A flu strain that most Americans have never encountered is spreading at an alarming pace. Hospitals are filling up. Children are dying. And experts say the worst may still lie ahead.
New data from the Centers for Disease Control and Prevention paints a sobering picture of an influenza season that has caught many off guard. With holiday gatherings now behind us and millions of travelers returning home, public health officials are bracing for what could become one of the most severe flu seasons in recent memory.
A Season Unlike Recent Years
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CDC estimates released on December 30 reveal a flu season gaining momentum faster than many anticipated. So far, at least 7.5 million Americans have fallen ill with influenza. Hospitals have admitted 81,000 patients. And 3,100 people have died.
Behind these numbers sits a culprit that has taken center stage in laboratories and hospital wards across the nation. A variant known as subclade K, part of the influenza A(H3N2) virus family, now accounts for nearly 90% of genetically characterized flu samples. H3N2 strains have long been associated with tougher flu seasons, particularly for older adults. But subclade K presents a fresh challenge.
Sharon Nachman, MD, division chief of pediatric infectious diseases at Stony Brook Children’s Hospital in New York, explained why so many people are getting sick. “This flu is something that most of us have not seen, this variant,” she said. “That means your antibodies that you had in the past aren’t going to help protect you so much.”
Subclade K did not arrive without warning. It circulated through other countries over the summer, driving significant flu spikes in Canada, Japan, and the United Kingdom. Yet its arrival in American communities has still managed to overwhelm health systems and catch families unprepared.
Geographic Spread Accelerates
Flu activity has reached “high” or “very high” levels in 32 jurisdictions, up from just 17 the previous week. Twenty states now report “very high” activity, including Arkansas, Colorado, Connecticut, Georgia, Idaho, Louisiana, Maryland, Massachusetts, Michigan, Minnesota, Missouri, New Hampshire, New Jersey, New Mexico, New York, North Carolina, Ohio, Rhode Island, South Carolina, and Tennessee.
Another nine states report “high” activity, among them Alabama, California, Florida, Indiana, Kansas, Maine, Nevada, Texas, and Virginia. Colorado, Louisiana, New York, and South Carolina have reached the highest activity levels recorded by CDC monitoring systems.
Andrew Pekosz, a virologist at Johns Hopkins Bloomberg School of Public Health, has noted something troubling about how quickly cases escalate once the virus enters a state or geographic area. Experimental data and genetic sequencing suggest mutations in the virus help it evade existing immunity in the population, allowing it to spread with unusual speed.
New York Shatters Records
No state illustrates the surge better than New York. Health officials there reported 71,123 flu cases in the week ending December 20, 2025, shattering the previous record for a single week since the state began tracking influenza in 2004.
Hospitalizations in New York jumped 63% in just one week, climbing to 3,666 from 2,251. To date, the state has recorded 189,312 positive flu cases for the season.
State Health Commissioner Dr. James McDonald acknowledged the strain on medical facilities while urging residents to take protective measures. New York has declared the flu “prevalent,” triggering requirements for unvaccinated health care workers to wear masks around patients.
Other states have felt the pressure as well. Massachusetts reported “high” flu activity, and Boston announced a 114% increase in recent cases, with children seeing the steepest rise. In Minnesota, a spike in cases prompted at least one school to shift to online learning before Christmas. State officials there recorded 113 flu-related hospitalizations in 2025, more than double the number from the same period last year.
Across the country, hospital admission rates continue climbing. During the week ending December 20, facilities reported 19,053 laboratory-confirmed flu-related hospitalizations, up from approximately 10,000 the prior week.
Weekly hospitalization rates rose to 6.2 per 100,000 population, up from 4.4 the previous week. Cumulative rates have reached 18.2 per 100,000, marking the third-highest figure at Week 51 since the 2010-11 season. Only the 2022-23 and 2023-24 seasons recorded higher cumulative rates at the same point.
Adults aged 65 and older face the greatest risk, with hospitalization rates of 53.4 per 100,000 population. Children under five follow at 21.5 per 100,000. Adults between 50 and 64 years old have rates of 16.4 per 100,000, while children aged 5 to 17 come in at 11.8 per 100,000.
Long-term care facilities have also seen increases. Nationally, hospitalization rates for residents with a positive influenza test in the prior ten days reached 22.9 per 100,000 residents during the week ending December 20.
