What is the 'Super Flu' Spreading in Europe and the United States? A Deep Dive into the 2025-26 Influenza Season
The influenza virus has been making headlines this fall, particularly in the United States and the United Kingdom, with some media outlets using the term 'super flu' to describe the severity of the outbreak. But what does this term really mean, and is it accurate? Let's explore the science behind the 2025-26 influenza season and the role of a new variant, subclade K.
The Rise of Subclade K
The term 'super flu' is not an official medical term, but it's being used to describe a new variant of the influenza A H3N2 virus called subclade K. This variant has multiple mutations in a protein called hemagglutinin, making it antigenically different from the variants used in existing vaccines. This means it can partially evade immunity acquired through previous infections or vaccines, making people more susceptible to infection.
The UK Health Security Agency's genetic analysis revealed that 87% of H3N2 viruses detected since late August 2025 are subclade K. This variant has already caused severe illness in the elderly and children, but it's important to note that the virus's inherent danger is not significantly different from the conventional H3N2 strain.
An Early and Severe Outbreak
The 2025-26 influenza season has been particularly severe, with the outbreak beginning earlier than usual in many parts of the world. In the US, the pandemic peaked in early February, with active epidemics occurring in 87.3% of the country. For 11 consecutive weeks, more than 50% of the country recorded high epidemic levels, leading to 287 child deaths. However, these figures reflect the scale of the epidemic, not an increase in lethality.
In Japan, the usual peak occurs between late December and February, but in 2025, the epidemic began in earnest at the end of September. Of the 23 H3 virus strains collected between September and November 5, 22 were subclade K. The early outbreak is attributed to declining population immunity due to COVID-19 countermeasures and record-breaking heat waves, which reduced physical strength.
Vaccine Efficacy and Prevention
Despite the severity of the outbreak, existing vaccines remain effective. The 2025-26 season's vaccine is based on the conventional J.2 lineage, which has different antigenicity from subclade K. Early data from the UK shows that 70-75% of vaccinated children and 30-40% of adults did not end up in the emergency room or hospitalized after infection. This highlights the vaccine's ability to prevent severe illness, even with some antigenic differences.
Basic prevention measures include vaccination from October to November, which takes effect about two weeks after administration. It is particularly recommended for people aged 65 and over, those with underlying medical conditions, pregnant women, children aged 6 months to 5 years, and healthcare workers. Other effective measures are thorough hand washing and disinfection, wearing masks in crowds, proper ventilation in rooms, and maintaining appropriate humidity levels.
Managing Symptoms and Recovery
If symptoms appear, it's best to wait at least 12 hours after the fever onset before visiting a medical institution. Anti-influenza medication is most effective when taken within 48 hours of symptom onset, with Xofluza and Tamiflu considered effective. People should refrain from going out for five days after symptom onset and two days (three days for children) after fever resolution, ensuring adequate rest and hydration.
A Calm Response is Key
Despite the term 'super flu,' the current outbreak is an extension of traditional influenza. A calm and scientifically informed response is crucial, as combining vaccination with basic infection control measures can significantly reduce the risk of severe symptoms. With consecutive high-severity seasons being rare, responsible choices based on accurate information will help protect public health.
This article was originally published in WIRED Japan and translated from Japanese.
