What Is the ‘Super Flu’ That Is Spreading in Europe and the United States?
The spread of the flu increased this fall, particularly in the United States and the United Kingdom. The U.S. Centers for Disease Control and Prevention has named the 2024-25 flu season the most severe season since 2017-18. In the UK, the spread started earlier than at any time since 2003-2004.
In this context, some media have started to use 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 variants used in existing vaccines. This allows it to partially evade immunity acquired from previous infection or vaccines, making people more vulnerable to infection. Genetic analysis carried out by the United Kingdom’s Health Security Agency found that 87% of H3N2 viruses detected since the end of August 2025 belong to the K subclade.
The epidemic started earlier than usual
The term “super flu” is not necessarily scientifically accurate. The H3N2 strain has already caused serious illness in the elderly and children, and the new mutant strain has not made it more deadly. Contrary to its name, the danger inherent in the virus would not be different from that of the conventional H3N2 strain.
In 2025, the influenza pandemic in the United States peaked in early February, with active outbreaks in 87.3% of the country. For 11 consecutive weeks, more than 50 percent of the country recorded high epidemic levels, an anomaly that resulted in the deaths of 287 children. However, these figures reflect the scale of the epidemic and do not imply an increase in the lethality of the virus itself.
The flu epidemic hit many parts of the world earlier this year. While the usual peak in Japan is between late December and February, in 2025 the epidemic really began at the end of September. According to the Ministry of Health, Labor and Welfare, of the 23 strains of the H3 virus collected in Japan between September and November 5 that could be analyzed, 22 belonged to the K subclade.
The reason for this early outbreak is believed to be the decline in population immunity due to measures against the new coronavirus infection (Covid-19), as well as a decline in physical strength due to the record heat wave. During the three years of the coronavirus pandemic, the influenza epidemic was largely under control. As a result, it is possible that the population’s immunity to the virus has been reduced. In fact, as Australia’s 2024 flu pandemic reaches its highest level in 19 years, it would not be surprising to see a similar trend in the Northern Hemisphere.
Existing vaccines are effective
There is also much interest in the effectiveness of the vaccine against this virulent strain. The vaccine for the 2025-26 season is based on the conventional J.2 (subclade) lineage, which has a different antigenicity than the K subclade. However, early data from the United Kingdom confirmed that 70 to 75 percent of vaccinated children and 30 to 40 percent of adults did not go to the emergency room or were not hospitalized after an infection. This means that even if the antigenicity is not completely identical, the vaccine is still effective in preventing serious illness.
Basic prevention measures are the same as for conventional flu. 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 pathologies, pregnant women, children aged 6 months to 5 years and medical personnel. In daily life, it is effective to wash and disinfect your hands thoroughly and wear a mask in crowds. Ventilating rooms and maintaining appropriate humidity levels are also important to suppress viral activity.
