New H3N2 flu strain raises global health concerns

(MENAFN) A newly identified strain of influenza A(H3N2), now classified as subclade K, is spreading internationally and raising concerns about a potentially severe flu season, particularly for older adults and young children, according to reports. The strain emerged after the 2025–2026 flu vaccine formulation was finalized, prompting questions about vaccine effectiveness and preparedness.

The virus is a mutated version of H3N2, one of the primary drivers of seasonal flu alongside H1N1 and influenza B. Experts note that it has undergone significant antigenic drift, accumulating roughly 10 mutations in the hemagglutinin gene, a key target of vaccines. “These mutations now make it different from the earlier, representative virus used to develop the vaccine,” noted CDC influenza surveillance officials.

The H3N2 subclade K strain has contributed to early and intense flu activity in several countries. In Japan, an influenza epidemic was declared unusually early in October. In the UK, health authorities warned of a “cruel winter,” with cases expected to triple compared to last year.

Canada has reported early influenza waves linked to the new strain. In the Southern Hemisphere, severe acute respiratory infections, including flu, rose 29%, prompting the Pan American Health Organization (PAHO) to call for strengthened preparedness. In the United States, H3N2 now dominates circulating influenza strains, accounting for 86% of influenza A cases, with 89% of those identified as subclade K, according to the CDC.

Historically, H3N2 seasons have led to higher hospitalizations and deaths, particularly among seniors and young children. So far this season, the CDC estimates 2.9 million illnesses, 30,000 hospitalizations, and 1,200 deaths in the US, while other reports suggest 4.6 million cases and 1,900 deaths, including at least three children.

Symptoms of the H3N2 strain are reported to be intense and fast-onset, including high fever, severe body aches, cough, rapid household spread, and vomiting in children. Pediatricians advise seeking medical attention if high fevers persist for more than four to five days.

Although subclade K emerged after vaccine strains were chosen, early data from the UK suggest the vaccine still provides meaningful protection, with 72%-75% effectiveness against emergency visits in children and 32%-39% in adults. Health authorities continue to recommend vaccination as the primary protective measure, complemented by early testing, antiviral treatment, respiratory hygiene, and staying home when ill. Experts stress that available antivirals remain effective against this strain.

Flu vaccination uptake has declined this season, with about 44 million doses administered in the US—over 3 million fewer than last year—partly attributed to a temporary government shutdown that limited public access to flu data.

Health authorities are also tracking avian influenza. A rare human case of H5N5 was reported in Washington state, while South Korea detected new outbreaks of highly pathogenic H5N1 at poultry farms, prompting quarantine measures and inspections.

Experts emphasize vigilance and early intervention, noting that preparedness, surveillance, and prompt medical care will determine the severity of this flu season.

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