America Faces Severe Flu Season with Limited Vaccine Protection

The United States is bracing for a potentially severe influenza season, according to Dr. Frederick G. Hayden, a prominent flu expert at UVA Health. He warns that the current flu vaccines may provide limited protection against the circulating strain, particularly the subclade K variant of the H3N2 virus. This variant is expected to gain momentum over the holiday period and increase in prevalence as schools resume after the winter break.

The H3N2 strain is of particular concern due to its historical significance. It belongs to the same lineage that caused the flu pandemic of 1968, which resulted in approximately 100,000 deaths in the U.S. and about a million globally. Dr. Hayden notes that H3N2 typically leads to severe outbreaks, with higher rates of hospitalizations and fatalities, especially among vulnerable populations such as the elderly and young children.

In his recent article published in JAMA, co-authored with British virologist Maria Zambon, Dr. Hayden emphasizes that the current flu season may see heightened severity. “The 2024–25 influenza season, primarily driven by influenza A H1N1 and H3N2 viruses, is associated with high severity across all age groups, including an exceptionally high number of pediatric deaths,” he states. He cautions that the emergence of the new variant could lead to further complications in managing the flu.

International Context and Vaccine Effectiveness

The researchers assessed the flu situation in other regions to forecast the potential impact in the U.S. The subclade K variant is causing significant issues in the United Kingdom, where the National Health Service has reported record high flu hospitalizations for this time of year. In contrast, Australia’s recent flu season was categorized as “moderate,” although the emergence of the K variant prolonged its effects.

Despite the low flu activity in the U.S. leading up to Thanksgiving, the Centers for Disease Control and Prevention (CDC) reported an acceleration in the spread of the virus as of December 11. Dr. Hayden and Dr. Zambon predict worsening conditions in the coming weeks.

A critical issue highlighted by the researchers is the mismatch between circulating viruses and the available vaccines. Vaccine manufacturers must create their products well in advance of flu season, necessitating educated guesses on which strains to include. Preliminary analyses in the UK indicated that adult vaccines have only 32–39% effectiveness in preventing emergency department visits and hospitalizations related to influenza. In contrast, vaccines for children, particularly the intranasal variant, showed better effectiveness at 72–75%.

Recommendations for Prevention

Despite the limitations of current vaccines, both doctors stress the importance of vaccination as a vital means of reducing the risk of severe illness. The CDC recommends flu vaccinations for everyone aged 6 months and older, with few exceptions. Vaccination is particularly crucial for high-risk groups, including children, pregnant individuals, and seniors, as older adults typically experience diminished immune responses.

In addition to vaccination, Dr. Hayden advises individuals to take common-sense precautions. This includes avoiding crowded places, wearing masks when necessary, and practicing good hand hygiene. For seniors and others at risk, he suggests considering antiviral medications for preventive measures if exposed to the virus, as timely treatment can shorten the duration of illness and mitigate complications.

“Getting vaccinated as soon as possible remains the single most important action people can take to protect themselves and their families,” Dr. Hayden asserts. As the flu season approaches, the emphasis on preventive measures will play a crucial role in safeguarding public health.

For more information, refer to the article by Maria Zambon et al., “Influenza A(H3N2) Subclade K Virus,” published in JAMA (2025). DOI: 10.1001/jama.2025.25903.