The Influenza A(H3N2), a subtype of the seasonal flu virussaw a significant increase in its circulation in 2025, driven by the emergence of a new subclass called K (formerly called J.2.4.1).
According to reports from the World Health Organization (WHO) and the Centers for Disease Control and Prevention (CDC) of the United States, this subclade dominated the detection of influenza in several regions of the Northern Hemisphere, contributing to an early onset of the flu season and a greater number of respiratory illnesses worldwide.
This Friday, December 19, the National Administration of Laboratories and Health Institutes (ANLIS) “Dr. Carlos G. Malbrán” confirmed this Detection of three cases of H3N2 corresponding to subclass K in the country, identified through genome sequencing techniques.
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The cases were detected by the National Network of Laboratories and Sentinel Units and confirmed by the National Reference Laboratory of INEI-ANLIS. These are two adolescents examined as part of the outpatient surveillance of acute respiratory infections in the province of Santa Cruz and a child admitted to a hospital in the Autonomous City of Buenos Aires. As officially reported, the disease progressed without complications in all patients.
The influenza A(H3N2) virus is one of the predominant subtypes in seasonal influenza epidemics. Influenza viruses continually evolve through genetic mutations, a process known as antigenic drift, which allows them to partially evade previous immunity acquired through vaccination or a previous infection. In 2025, the K subclass emerged after the accumulation of multiple mutations in hemagglutinin (HA), the key surface protein for the virus to enter cells.
These genetic changes, identified by the CDC in August 2025, include amino acid substitutions that distinguish subclade K from the strains included in the 2025-2026 seasonal vaccine. As a result, this subclade represents up to 89% of H3N2 viruses characterized in the United States since September 2025, and similar proportions in Europe and Asia, according to data from the Global initiative to share all influenza data (GISAID) and the WHO.
Subclade K emerged at the end of the southern hemisphere flu season (Australia and New Zealand) and spread rapidly to the north from September 2025. The WHO reports a rapid increase in detections since August, with countries in Europe, North America and Asia predominating. In Europe, it accounts for up to 90% of confirmed cases in some regions, putting pressure on health systems, although there is no evidence of greater clinical severity compared to previous H3N2 strains.
In the United States, the CDC points this out H3N2 is the most common virus this seasonwith increasing activity in most states. Globally, influenza activity remains within expected seasonal limits, although early surges are occurring in several areas, contributing to the overall increase in respiratory infections.
The 2025-2026 seasonal vaccine contains an H3N2 component based on earlier strains (subclade J.2) and selected before the emergence of subclade K. This leads to partial antigen mismatch and reduces effectiveness against infections, but preliminary data from Europe and the United States confirm this Vaccination still provides significant protection against serious illness, hospitalization and death.
Symptom Guide
Symptoms of influenza A (H3N2) infection, including subclass K, are similar to those of other seasonal flu and have no significant clinical differences in severity, according to the WHO and CDC. They generally occur suddenly and include:
– High fever or chills.
— Persistent cough.
– Sore throat.
– Stuffy nose or discharge.
— Severe muscle and body pain.
– Headache.
— Extreme tiredness.
— In some cases, vomiting or diarrhea (more common in children).
The typical duration is 3 to 7 days, although fatigue can last for weeks. Complications such as pneumonia are more common in older adults, young children, pregnant women and people with chronic illnesses. Seasons dominated by H3N2 historically have a greater impact on older adults.
Prevention measures in view of the increase in global respiratory diseases
Given the increase in respiratory infections caused by subclass K viruses and others in 2025, health authorities such as the WHO, the CDC and the Pan American Health Organization (PAHO) recommend:
— Annual vaccination: It is the most effective measure. Although there is a partial imbalance, it reduces the risk of serious illness and hospitalization. It is recommended for everyone over 6 months, especially at risk groups.
— Hand hygiene: Frequent washing with soap and water or using alcohol-based disinfectant.
— Respiratory label: Cover your mouth and nose with a disposable tissue or elbow when coughing or sneezing.
— Avoid close contact: Keep your distance in public spaces if symptoms occur.
— Using a mask: In enclosed environments or when respiratory symptoms are present to reduce transmission.
— Stay home if you are sick: To prevent spread.
— Monitoring and early treatment: In risky cases, consult a doctor; Antivirals such as oseltamivir are effective when used early.
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