Flu Season Intensifies Across US Amid Spread of New Subclade K Variant

The United States is experiencing an early and intensifying flu season fueled by a new influenza virus variant known as subclade K. For the week ending December 6, doctor’s visits for flu-like symptoms exceeded the epidemic threshold at 3.2%, signaling an official start to the season, according to the CDC.

This year’s flu activity is concentrated mostly in the Northeast, with New York City reporting very high levels and other states like New Jersey, Massachusetts, and Louisiana showing elevated flu cases. New York State health officials noted that flu hospitalizations have doubled weekly over the last two weeks, indicating a sharp rise in severe cases.

Subclade K is a variant of the H3N2 influenza A strain, which is known for causing more severe illness, especially in older adults. This variant has already driven record flu seasons in Australia and is contributing to early and high flu activity in countries including Japan, the UK, and Canada. Experts warn that the US may also face a challenging season.

The current flu vaccines do not specifically target subclade K, as the variant emerged after the vaccine composition was finalized. However, studies from the UK show that this year’s vaccines still reduce the risk of hospitalization and emergency visits due to H3N2 strains by 30% to 75%, especially offering better protection for children than adults.

Despite limitations, health professionals emphasize the importance of vaccination, recommending people get their flu shot immediately if they have not yet done so. Dr. Alex Greninger from the University of Washington noted that immunity builds about seven days after vaccination, making prompt action key this season.

In addition to vaccination, experts advise adopting multiple preventive measures such as wearing masks in crowded areas and improving ventilation by opening windows or using air purifiers. These combined strategies help reduce flu transmission and protect vulnerable populations during this busy flu season.

Clinicians encourage timely flu testing for symptomatic individuals, as antiviral treatments started within 48 hours can lessen illness severity and shorten recovery. With an active subclade K variant driving increased flu burden, ongoing vigilance and preventive efforts remain critical to managing this developing public health challenge.