Flu Shots Remain Effective Against H3N2 Subclade K

The Pan American Health Organization (PAHO) recently released a briefing note informing countries about the increasing circulation of the influenza A(H3N2) subclade K (J.2.4.1) virus.

The PAHO noted in a media release that the genetic evolution observed in subclade K is part of the natural variation in seasonal influenza viruses.

As of December 12, 2025, in North America, particularly in the United States and Canada, there has been a gradual increase in detections of subclade K. As of now, similar circulation levels have not been reported in South American countries.

While evidence on vaccine effectiveness for the current season remains limited, preliminary data from Europe suggest that vaccination continues to offer comparable protection against severe disease, including hospitalization, wrote the PAHO.

The ECDC previously wrote that compared to previous years, influenza is increasing unusually early in the EU/EEA, with A(H3N2) driving the increases in recent weeks. Even if a less well-matched A(H3N2) virus dominates this winter, the vaccine is still expected to protect against severe disease.

The U.S. CDC wrote last week that subclade J.2.4.1, renamed "H3N2 subclade K," was identified in August 2025. Of the 401 influenza A viruses subtyped during Week #49 in the USA, 14.% were influenza A(H1N1)pdm09, and 86.0% were A(H3N2).

These viruses have slight changes in their hemagglutinin gene and have been antigenically characterized as "antigenically drifted" in comparison to the virus selected as the A(H3N2) component of the U.S. 2025-26 seasonal influenza vaccines.

The CDC reaffirmed its recommendation that most people in the USA, and those traveling abroad for the winter holidays, get an annual flu shot.