Mutated H1N1 Strain Drives Early US Flu Season and West Coast Surge
- National flu activity is tracking four weeks ahead of expectations as a mutated variant of the H1N1 virus drives an early surge in hospitalizations across the United States.
- The early wave of influenza has broken traditional geographic patterns by striking the West Coast first instead of starting in southern states and moving northward.
- State health departments across the region report sharp increases in test positivity.
National flu activity is tracking four weeks ahead of expectations as a mutated variant of the H1N1 virus drives an early surge in hospitalizations across the United States. While the Centers for Disease Control and Prevention typically reports the start of flu season in October, infections began climbing in late August 2026, according to the Centers for Disease Control and Prevention.
Surge Reaches Pacific States First
The early wave of influenza has broken traditional geographic patterns by striking the West Coast first instead of starting in southern states and moving northward. In Washington state, approximately 1 in 10 people tested for the flu last week returned positive results, prompting Dr. Alex Greninger, who leads the Division of Infectious Disease Diagnostics at the University of Washington’s Department of Laboratory Medicine, to point out that local clinics are experiencing January levels of flu
much earlier than expected.
State health departments across the region report sharp increases in test positivity. California public health officials indicate that test positivity has reached nearly 10 percent across most of the state, while Hawaii recorded a test positivity rate of 25.7 percent as of October 17, 2026. Nationally, the percentage of positive influenza tests climbed from 1.3 percent in early September to nearly 6 percent by mid-October, and CDC surveillance data shows flu hospitalizations have more than doubled to more than 4,800 cases.
Mutated H1N1 Strain Evades Immunity
The primary driver behind the unusual timeline is a genetic mutation designated G155E on the surface of the influenza A (H1N1) protein, which accounts for more than 96 percent of subtyped samples. Dr. Peter Chin-Hong, an infectious-disease specialist and professor of medicine at the University of California San Francisco School of Medicine, explained that this mutation makes it more difficult for antibodies from previous infections or prior vaccinations to recognize and block the virus.
Researchers trace the origins of this variant back to Asia, where the strain previously caused outbreaks and school and daycare closures in Japan after being identified there as early as May 2026. Dr. Thomas Russo, an infectious-diseases expert at the Jacobs School of Medicine and Biomedical Sciences at SUNY Buffalo, noted that international travel likely carried the mutation eastward, making the western United States its first continental stop.
Vulnerability Differs Among Age Groups
Public health authorities continue to recommend annual vaccination as the most effective defense against severe outcomes. Even though the G155E mutation arose after the creation of the 2026–2027 flu shots, the CDC anticipates that the immunizations will still deliver meaningful protection against serious disease this season.
Additional Regional Impacts and Clinical Guidance
This makes sense because older people have seen a lot more influenza, so they’re more likely to develop prior antibodies that have some cross-protection, even though it’s a new variant,
Russo says regarding age-related susceptibility.
Dr. Russo reports from New York: Here in Buffalo, we’re seeing more patients testing positive and landing in the hospital.
Greninger also explains that because current vaccines target a version of H1N1 and the mutation cropped up after strains were already selected, the vaccines should still offer some protection, but the virus can still break through.