Health officials are raising alarms as cases of flu and norovirus escalate just ahead of the busy holiday travel period. This season’s flu outbreak is primarily driven by the newly identified H3N2 subclade K strain, which has shown greater transmissibility and a reduced effectiveness against vaccines. At the same time, norovirus infections are also on the rise across the United States, complicating the health landscape as families prepare to gather for celebrations.
The current wave of influenza is largely attributed to the H3N2 subclade K strain, which has become the dominant variant in the U.S. According to genetic sequencing data from the Centers for Disease Control and Prevention (CDC), this strain accounted for approximately 89 percent of all H3N2 infections tested since late September. This emergence occurred after the formulation of the 2025-26 flu vaccine, raising concerns about its effectiveness. Typical flu symptoms such as fever, cough, and body aches remain unchanged, but the mismatch between the circulating strain and the vaccine heightens the risk of severe cases.
As of now, the CDC reports an estimated 4.6 million flu cases, 49,000 hospitalizations, and approximately 1,900 deaths in the U.S. for this season. Data indicates a sharp increase in cases from the week ending on December 6 to the week ending on December 13, with the percentage of positive cases rising from 8.1 percent to 14.3 percent.
Norovirus Outbreaks Intensify
In tandem with the flu surge, norovirus, often referred to as the “winter vomiting disease,” is spreading rapidly. Wastewater analysis conducted by WastewaterSCAN indicates a 69 percent increase in viral concentrations since October. The most significant outbreaks are concentrated in the Northeast and Midwest, with clusters reported in states such as Illinois and Texas. The CDC noted that positivity rates for norovirus in clinical tests reached nearly 14 percent by mid-November, a doubling compared to three months prior.
The impact of these viruses has been particularly notable among children. In Marin County, California, around one-third of students were absent from school due to flu-like symptoms, while schools in suburban Chicago resorted to remote learning days in response to large outbreaks. New York City schools also reported declining attendance, prompting many institutions to undertake extensive cleaning measures.
Challenges of Holiday Travel
The upcoming holiday travel period, projected to be one of the busiest on record, poses additional risks for viral transmission. The American Automobile Association (AAA) estimates that approximately 122 million Americans will travel from December 20 to January 1. Crowded airports and family gatherings create ideal conditions for the spread of respiratory viruses. Health officials are urging individuals to remain vigilant, advising anyone who feels unwell to stay home, practice thorough hand hygiene, and get vaccinated against both influenza and COVID-19.
Dr. Farrell Tobolowsky, medical director for communicable diseases at the San Francisco Department of Public Health, emphasized the importance of seeking medical attention if symptoms arise, particularly for those at higher risk for severe illness. “We would encourage people, if you’re sick, especially if you’re at higher risk of severe illness, to promptly get to your health care provider to get tested and get treatment,” Tobolowsky stated.
Virologist Andy Pekosz from the Johns Hopkins Bloomberg School of Public Health noted that while the symptoms remain consistent, the increased infectivity of this year’s strain will likely result in a higher number of cases. Dr. Robert Hopkins Jr., from the National Foundation for Infectious Diseases, expressed concern regarding the potential severity of H3N2, especially given its mutation.
Looking ahead, the CDC and local health agencies are closely monitoring trends as Americans return from holiday travels, anticipating further spikes in cases. Public health officials continue to stress the importance of vaccination and preventive measures to mitigate the effects of these seasonal outbreaks.
