Although the flu season seems off to a quiet start in the US, warnings from abroad suggest that a new strain may yet shake up the US healthcare system.

New data reported in the British Medical Journal showed a significant increase in flu rates in the UK, largely among young adults and school-aged children. The cases are dominated by a new H3N2 strain, which mutated several times over the summer, increasing in severity, according to experts.

Sometimes a strain develops that, while not entirely new, is new enough to spread more efficiently because it is not well matched to current community immunity from previous infections and vaccinations, according to Antonio Ho, MD, clinical senior lecturer and consultant in infectious diseases at the University of Glasgow, Glasgow, Scotland.

“The current circulating influenza A (H3N2) virus acquired seven new mutations over the summer, which means the virus is quite different to the A (H3N2) strain included in this year’s vaccine,” Ho said in a media statement.

The emergence of seasonal influenza A (H3N2) variants was reported in the Journal of the Association of Medical Microbiology and Infectious Disease Canada.

During the southern hemisphere’s 2025 influenza season, a variant A (H3N2) virus emerged that did not match well with the current vaccine, and this variant, now renamed subclade K, is projected to dominate the northern hemisphere’s 2025-2026 season, the researchers wrote. In October, Japan declared a flu epidemic, as reported in the journal Nature, but the proportion of cases caused by the subclade K variant and the impact on other countries remains unclear.

Silence From CDC, Local Data Arriving 

Between the recent government shutdown and ongoing changes in the makeup of the CDC, up-to-date information on flu activity on the national level is lacking; the most recent flu activity from the CDC’s website indicates low levels as of the last week in September.

“National data on influenza and pertussis are delayed, though organizations that work to collate data from local and state resources are reporting flu activity is beginning in the South,” said Lori Handy, MD, MSCE, associate director of the Vaccine Education Center at the Children’s Hospital of Philadelphia, and assistant professor of clinical pediatrics at the Perelman School of Medicine, Philadelphia, in an interview.

Even a mismatched vaccine has protective benefits, Handy said. “With flu season about to begin, those who are not yet vaccinated should get vaccinated as soon as possible,” she added. “Many people may still get sick, but vaccination will keep them out of the emergency department and hospital,” Handy emphasized.

Handy disclosed no financial conflicts of interest.