Australia’s 2025 flu season lasted weeks longer than usual, with Hong Kong experiencing an early outbreak before the rollout of seasonal flu shots. Meanwhile, New York has seen record-breaking flu hospitalizations in recent weeks. This flu season has been dominated by a variant known as subclade K, a member of the H3N2 family of viruses that can cause more severe illness, particularly in older individuals.
Subclade K first emerged in New York in June, after the decision had been made on which strains to include in the 2025-2026 Northern Hemisphere flu shot. This means that this year’s vaccine may not provide optimal protection against H3N2. The Centers for Disease Control and Prevention (CDC) reported that nearly 11 million people have been sick with the flu this season, with 120,000 hospitalizations and 5,000 deaths, including nine children.
The mutations in subclade K make it adept at evading the immune response, leading to widespread infection across the country. While the flu season has not been exceptionally early in most parts of the U.S., some countries like Japan, Hong Kong, and the U.K. experienced early outbreaks. The timing and severity of flu seasons can vary widely from year to year, making it challenging to predict the course of the illness.
Despite the overwhelming number of cases, there is no evidence to suggest that subclade K causes more severe illness on an individual level. Antiviral drugs remain effective against this variant, providing an important treatment option for high-risk individuals. The effectiveness of this year’s flu vaccine against subclade K is still being evaluated, with preliminary data showing moderate protection in children and teens.
However, vaccination rates have declined in recent years, leaving a large portion of the population vulnerable to the flu. While the majority of those who received the flu shot may have some degree of protection, many are facing this new H3N2 variant without the benefit of vaccination. As the flu season continues, public health agencies will closely monitor the effectiveness of the vaccine and the impact of subclade K on the population.
