Seasonal viruses—including influenza and norovirus—are surging across country ahead of one of the busiest holiday travel weeks, increasing the risk of outbreaks among families gathering to celebrate.
Health officials warned that this season’s flu is being driven largely by the newly identified H3N2 subclade K strain, which shows signs of increased transmissibility and reduced vaccine match. Simultaneously, norovirus infections are reaching high levels in numerous states, with COVID-19 continuing to persist, threatening the health care system’s capacity.
What To Know
This year’s “super flu” surge is tied to a mutated strain of influenza A, called H3N2 subclade K. The strain has become dominant in the United States, responsible for about 89 percent of all H3N2 infections tested since late September, according to genetic sequencing data from the Centers for Disease Control and Prevention (CDC).
The subclade emerged after the 2025-26 flu vaccine was designed, prompting concerns about a weaker immune response for vaccinated individuals.
However, typical symptoms remain unchanged—fever, chills, severe fatigue, cough, body aches—but the relative vaccine mismatch raises the risk of more infections and severe cases.
According to CDC data published on Friday, an estimated 4.6 million flu cases, 49,000 hospitalizations, and 1,900 deaths have been recorded in the U.S. so far this season.
The number of cases sharply increased from the week ending on December 6 to the week ending on December 13, the data shows, with the percentage of positive cases growing from 8.1 percent to 14.3 percent.
Data collected over the last few months shows test positivity and hospitalizations have risen since November, with some school districts closing due to large outbreaks among students and staff.
Norovirus Spreads Rapidly Across the U.S.
Norovirus, commonly called the “winter vomiting disease,” is also surging across much of the country, with wastewater data showing a 69 percent nationwide increase in viral concentrations since October, according to WastewaterSCAN, which categorized the national risk as “high” this month.
Outbreaks have been most pronounced in the Northeast and Midwest, along with local clusters in Illinois and Texas. Norovirus cases have led to school closures and increased emergency room visits. The CDC reported nearly 14 percent positivity in recent clinical tests by mid-November, double what was seen three months before.
Impact on Children and Schools
Children have been heavily affected by flu and norovirus outbreaks. In Marin County, California, about one-third of a school’s students were absent due to flu-like symptoms, while similar impacts led to e-learning days in suburban Chicago.
New York City schools reported declining attendance amid hundreds of pediatric flu cases. Many schools responded by deep cleaning and disinfecting facilities ahead of the winter break.
Holiday Travel and Gathering Risks
The holiday travel period is expected to be the busiest on record, with AAA projecting 122 million Americans on the move from December 20 to January 1.
Crowded airports, transit terminals, and family gatherings create ideal conditions for viral transmission.
Health officials encourage people to stay home if symptomatic, practice thorough handwashing (especially for norovirus prevention), and get vaccinated against influenza and COVID-19—even with suboptimal strain matches, vaccines reduce the likelihood of severe illness, hospitalization, and death.
What People Are Saying
Dr. Farrell Tobolowsky, medical director for communicable diseases, San Francisco Department of Public Health, told SFGATE on Friday: “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.”
Virologist Andy Pekosz, Johns Hopkins Bloomberg School of Public Health, told Gothamist on Friday about this year’s strain of flu: “Symptom-wise, [there’s] nothing that we know that’s different about this virus outside of the fact that it’s just going to be able to infect more people and therefore we’ll see more cases.”
Dr. Robert Hopkins Jr., National Foundation for Infectious Diseases, told Today.com on Friday: “Knowing that there’s a new mutated strain out there and H3N2 generally causes more severe disease is concerning.”
What Happens Next
The CDC, state, and local health agencies will continue to monitor viral trends closely as Americans return home after the holidays, anticipating additional spikes in cases.