Influenza Severity Increases This Fall
This fall, the spread of influenza has intensified, especially in the United States and the United Kingdom. The US Centers for Disease Control and Prevention (CDC) has labeled the 2024-25 flu season as the most severe since the 2017-18 season. In the UK, influenza is spreading earlier than it has since the 2003-04 season.
In light of this situation, some media outlets have begun referring to a “super flu.” Yet, this term isn’t medically recognized. The actual variant in question is called “subclade K,” a new type of influenza A H3N2.
This variant features several mutations in a protein on the virus’s surface known as hemagglutinin, which makes it antigenically distinct from the strains used in current vaccines. As a result, it can partially escape immunity from past infections or vaccinations, leaving individuals more vulnerable. According to genetic analysis from the UK Health Security Agency, a significant 87 percent of H3N2 viruses observed since late August 2025 belong to subclade K.
Unusual Early Outbreak
The term “super flu” might not be the most scientifically precise. The H3N2 strain has already caused severe cases among the elderly and children, and the new mutant variant hasn’t changed its lethality. So, despite the name, it seems that the virus isn’t inherently more dangerous than the traditional H3N2 strain.
In 2025, the flu pandemic in the US peaked in early February, affecting 87.3 percent of the nation. For over 11 weeks, more than half of the country experienced high epidemic levels, an unusual trend that resulted in 287 child fatalities. However, these statistics reflect how widespread the epidemic was rather than indicating a deadlier virus.
This year’s influenza epidemic appears to be starting earlier across various regions. Typically, Japan sees its peak between late December and February, but in 2025, the outbreak began in earnest by late September. The Ministry of Health, Labor, and Welfare noted that, out of 23 analyzed strains collected in Japan between September and November 5, 22 were identified as subclade K.
The early outbreak might be linked to reduced population immunity as a result of anti-COVID measures and diminished physical health due to an unprecedented heatwave. The flu epidemic was significantly suppressed during the three years of the pandemic, which could have led to decreased immunity among the population. Notably, with Australia seeing its highest flu season in 19 years, a similar pattern might emerge in the Northern Hemisphere.
Vaccine Effectiveness
Amid the concerns about this aggressive strain, questions have arisen regarding vaccine effectiveness. The vaccine for the 2025-26 season is based on the conventional J.2 lineage (subclade), which bears different antigenic characteristics compared to subclade K. Early data from the UK shows that 70-75 percent of vaccinated children and 30-40 percent of adults avoided emergency visits or hospitalization after infection, suggesting that, while not a perfect match, the vaccine still aids in preventing severe illness.
The general prevention guidelines remain consistent with standard practices for conventional influenza. Vaccination is recommended from October to November prior to the onset of the epidemic, with effects typically starting around two weeks post-vaccination. It’s especially advised for those aged 65 and older, individuals with underlying health issues, pregnant women, children aged 6 months to 5 years, as well as healthcare workers. In daily life, thorough handwashing, disinfection, and wearing masks in crowded spaces are effective. Adequate ventilation and humidity control in indoor environments are important for reducing viral transmission as well.
