A New Flu Variant is on the Rise

Fast-spreading Subclade K is a strain to watch heading into the winter

November 26, 2025

A new flu variant has public health researchers on alert heading into the winter holidays.

While flu activity remains low in Pennsylvania and the U.S., there are a few early signs that this could be an eventful respiratory virus season. Here’s what you need to know:

• Background:  The flu is constantly evolving, and public health surveillance helps predict the ways this year’s trivalent vaccine will match the most commonly circulating strains.
• The big news:  In a recent report, Canadian researchers indicated that a new flu A variant emerged in the southern hemisphere, and is likely to be circulating this flu season in the U.S. The variant, known as subclade K, may be the predominate flu A subtype this year.
  • Early reports from the United Kingdom and Japan show “unusually sharp and early upswing in influenza indicators, foremost due to A(H3N2) of which most (>85%) in the United Kingdom” are related to subclade K.
• Stay protected:  The flu shot offers important protection, even when the circulating virus drifts from the strains included in this year’s vaccine.
  • “Getting your child, and yourself, vaccinated against the flu annually reduces your own risk of infection and hospitalization and also helps protect people in your community at high risk for severe illness related to the virus,” noted Dr. Debra Bogen, Pennsylvania’s health secretary earlier this year.
• Pennsylvania perspective:  Ahead of the holidays, Pennsylvania emergency rooms have reported “low” levels for the flu, but activity tends to peak during the upcoming winter months.
• Bottom line:  “While mismatched vaccines can still provide protection against circulating variants, enhanced surveillance is warranted including timely influenza virus sequencing and vaccine effectiveness monitoring to inform risk assessment and response,” the Canadian researchers noted.

HAP continues to monitor the latest public health trends and provide updates to members. Additional information about the flu strain is available online.