A mutated H3N2 variant is fueling surging hospitalizations and pediatric deaths across the country.
The 2025–26 flu season is shaping up to be one of the harshest in recent memory, driven by a fast-spreading mutated strain of influenza A that has earned the informal label “super flu.” Federal surveillance data show that hospitalizations nearly doubled in a single week in late December, and doctors across the country say the season is far from its peak.
By late December, the CDC estimated that at least 7.5 million Americans had fallen ill with influenza since October, with approximately 3,100 deaths recorded. Flu activity reached “high” or “very high” levels in 32 states and jurisdictions during the week ending December 20—up from 17 just one week earlier. Hospital admissions surged from roughly 9,900 to more than 19,000 in that same period. New York State reported its highest single-week case count on record, with over 71,000 confirmed infections.
At the center of the surge is a mutated version of the H3N2 virus known as subclade K. The variant emerged earlier in 2025 and quickly became the dominant circulating strain in the Northern Hemisphere, triggering severe outbreaks in Japan, the United Kingdom, and Canada before gaining dominance in the United States. Virologists say subclade K carries mutations that help it evade existing immunity from prior infections and vaccinations, making populations more susceptible.
Children and older adults are bearing a disproportionate burden. Pediatric flu deaths climbed to eight by late December, and doctors reported seeing unusually high case volumes in young children under four years old. Emergency departments in hospitals across the Northeast, Midwest, and South have been stretched thin as flu, COVID-19, RSV, and norovirus cases overlap during what physicians describe as a punishing winter respiratory season.
The season’s severity is complicated by a partial mismatch between the circulating virus and the 2025–26 flu vaccine. Because subclade K emerged after vaccine formulations were finalized, the vaccine is not an optimal match for the dominant strain. However, preliminary data from England suggest the shot still provides meaningful protection—approximately 70–75 percent effectiveness against hospitalization in children and 30–40 percent in adults. Experts emphasize that even imperfect vaccines reduce the risk of severe illness and death.
Adult flu vaccination rates have hovered around 43 percent nationally, and childhood rates are similar—both insufficient to blunt a severe season. Public health officials have urged anyone who has not yet been vaccinated to do so, noting that it takes roughly two weeks to build protective immunity. Antiviral treatments like Tamiflu and the newer single-dose drug Xofluza remain effective when administered within 48 hours of symptom onset.
Infectious disease specialists expect flu activity to continue rising through January and possibly into February before subsiding. The combination of a highly transmissible variant, moderate vaccine coverage, and holiday travel and gatherings has created conditions for sustained, widespread transmission that may not ease for weeks.
Sources
1. “Flu is surging nationwide. Hospitalizations nearly doubled in a single week.” — NBC News, December 30, 2025. https://www.nbcnews.com/health/health-news/flu-season-virus-symptoms-surge-nationwide-variants-vaccines-children-rcna251534
2. “A New, Potentially Severe Flu Variant Is Spreading in the US” — Today.com, January 16, 2026. https://www.today.com/health/cold-flu/new-flu-strain-2025-symptoms-rcna243681
3. “New York State Department of Health Confirms Most Flu Cases Ever Recorded in One Week” — New York State Department of Health, December 26, 2025. https://www.health.ny.gov/press/releases/2025/2025-12-26_flu_cases.htm
4. “How Bad Will This Winter Be for Flu, COVID, RSV, and Measles?” — Johns Hopkins Bloomberg School of Public Health, January 5, 2026. https://publichealth.jhu.edu/2025/virus-transmission-trends-winter-2025-26
5. “10 things to know about this year’s surprisingly fierce flu season” — AAMC, January 2026. https://www.aamc.org/news/10-things-know-about-year-s-surprisingly-fierce-flu-season
