The spread of influenza has increased this fall, especially in the United States and the United Kingdom. The U.S. Centers for Disease Control and Prevention does appointed 2024–25 flu season as the hardest season from 2017-18. In the UK, the spread of the disease began earlier than at any time since 2003–04.
Some media against this background sockets have started using the term “super flu”. However, this term is not an official medical term. The actual name is “subclade K” and it is the fresh H3N2 variant of the influenza A virus.
This variant is characterized by multiple mutations in a protein on the surface of the virus called hemagglutinin, which makes it antigenically different from variants used in existing vaccines. This allows for partial bypass of immunity acquired as a result of previous infection or vaccines, which increases people’s susceptibility to infection. Genetic analysis by the UK Health Security Agency revealed that 87 percent of H3N2 viruses detected since overdue August 2025 are of the K subclade.
The epidemic started earlier than usual
The term “super flu” is not necessarily scientifically true. The H3N2 strain has already caused stern illness in elderly people and children, and the fresh mutant strain did not boost its mortality rate. Despite the name, the inherent danger of the virus is believed to be no different from the conventional H3N2 strain.
In 2025, the U.S. influenza pandemic peaked in early February, with vigorous outbreaks occurring in 87.3% of the country. Over 50% for 11 consecutive weeks. country recorded a high level of epidemicanomaly that led to 287 child deaths. However, these numbers reflect the scale of the epidemic and do not mean an boost in the mortality rate of the virus itself.
An influenza epidemic hit many parts of the world earlier this year. Although the peak of disease in Japan usually occurs between overdue December and February, in 2025 the epidemic began in earnest in overdue 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 belonged to the K subclade.
The cause of the early outbreak of the epidemic is believed to be a decline in population immunity as a result of countermeasures 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 flu epidemic was largely suppressed. As a result, it is possible to reduce the population’s resistance to the virus. In fact, with the 2024 flu pandemic in Australia at the highest level for 19 years, it would not be surprising to see a similar trend in the northern hemisphere.
Existing vaccines are effective
The effectiveness of the vaccine against this virulent strain has also generated great interest. The 2025-2026 vaccine is based on the conventional J.2 lineage (subclade), which has a different antigenicity from subclade K. However, early data from the UK confirmed that 70-75 percent of vaccinated children and 30-40 percent of adults did not go to the emergency room or were hospitalized after infection. This means that even if the antigenicity is not completely identical, the vaccine remains effective in preventing severe disease.
The basic preventive measures are the same as for conventional flu. Vaccination is recommended from October to November preceding the epidemic, and the effect appears about two weeks after vaccination. It is especially recommended for people over 65 years of age, people with comorbidities, pregnant women, children aged 6 months to 5 years and medical workers. In everyday life, thorough hand washing and disinfection is effective, and wearing a mask in crowds is effective. Indoor ventilation and maintaining appropriate humidity levels are also significant in suppressing virus activity.