Recent data suggests that the flu season in the Northern Hemisphere may be starting earlier than usual, which has led to discussions about when individuals should receive their annual influenza vaccines. Reports indicate that the percentage of positive flu tests is unusually high at this point in the year, particularly in regions like the United Kingdom and along the U.S. West Coast. In Japan, there were outbreaks in August and early September that resulted in some schools closing, just as they were reopening after summer break.
According to Scott Hensley, a virologist at the University of Pennsylvania in Philadelphia, if the current rate of flu cases on the West Coast continues, it could indeed be an exceptionally early flu season for the United States.
Interestingly, the influenza subtype that appears most prevalent in the U.S. this year is H1N1, a shift from the previous year’s dominance of H3N2. Additionally, sequencing from these early cases has detected a concerning mutation in H1N1 that has specialists on alert.
While the number of positive tests currently is relatively small compared to what we might see during peak flu season, experts warn against hastily predicting the outbreak’s overall direction. James Hay, an epidemiologist at the University of Oxford, UK, reminds us that an early season doesn’t automatically lead to a more severe one. Still, as Caitlin Rivers, an epidemiologist at Johns Hopkins University, noted in her newsletter, “there is something afoot.” This shift has prompted her to reconsider when to get her vaccine; typically, she’d wait until mid-to-late October, but now she’s considering an earlier shot.
An emerging mutation
Seasonal flu generally arises from a mix of three virus types: the two subtypes of influenza A (H1N1 and H3N2) and influenza B. The dominance of these viruses can vary each flu season due to the genetic mutations of the viruses and the changes in people’s immunity based on vaccinations and prior infections. Recent U.S. data shows that H1N1 made up about 96% of positive cases for which the subtype was tested, while H3N2 constituted about 4%, and influenza B was undetected.
Last year, a new variant of H3N2 called subclade K emerged, becoming globally dominant due to its mutations that made it more successful in infecting individuals.
This year, the new H1N1 mutation raised eyebrows is G155E. Alex Greninger, a virologist from the University of Washington in Seattle, reported that his lab identified this mutation in 13 out of 17 H1N1 samples collected from local hospitals and clinics over the past few months.
This particular mutation has been highlighted in analyses led by Jesse Bloom, a virologist at the Fred Hutchinson Cancer Center in Seattle, which was shared on a preprint server recently. Bloom’s team examined how effectively antibodies in blood samples from 260 individuals in Australia and the United States recognized and blocked infection from various circulating influenza A strains by mid-2026. Their findings indicated that some strains with the G155E mutation are less recognizable to existing human antibodies, particularly among teenagers and young adults. This could be particularly troubling since many in this age group haven’t encountered H1N1 viruses with similar traits previously.
