (Copy link to clipboard)     Link  (Share with Threads)    Threads    Link Copied!   (Open social share)    (Close social share)         Follow              Ad Feedback                     Paid Content    Paid Content        Ad Feedback       (Search CNN...) 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