A mutated virus strain is jumpstarting a worrisome flu season
Despite concerns about the flu, early evidence suggests this year’s vaccine may still offer meaningful protection against the new strain, experts say.
A mutated viral strain has prompted a start to flu season in the United States, as well as a spike in cases on the West Coast.
Despite concerns about the flu, early evidence suggests this year’s vaccine may still offer meaningful protection against the new strain, experts say.
The G155E mutation emerged in an H1N1 influenza strain that is now driving a seasonal surge in cases in the U.S., said Samuel Scarpino, executive director of the Institute for Experiential AI and a professor of public health and health sciences in the Bouvé College of Health Sciences at Northeastern University.
“What we’re seeing in the U.S. right now is almost entirely H1N1, carrying a mutation that helps it evade some existing immunity,” Scarpino said.
H1N1, a subtype of influenza A commonly known as swine flu, rose to global prominence in 2009, when a new strain triggered a pandemic. The G155E mutation occurs in a specific gene that encodes a protein on the surface of the influenza virus, changing the manner in which it attaches to and infects cells.
Flu season ordinarily begins in the fall and winter months, with surveillance data for the seasonal period tracked from about the 40th week of the year through the 20th week of the following year, according to the Centers for Disease Control and Prevention, which noted that the public health impacts vary from season to season.
Scaprino said flu season is starting earlier than usual, but that “[it is] still within that window.”
“This H1N1 variant appears to be behind the current surge and the early start,” Scarpino said.
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The mutation was detected in Japan as early as May 2026, and has been spreading gradually from August onward, according to Japan’s national infectious disease monitoring agency. The surge there led to school closures as hospitals reported a jump in flu patients to roughly 11.35 per clinic for the week through Oct. 4, which was about a seven-fold increase from the previous year.
In the U.S., the CDC’s surveillance data for the week ending Sept. 26, 2026, showed that flu activity remained low nationally but was beginning to rise. About 3.4% of respiratory specimens tested in the nation’s clinical labs were positive for influenza, representing about an eight-fold increase compared to this time last year, data shows.
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.
During the 2022-23 season, when vaccine effectiveness was about 30%, the CDC estimated that flu vaccination prevented more than 70,000 hospitalizations and roughly 3,700 deaths, Scarpino said. This year’s vaccine was updated to better match the H3N2 variant that circulated last season, also a subtype of influenza A, he added.
Scarpino noted that public health officials won’t have data on the effectiveness of the vaccine until midwinter.
“Still, early data shows the vaccine should provide meaningful protection, especially against severe illness, and it was updated against the H3N2 strain that may come later in the season,” he said. “Now is the time to get vaccinated.”
Neil Maniar, director of Northeastern’s master of public health program, said that public health officials always recommend getting vaccinated as soon as possible — especially when infections start cropping up earlier than expected.
“Going into the holidays, that means you’ll have built up some immunity,” Maniar said. “Influenza causes a significant number of hospitalizations and deaths, so we should be taking this very seriously.”
