What is the “super flu” spreading in Europe and the United States?

The spread of influenza has become more severe this fall, especially in the United States and the United Kingdom. The US Centers for Disease Control and Prevention has identified the 2024-2025 flu season as the most severe since 2017-2018. In the United Kingdom, the spread began earlier than at any time since 2003-2004.

Against this background, some media outlets have begun to use the term “super influenza.” However, this term is not an official medical term. The actual name is “subclade K”, which is a new type 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 through previous infections or vaccinations, making people more susceptible to infection. Genetic analysis by the UK’s Health Security Agency revealed that 87 percent of H3N2 viruses detected since late August 2025 belong to subgroup K.

The outbreak began earlier than usual

The term “super flu” is not necessarily scientifically accurate. The H3N2 strain has already caused severe illness in the elderly and children, and the new mutant strain does not make it more deadly. Contrary to the name, the danger inherent in the virus is said to be no different from the traditional H3N2 strain.

In 2025, the influenza pandemic in the United States peaked in early February, with active epidemics occurring in 87.3 percent of the country. For 11 consecutive weeks, more than 50% of the country recorded high epidemic levels, an anomaly that led to the deaths of 287 children. However, these numbers reflect the scale of the epidemic and do not mean an increase in the lethality of the virus itself.

An influenza epidemic began 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 analysed, 22 belonged to subcategory K.

The early outbreak is believed to be caused by the population’s low immunity due to anti-measures against the new coronavirus (Covid-19) infection, as well as low physical strength due to the record heat wave. During the three years of the coronavirus pandemic, the influenza epidemic has been largely suppressed. As a result, the population’s immunity to the virus is likely to have decreased. In fact, with the 2024 influenza pandemic in Australia reaching 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 was also great interest in the vaccine’s effectiveness against this deadly strain. The vaccine for the 2025-2026 season is based on the traditional J.2 strain (subclass), which has a different antigen than subclass K. However, early data from the UK confirmed that 70-75% of vaccinated children and 30-40% of adults did not end up visiting the emergency room or being hospitalized after infection. This means that even if the antigens are not a perfect match, the vaccine is still effective in preventing severe disease.

Basic prevention measures are the same as for traditional influenza. Vaccination is recommended in the period from October to November before the outbreak of the epidemic, and the effect appears about two weeks after vaccination. It is especially recommended for people ages 65 and older, people with underlying medical conditions, pregnant women, children ages 6 months to 5 years, and medical personnel. In daily life, it is helpful to wash and disinfect your hands well and wear a mask when in crowds. Ventilation in rooms and maintaining appropriate humidity levels are also important in suppressing viral activity.