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The H3N2 influenza virus subclade K is a recently emerged variant of the influenza A(H3N2) subtype, characterized by seven genetic mutations that distinguish it from the J.2 subclade strain included in the 2025/2026 seasonal flu vaccines.
This subclade represents genetic drift rather than a major change, meaning it does not indicate a new pandemic strain, and its symptoms are similar to those of typical seasonal flu, including fever, cough, sore throat, body aches, and fatigue.
It has rapidly become dominant at the beginning of the 2025/2026 flu season, accounting for 86% to 89% of sequenced H3N2 cases globally, with notable circulation in the UK, Japan, Canada, the US, and parts of the EU.
There is no evidence of increased virulence or severity beyond typical H3N2 patterns, nor of resistance to antivirals such as neuraminidase inhibitors or baloxavir.
Subclade K has driven an early start to the 2025/2026 flu season, beginning 3 to 5 weeks earlier than recent averages in regions such as the UK and East Asia.
This has led to a rapid increase in cases, particularly among children, adolescents, and young adults, who are typically the first affected and the main drivers of community transmission.
In the UK, for example, 98% of recent influenza A cases were H3N2, predominantly from subclade K, contributing to spikes in test positivity and healthcare visits.
Globally, H3N2 seasons like this one tend to lead to higher overall morbidity, with up to 50 million symptomatic cases and between 15,000 and 70,000 deaths annually in the EU/EEA alone.
Early data suggest it may be more infectious in some populations, but overall indicators of transmission and severity remain low to moderate through the end of 2025.
Cocirculation with other respiratory viruses such as RSV and SARS-CoV-2 could amplify the strain on healthcare systems.
Historically, H3N2 strains, such as subclade K, cause more severe flu seasons than H1N1. Examples like the winter of 2022–2023 in the UK saw flu-related deaths double (16,000 compared to 8,000 the previous year).
The main consequence is a potentially larger epidemic due to lower population immunity resulting from less recent exposure to similar strains, especially among young children, leading to a higher overall number of cases.
This could result in increased use of healthcare, from primary care to ICU beds, and greater absenteeism from school and work.
In closed settings, such as long-term care facilities, outbreaks can cause high morbidity and mortality.
An antigenic mismatch with current vaccines implies reduced antibody recognition, which could decrease vaccine efficacy against infection, although protection against severe disease remains significant (estimated at 30%–40% in adults and 70%–75% in children aged 2–17 years, based on preliminary UK data).
This mismatch occurred because subclade K emerged after the WHO selected vaccine strains. In the US, the lack of timely CDC surveillance data since September 2025 adds uncertainty to monitoring and response efforts.
However, the strain is not associated with unusual severity or antiviral resistance, which mitigates some risks.
Health authorities conclude that, while subclade K poses a moderate risk to the general population and a high risk to vulnerable groups, it does not represent a significantly new threat, and existing tools, such as vaccines and antivirals, remain effective against severe outcomes.
Vaccination is strongly recommended for everyone, especially those at risk, as it provides significant protection despite the mismatch, and early vaccination is crucial given the early start of the season.
Other measures include immediate antiviral treatment for high-risk cases, improved infection control (masks, ventilation in healthcare facilities), and ongoing surveillance to monitor the evolution and effectiveness of the vaccine.
Experts emphasize preparedness and vaccination to reduce hospitalization and mortality rates, noting that, while uncertainties exist regarding immunity and the exact performance of the vaccine, the overall outlook aligns with typical H3N2-predominant seasons.
