The spread of influenza became more severe this fall, particularly in the United States and the United Kingdom. The US Centers for Disease Control and Prevention has designated the 2024-25 flu season as the most severe season since 2017-18. In the UK, the spread has begun earlier than at any time since 2003-04.
Against this backdrop, some media outlets have begun using the term “super flu.” However, this term is not an official medical term. The actual name is “subclade K,” a new variant of influenza A H3N2.
This variant has multiple mutations in a protein on the surface of the virus called hemagglutinin, making it antigenically different from the variants used in existing vaccines. This allows it to partially evade immunity acquired through previous infection or vaccines, making people more susceptible to infection. Genetic analysis by the UK Health Security Agency has revealed that 87 percent of H3N2 viruses detected since late August 2025 are subclade K.
The Outbreak Began Earlier Than Usual
The term “super flu” is not necessarily scientifically accurate. The H3N2 strain already caused severe illness in the elderly and children, and the new mutant strain has not made it more deadly. Contrary to the name, the virus’s inherent danger is said to be no different from the conventional H3N2 strain.
In 2025, the US influenza pandemic peaked in early February, with active epidemics occurring in 87.3 percent of the country. For 11 consecutive weeks, more than 50 percent of the country recorded high epidemic levels, an anomaly that led to 287 child deaths. However, these figures reflect the scale of the epidemic and do not imply an increase in the lethality of the virus itself.
The influenza epidemic is hitting earlier this year in many parts of the world. While the usual peak in Japan occurs between late December and February, in 2025 the epidemic began in earnest at the end of September. According to the Ministry of Health, Labor, and Welfare, of the 23 H3 virus strains collected in Japan between September and November 5 that could be analyzed, 22 were subclade K.
The reason for the early outbreak is thought to be the decline in immunity of the population due to the countermeasures against new coronavirus infection (Covid-19), as well as a decline in physical strength due to the record-breaking heat wave. During the three years of the coronavirus pandemic, the influenza epidemic was largely suppressed. As a result, it is possible that population immunity to the virus was reduced. In fact, with the 2024 influenza pandemic in Australia at its highest level since 19 years, it would not be surprising to see a similar trend in the Northern Hemisphere.
Existing Vaccines Are Effective
There has also been much interest in vaccine efficacy in the face of this virulent strain. The vaccine for the 2025-26 season is based on the conventional J.2 lineage (subclade), which has different antigenicity from subclade K. However, early data from the UK has confirmed that 70-75 percent of vaccinated children and 30-40 percent of adults did not end up visiting the emergency room or being hospitalized after infection. This means that even if the antigenicity is not completely identical, the vaccine remains effective in preventing severe illness.
The basic prevention measures are the same as for conventional influenza. Vaccination is recommended from October to November before the epidemic, and the effect appears about two weeks after vaccination. It is particularly recommended for people aged 65 and over, people with underlying medical conditions, pregnant women, children aged 6 months to 5 years, and medical workers. In daily life, it is effective to thoroughly wash and disinfect your hands, and wear a mask when in crowds. Ventilation in rooms and maintaining appropriate humidity levels are also important in suppressing viral activity.
