Flu season has arrived earlier than usual in the U.K., with cases beginning to rise in October, over a month ahead of typical expectations. Health authorities say the surge is being driven by a rapidly evolving variant that has accumulated an unusually high number of mutations.
The strain, known as influenza A subtype H3N2 subclade K—or “subclade K”—has also been detected in Japan, where officials declared a flu epidemic last month. Jamie Lopez Bernal, consultant epidemiologist at the U.K. Health Security Agency (UKHSA), said the variant is changing faster than usual.
“This one has evolved a bit more rapidly, with more changes than we normally see,” Lopez Bernal said. Scientists are closely monitoring the mutations to determine whether current vaccines remain effective. Canadian researchers warned in late October that the changes in subclade K warranted ongoing surveillance, including genetic sequencing and vaccine effectiveness checks.
Preliminary data released by the UKHSA on 12 November suggests that vaccination is still providing significant protection against severe illness and hospitalisation, even with the variant’s mutations. However, the level of protection is similar to what is typically seen at the end of a flu season, when vaccines are slightly less effective due to virus evolution.
Health experts warn that the early start and presence of this new variant could make the season more intense than usual. “Flu is notoriously unpredictable, so it’s very difficult for us to say what’s going to happen,” Lopez Bernal said. He added that the earlier onset, combined with the concerning subclade, increases the likelihood of a more severe season.
Dr. Antonia Ho, professor and consultant in infectious diseases at the University of Glasgow, noted that early flu seasons often result in more cases overall and tend to affect older adults more severely, particularly when driven by H3N2 strains.
In the U.S., flu surveillance data have been limited due to the government shutdown. The last CDC update was from late September, showing low activity. However, New York State reported a 49% rise in lab-confirmed flu cases and a 71% increase in hospitalisations for the week ending 1 November, matching levels from the same point last year. Subtyping data is not yet available to confirm whether subclade K has arrived.
Health officials emphasise vaccination as the most effective protection. Lopez Bernal highlighted the importance of children getting vaccinated, noting that it also protects vulnerable family members. The vaccine takes up to two weeks to offer full protection, making early immunisation crucial.
“It remains our best tool to protect each other,” Dr. Ho said, urging the public to receive the flu shot as soon as possible.
