An unusually early start to the 2026–27 flu season is spreading across parts of the Northern Hemisphere, while researchers investigate whether a mutated influenza strain is helping the virus evade existing immunity.
Flu activity began rising in early September, several weeks ahead of the usual October or November start, with increases reported in the United States, the United Kingdom and Japan. In the United States, the earliest surge has been concentrated in Western states, including California, Washington, Hawaii and Alaska, an unusual pattern because flu seasons more often begin in the South. Scientists are examining a mutated form of influenza A that appears better able to bypass immunity from previous infections or vaccinations, particularly among teenagers and young adults.
However, experts caution that the mutation may explain increased transmission without necessarily making the illness more severe. Early case numbers remain relatively small, and researchers say it is too soon to predict whether the season will become unusually dangerous. The U.S. Centers for Disease Control and Prevention currently says there is no evidence that a severe flu season is more likely than usual, although its assessment could change as surveillance data accumulate.
Health specialists are urging people who have not been vaccinated to consider getting a flu shot soon, saying vaccination should still reduce the risk of serious illness even if protection against infection is imperfect.