WHO Warns of Global Spread of Influenza A(H3N2) Subclade K

The Pan American Health Organization/World Health Organization (PAHO/WHO) reports a rapid increase in the circulation of the influenza A(H3N2) subclade K in the European Union, East Asia, and parts of North America. The variant has accumulated genetic changes, but available data show no rise in clinical severity, with no significant increases in hospitalizations, intensive care admissions, or deaths.

A(H3N2) subclade K viruses have evolved from related J.2.4 strains and now show several amino acid changes in their hemagglutinin. They have been increasingly observed across many regions worldwide, with South America remaining the main exception so far. The subclade became especially noticeable in Australia and New Zealand starting in August 2025, and has since been identified in more than 34 countries over the past six months.

“Current epidemiological data do not indicate an increase in disease severity, although this subclade marks a notable evolution in influenza A(H3N2) viruses,” says WHO.

Europe experienced an early influenza season in which subclade K accounted for nearly half of reported sequences, while the United States and Canada also saw a steady increase in detections.

Despite the antigenic differences observed in the subclade, preliminary vaccine-effectiveness data indicates that protection against hospitalization remains comparable to previous seasons. Estimates suggest effectiveness of about 70%–75% in children and 30%–40% in adults. PAHO/WHO issues this information note to update member states on the global situation and to reinforce recommendations included in its December 2025 epidemiological alert for the Americas.

Mexico’s Ministry of Health reported that no cases of the influenza A(H3N2) subclade K have been detected in the country, as national epidemiological surveillance systems continue routine monitoring ahead of the winter season.

For the Americas, health authorities stress the need to closely monitor the virus through genomic surveillance, maintain high vaccination coverage, and ensure timely treatment as countries prepare for potentially early or intensified influenza activity in the 2025–2026 season. The guidance highlights the importance of vaccination, especially for older adults and people with risk factors, to protect individuals and reduce pressure on hospital services.

According to the Ministry of Health, the National Epidemiological Surveillance System (SINAVE) has not identified the variant. Authorities say that H3N2 is among the influenza strains that circulate each year globally, and monitoring efforts are designed to detect any changes in its behavior or spread.

Health authorities urge the population to seek vaccination at health centers and authorized medical units. The national vaccination campaign for the winter period includes COVID-19, influenza, and pneumococcal vaccines, which are administered according to each individual’s eligibility. The ministry says that these vaccines play a role in reducing risks and preventing complications, particularly among children, older adults, pregnant women, healthcare workers, and people with chronic conditions.

The Ministry of Health reiterates its commitment to public health protection and says that it will continue to provide timely updates should any relevant developments arise.