Influenza cases have risen sharply in the United States and the United Kingdom this fall, prompting some media outlets to dub the circulating strain a "super flu." However, health officials clarify that "super flu" is not an official medical term. The dominant strain is a new variant of influenza A H3N2, specifically subclade K.

The US Centers for Disease Control and Prevention (CDC) has characterized the 2024-25 flu season as the most severe since 2017-18. In the UK, the influenza season began earlier than any time since 2003-04.

Subclade K possesses multiple mutations in the hemagglutinin protein, located on the virus's surface. These mutations cause the virus to be antigenically different from the strains used in current vaccines. This difference allows the virus to partially evade immunity acquired through previous infections or vaccinations, increasing susceptibility to infection.

According to genetic analysis conducted by the UK Health Security Agency, subclade K accounted for 87 percent of H3N2 viruses detected since late August 2025.

While the term "super flu" is being used, experts caution against exaggerating the threat. The H3N2 strain has historically caused severe illness, particularly in the elderly and children. The primary concern with subclade K is its ability to circumvent existing immunity, potentially leading to a higher number of infections.

Health officials continue to monitor the spread of subclade K and encourage individuals to take preventative measures, including vaccination, frequent handwashing, and staying home when sick. They emphasize that while the new variant presents a challenge, established public health practices remain effective in mitigating the impact of influenza.