Scientists are studying G155E, a change in the H1N1 flu virus, as activity rises early on the U.S. West Coast, but any effect on illness among younger people is unverified, Reuters reported.
Of 251 influenza A specimens subtyped by public health laboratories during the week ended Oct. 10, 242 were H1N1, the Centers for Disease Control and Prevention (CDC) said. That count covers the specimens assigned a subtype, rather than every influenza A positive. The CDC also reported 5% clinical laboratory test positivity nationwide in the surveillance update covered earlier by EconoTimes, while assessing national flu activity as low.
On the West Coast, the share of clinical tests returning positive reached 10.7%, compared with 0.5% a year earlier, according to CDC data cited by Reuters. The CDC said the national increase remained within expected seasonal variation.
Jeanne Marrazzo, the Infectious Diseases Society of America's chief executive, told Reuters that small studies found antibodies from people exposed to flu in 2025 recognized G155E poorly, particularly in samples from teenagers and young adults. She said the findings deserved attention but were no cause for alarm. The studies do not establish whether the change leads to more illness or greater severity in younger people.
A separate CDC experiment tested selected H1N1 viruses against ferret antibodies raised using a vaccine reference virus. The antibodies recognized 70 of 74 viruses collected since May 24. Those samples differ from the 251 viruses subtyped in the latest surveillance week, and the ferret results cannot establish how well vaccination prevents illness or hospitalization in people.
Johns Hopkins epidemiologist Caitlin Rivers told Reuters that last season's H3N2 dominance may have left an H1N1 immunity gap. She said the reason flu activity began rising in the West was unclear.
The 2026-27 U.S. flu vaccine includes H1N1, H3N2 and an influenza B virus. The American Medical Association recommends annual vaccination for people six months and older who have no contraindications. The CDC identifies September and October as the preferred vaccination period for most people; its weekly surveillance reports will provide the next measure of how flu activity is changing.
