A new version of the common flu is spreading globally, and health officials are monitoring this evolving strain of influenza A(H3N3) subclade K, which has been increasingly detected worldwide.
Seasonal influenza activity has increased globally in recent months, with influenza A viruses accounting for the majority of detections, according to the World Health Organization.
Health officials are closely monitoring a growing subgroup of influenza A(H3N2) viruses known as J.2.4.1, also referred to as subclade K.
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The WHO says detections of this subclade have risen rapidly since August 2025 based on genetic sequence data shared through the global GISAID database.
Current epidemiological data does not indicate increased disease severity associated with subclade K, the WHO says, though its spread reflects the continued evolution of seasonal influenza viruses.
What is subclade K?
Subclade K is a genetically distinct subgroup of influenza A(H3N2) viruses, according to the WHO.
These viruses have drifted genetically from related J.2.4 viruses and carry several amino acid changes in the haemagglutinin protein, which plays a key role in the virus’ attachment to human cells.
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Influenza viruses commonly undergo such changes over time. Global surveillance tracks these shifts to assess potential impacts on transmission, severity and vaccine effectiveness.
Where is subclade K circulating?
The WHO reports that subclade K viruses were first detected at increased levels beginning in August 2025, particularly in Australia and New Zealand.
Since then, the viruses have been identified in more than 34 countries over the past six months, including the U.S.
Detections are increasing in many regions of the world, except in South America so far, according to the WHO.
What is happening in America?
Data from the U.S. Centers for Disease Control and Prevention shows that influenza activity in North America remains relatively low but is increasing, driven mainly by detections of influenza A viruses.
During the 2025 southern hemisphere influenza season in the Americas, transmission exceeded the seasonal threshold in mid-March and mainly remained at low to moderate levels, the CDC says.
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The CDC reports a predominance of influenza A(H3N2) in both the United States and Canada, with growing detections of the A(H3N2) subclade K.
Are symptoms different?
The WHO says there is no evidence that infections caused by subclade K produce symptoms that differ from or are more severe than those caused by other seasonal influenza viruses.
Seasonal influenza symptoms typically include fever, cough, sore throat, runny or stuffy nose, muscle or body aches, headache and fatigue. Severity can vary based on age, underlying health conditions and immune status.
Do vaccines still protect?
Early estimates cited by the WHO suggest that seasonal influenza vaccines continue to protect against severe illness and hospitalization in both children and adults.
While effectiveness against symptomatic infection may vary from season to season, health officials say vaccination remains one of the most effective public health measures, particularly for people at higher risk of influenza complications and their caregivers.
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Even when circulating viruses differ genetically from vaccine strains, vaccines may still reduce the risk of severe outcomes, the WHO says.
What happens next?
The WHO says it continues to monitor global influenza activity and viral evolution, while supporting countries in surveillance efforts and updating guidance as new data emerges.
Health officials note that changes in circulating influenza viruses are expected each season and are routinely assessed through international monitoring systems.