Daniel Kuritzkes, a senior infectious disease physician for Mass General Brigham, offered some measured perspective on the situation. “This could be a much bigger flu season than what we’ve seen in the last several years, but so far we don’t have evidence that on a per-patient basis it’s more severe,” he said.
CDC officials echoed that assessment, noting that while activity continues to rise, severity indicators remain low for now. Still, they caution that flu activity will likely continue for several more weeks, and the picture could change as more data arrives.
Children at Risk
Perhaps the most heartbreaking aspect of any flu season involves pediatric deaths. Five flu-related deaths among children were reported in the week ending December 20, bringing the season total to eight.
Four of the five recent pediatric deaths were linked to A(H3N2) viruses, with one associated with A(H1N1). All five occurred during weeks 49, 50, and 51.
Last season serves as a grim reminder of what severe flu years can bring. A total of 288 children died from influenza during the 2023-24 season, nearly all of them unvaccinated, according to CDC studies. That figure matched the number of pediatric deaths during the 2009 H1N1 pandemic and represented the highest toll since mandatory tracking of pediatric flu deaths began in 2004.
Health officials hope to avoid a similar outcome. But with vaccination rates lagging behind last year’s pace, the risk remains elevated for the youngest and most vulnerable.
Vaccine Mismatch Complicates Protection
About 130 million doses of flu vaccine have been distributed so far, compared to more than 143 million at the same point last season. Only approximately 42% of adults and children have received a flu shot.
Compounding the challenge, mutations in subclade K have created a mismatch between the circulating virus and the current vaccine composition. CDC testing found that only 8.6% of tested A(H3N2) viruses were well-recognized by antibodies generated against the vaccine reference virus.
Despite the mismatch, experts and health officials continue urging vaccination. Early-season data from the United Kingdom suggest that vaccines still help prevent severe illness, particularly in children. Richard Webby, director of the World Health Organization’s Collaborating Centre for Studies on the Ecology of Influenza in Animals and Birds, noted that while experts expect an H3N2 mismatch, the vaccine’s protection against H1N1 and influenza B strains should be good.
Jesse Bloom, a scientist at the Fred Hutchinson Cancer Center who studies viral evolution, emphasized that vaccination remains worthwhile even when the match is imperfect. Getting vaccinated will not eliminate the chance of infection, but it can help the body fight off disease and reduce the risk of severe outcomes.
Treatment Options Remain Effective
For those who do get sick, antiviral medications offer an important line of defense. Prescription drugs, including oseltamivir, marketed as Tamiflu, zanamivir, peramivir, and baloxavir, remain effective against circulating strains. CDC testing found no evidence of reduced susceptibility among any of the 447 viruses tested for neuraminidase inhibitor resistance or the 429 tested for baloxavir resistance.
Health officials stress that antivirals work best when started early, ideally within 48 hours of symptom onset. Patients at higher risk for complications stand to benefit most from prompt treatment.
Dr. Nachman encouraged physicians to keep testing and treatment top of mind, particularly when vulnerable household members could be exposed. An older sibling bringing flu home from daycare to a newborn baby, for example, represents a scenario where early testing and antiviral treatment can make a real difference.
Staying Safe as Cases Keep Climbing
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Cameron Wolfe, professor at Duke University, summed up the trajectory many experts see unfolding. Holiday gatherings have likely accelerated transmission, and officials have yet to see any slowdown in the rate of new cases.
Yet amid the concerning numbers, Dr. Nachman offered a reminder that clinicians and public health officials are not facing an unknown threat. “We know what the virus is. We know how to diagnose it. We know how to treat it. And we know somewhat how to prevent it,” she said. “We know what to do. That’s why we do science.”
For individuals looking to protect themselves and their families, CDC recommendations remain straightforward. Everyone six months and older should get vaccinated if they have not already done so. Frequent handwashing with soap and water for at least 20 seconds helps reduce transmission. Those who feel sick should stay home and avoid contact with others. Covering coughs and sneezes with tissues that are immediately discarded, along with regular cleaning of frequently touched surfaces, can further limit spread.
Flu season still has weeks to run. With subclade K continuing its march across the country and vaccination rates trailing last year’s pace, the coming weeks will test health systems and communities alike. But with effective treatments available and clear guidance on prevention, Americans have tools at their disposal to weather what may prove to be a punishing influenza season.
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