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America could be in for a “severe” influenza season in which flu vaccines offer limited protection, a top UVA Health flu expert is warning.

Frederick G. Hayden, MD, Professor Emeritus, UVA School of Medicine, says available vaccines appear to be a poor match for the subclade K virus that already has started circulating in the United States. The virus likely will gain traction over the holidays and spread further when schools reopen after winter break, Dr. Hayden says.

The variant is of the same influenza A lineage that caused the flu pandemic of 1968, killing an estimated 100,000 Americans and a million people worldwide. The strain, H3N2, has circulated since and often sparks severe epidemics that are more likely to cause deaths and hospitalizations than other strains, especially among the elderly and very young.  

“The 2024-25 influenza influenza season, largely due to influenza A H1N1 and H3N2 viruses, was associated with high severity across all age groups, including an exceptionally high number of pediatric deaths. Unfortunately, we are seeing the circulation of a new variant of H3N2 virus for which our vaccines may be somewhat less effective,” says Dr. Hayden, part of UVA Division of Infectious Diseases and International Health. “However, available vaccines will reduce the risk of serious illness and potentially the spread of this virus.” 

Predicting Flu Season

Dr. Hayden raises his concerns about the upcoming flu season in a new article in the Journal of the American Medical Association co-authored by British virologist Maria Zambon, PhD. As one consideration in determining what to expect, the scientists looked to other countries to see how they have fared during their flu seasons. The fast-spreading subclade K is hitting the United Kingdom hard right now. The UK National Health Service has warned that flu hospitalizations in that country are the highest they’ve ever been this time of year.

Australia, meanwhile, has already gone through flu season in the Southern Hemisphere. The overall flu burden there was considered “moderate,” but Dr. Hayden notes the emergence of the K subclade variant gave the country’s flu season extra legs, stretching out its impact longer than usual.

In the United States, flu activity remained low up through Thanksgiving, even with the K variant already starting to circulate. But a Dec. 11 update from the federal Centers for Disease Control and Prevention suggests the spread of the virus already is accelerating, and Dr. Hayden and Zambon expect it to worsen in the coming weeks.

Flu Vaccine Mismatch

Of particular concern, the scientists say, is the mismatch of the virus and the available vaccines. Vaccine makers must prepare their vaccines well before flu season, so public health agencies like the CDC and WHO must make educated guesses about the strains to include. In the UK, an early analysis suggests injected vaccines for adults were only 32%-39% effective in preventing influenza-associated emergency department visits and hospitalizations so far this year, while children’s vaccines — primarily the intranasal one — fared better, at 72%-75% effectiveness.

While the available vaccines may not be as effective as we’d like, they still offer important protections from severe illness. The CDC recommends influenza vaccination for everyone aged 6 months and older with rare exceptions. Vaccines are particularly important for children, those with underlying health conditions including pregnancy, and for seniors, as the immune system’s ability to fend off severe illness fades with age.

In terms of prevention, Dr. Hayden also urges common-sense precautions such as keeping your distance from sick people by avoiding crowds, wearing a mask when that is unavoidable, and handwashing. In addition to vaccination, seniors and other people at risk also should consider taking flu antivirals for prevention if they are exposed to someone with influenza in their home or place of residence, Hayden says. If illness develops, prompt antiviral treatment can reduce the duration of the illness and the risk of complications. Such measures also can help prevent hospitals from becoming overburdened.

“Getting vaccinated as soon as possible remains the single most important action people can do to protect themselves and their families,” Dr. Hayden says. 

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