JONATHAN M. METSCH, Dr.P.H. – Tracking Emerging Public Health Challenges – October 22, 2026 – Seasonal Flu
“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.”
““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 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 Force of Infection newsletter.
The wave of flu hitting the Pacific states came from Asia, Greninger said, and is dominated by the H1N1 virus, which sparked a pandemic when it first emerged in 2009.
It represents about 98% of the samples his lab is getting now, he said.
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.” (1)
“Doctors are urging Americans aged 6 months and older to get vaccinated against the flu after an unusual early rise in cases on the West Coast that virologists are still working to understand.
It could be the strain itself. The predominant flu strain this year is H1N1, which caused the 2009 swine flu pandemic and is now a standard part of the flu shot. But this year’s version of the virus has a new mutation called G155E.
People who have been previously exposed to H1N1, either through infection or vaccination, would only have some residual protection from the older versions, scientists say.
“What people are a bit worried about is that the H1N1 strain this year hasn’t been seen by a lot of younger folk, so it may be more severe in them,” said Dr. Jennifer Duchon, a pediatric infectious disease expert at Mount Sinai Children’s Hospital in New York, adding that her hospital has started to see some flu cases.
The 2026-2027 flu vaccines, approved by the FDA in August, target H1N1, H3N2 — the dominant strain last flu season — and one influenza B virus.
While most children recover, flu can be deadly, especially for the very youngest and those with underlying health conditions.”
Scott Hensley, a microbiologist at the University of Pennsylvania, said two of the three components of this year’s vaccine appear to be very well matched to circulating flu strains.
“However, the H1N1 mutation might affect antibodies in younger people, and with flu season picking up steam, “it is probably a good idea to get vaccinated early,” Hensley said via email.” (2)
“Scarpino said preliminary lab testing suggests that the current vaccine should still recognize this strain, but added that tests of human blood samples indicate that “some people,” like teenagers and young adults, may have less protection.
“The likely reason is a bit technical,” Scarpino said. “Our immune responses to flu are shaped by the first strains we encounter as children, and the 2009 H1N1 pandemic replaced the H1N1 strain that had been circulating. Younger people’s first exposures were to that new strain, and this mutation hits a spot their immune systems tend to focus on.”
A single strain, such as the mutated H1N1 variant, can drive outbreaks, Scarpino said. Last year’s flu season was particularly severe, driven by an H3N2 variant that partially evaded existing immunity, including immunity from vaccination, he said.
The flu vaccine was about 30% to 40% effective at preventing doctor visits and hospitalizations, lower than in recent years. Still, even vaccines with modest effectiveness can prevent substantial numbers of severe cases and deaths, he added.” (3)
Flu Arrived Early This Year. Should Clinicians Do Anything Differently?, by Niranjana S Rajalakshmi, https://www.medscape.com/viewarticle/flu-arrived-early-this-year-should-clinicians-do-anything-2026a10013qy
1.A mutated virus is driving the US into an early flu season, especially on the West Coast, By Brenda Goodman, https://www.cnn.com/2026/10/22/health/flu-2026-mutation?cid=ios_app
2.Early flu season, current strain may signal risk for younger people; doctors urge shots, by Michael Erman and Julie Steenhuysen, https://www.ksl.com/article/51638559/early-flu-season-current-strain-may-signal-risk-for-younger-people-doctors-urge-shots#:~:text=Early%20flu%20season
3.A mutated virus strain is jumpstarting a worrisome flu season, by Tanner Stening, https://news.northeastern.edu/2026/10/22/mutated-flu-strain-flu-season/
curated by Jonathan M. Metsch, Dr.P.H.
Clinical Professor of Environmental Medicine, Icahn School of Medicine at Mount Sinai