As winter approaches and holiday gatherings bring people together, health experts are sounding the alarm about a potentially tough flu season in the United States. A new variant of the influenza A (H3N2) virus, known as subclade K, is spreading rapidly across the country and around the world. This strain has already caused early and severe outbreaks in places like Japan, the United Kingdom, Canada, Australia, and parts of Europe. In the U.S., flu activity is picking up faster than usual in some areas, with the majority of tested viruses being this subclade K variant.
According to the latest data from the Centers for Disease Control and Prevention (CDC), flu season officially began in early December 2025, when the percentage of doctor’s visits for flu-like symptoms crossed the national epidemic threshold. As of the week ending December 6, at least 14 states and territories reported moderate to high flu activity, concentrated in the Northeast, South, and parts of the West, including high levels in New York, New Jersey, Louisiana, and Colorado. The CDC has also reported the season’s first pediatric flu death, a somber reminder of the virus’s dangers.
What makes subclade K particularly worrisome is how it evolved. Influenza viruses mutate naturally as they spread, and this variant developed several genetic changes after vaccine manufacturers had already selected the strains for the 2025-2026 flu shot. These mutations allow it to partially evade the immune response triggered by the current vaccine, creating what experts call a “mismatch.” As one infectious disease specialist explained, it’s like using an outdated address—the body’s defenses get close but may not fully stop the virus.
Despite this mismatch, experts strongly recommend getting the flu vaccine anyway. Early data from the UK shows that the shot still reduces the risk of severe illness, hospitalization, and death by 30-40% in adults and up to 75% in children, even against subclade K-dominated strains. In the U.S., where most vaccines are egg-based, protection might be slightly lower, but any reduction in severity is valuable. The vaccine also guards against other circulating flu strains and helps limit spread in the community.
Adding to the concern is a drop in vaccination rates. Nationwide, only about 38% of children have received their flu shot so far this season—the lowest in recent years. Overall adult rates are also lagging. Misinformation about vaccines, lingering from the COVID-19 era, has contributed to this decline. When fewer people are vaccinated, viruses spread more easily, leading to more illnesses, hospitalizations, and deaths.
The flu remains a serious threat every year. Even in average seasons, it causes tens of thousands of deaths in the U.S., with the very young, elderly, and those with underlying conditions at highest risk. Last season (2024-2025) was especially severe, with record-high pediatric deaths—nearly 280 children lost their lives, many of whom were previously healthy and unvaccinated.
To stay safe this holiday season, experts advise:
- Getting vaccinated if you haven’t already—it takes about two weeks for full protection to kick in.
- Practicing good hygiene: washing hands frequently, covering coughs, and staying home when sick.
- Considering masks in crowded indoor spaces, especially airports or large gatherings.
- Improving ventilation by opening windows or using air purifiers.
- Testing promptly if symptoms appear and asking about antiviral treatments like Tamiflu, which work best when started early.
Flu symptoms hit suddenly and include high fever, chills, cough, sore throat, body aches, and fatigue. While most people recover at home, complications like pneumonia can turn dangerous quickly.
Health officials urge everyone aged 6 months and older to get their flu shot now. As Dr. Michelle Barron from UCHealth noted, “It’s never too late… You want to be able to go out and enjoy the holidays” without ending up sick in bed. With subclade K on the rise and vaccination rates down, taking these steps can help protect yourself, your family, and vulnerable loved ones from a rough winter ahead.
