Last winter marked a significant shift in the landscape of seasonal illnesses, as influenza reclaimed its position as the predominant viral threat following the Covid-19 pandemic, leading to one of the most severe flu seasons in recent memory. With the upcoming 2025-2026 flu season on the horizon, experts are now raising alarms about a new variant within the H3N2 strain, known as subclade K, which has exhibited genetic changes that could complicate vaccination efforts.
Subclade K first began spreading in June and July, after vaccine manufacturers had already determined the strains to include in this year’s flu shots. As a result, there are concerns that this variant could diminish the effectiveness of the vaccine, increasing the risk of infection. The H3N2 strain has been historically recognized for its severity, and the prospect of a vaccine mismatch has experts closely monitoring the situation.
Scott Hensley, a microbiologist at the University of Pennsylvania, emphasizes the importance of vaccination this year, stating, “If there’s ever a year to get a flu vaccine, this is the year.” Evidence suggests that the mutations in subclade K may help it evade antibodies from previous infections or vaccinations. However, experts like Kanta Subbarao from Laval University advise against skipping the flu shot altogether. The vaccine still offers protection against other strains, such as H1N1, and even with a potential mismatch, vaccinated individuals have previously shown a reduced likelihood of severe illness.
Historical data supports the idea that vaccines can provide some level of protection against worse outcomes. During the 2014-2015 flu season, despite a notable vaccine mismatch with the H3N2 strain, those vaccinated were less likely to require hospitalization.
The emergence of H3N2 subclade K has raised the stakes, particularly as flu season begins in the United Kingdom and Japan, where the variant is now prevalent. Initial studies from the U.K. Health Security Agency indicate that the current vaccine could still offer reasonable protection against subclade K, with estimates showing 70% to 75% efficacy in children and 30% to 40% in adults against hospitalization due to flu.
However, experts like Danuta Skowronski express caution, indicating uncertainty about the vaccine’s effectiveness over time, especially as immunity may wane. Preliminary reports from the Centers for Disease Control and Prevention suggest that flu activity remains low in the U.S., but a significant portion of circulating viruses are H3N2 strains, with many being subclade K.
While the flu season remains unpredictable, experts like Ian Barr from the World Health Organization’s Collaborating Centre for Reference and Research on Influenza in Melbourne report that despite the emergence of subclade K, there are no current indications of more severe illness linked to this variant.
As influenza experts navigate the complexities of predicting flu seasons, there remains a cautious optimism that vaccination efforts will provide critical protection. Engaging in preventive measures, including getting vaccinated, could mitigate the potential burden of flu this winter, offering hope for a healthier season ahead.