Flu Surge Pushes 2025–26 Season to ‘Moderately Severe’ as Mutated H3N2 Strain Spreads

Crowded ERs signal a harsh start

The children’s emergency department at a major Atlanta hospital was standing room only by the first week of January. Parents cradled feverish toddlers in winter coats, teenagers in masks scrolled on their phones, and triage nurses moved stretcher to stretcher in a hallway lined with portable oxygen tanks.

“We’re seeing flu like we haven’t seen it in years,” one pediatrician said, asking not to be named because she was not authorized to speak to reporters. “It feels like the peak of COVID winters, but it’s flu driving this, not SARS‑CoV‑2.”

National data back up what doctors are seeing.

By the week ending Dec. 27, the Centers for Disease Control and Prevention estimated at least 11 million people in the United States had come down with influenza so far this season, along with about 120,000 hospitalizations and 5,000 deaths. In its weekly FluView report released Jan. 5, the agency said this winter’s influenza season had reached the level of a “moderately severe season” for the first time.

At the same time, 8.2% of all outpatient visits tracked by the CDC’s surveillance network were for respiratory illness—more than double the national baseline and the highest share in decades.

A mutated flu strain, a partially mismatched vaccine and falling vaccination rates are converging to make the 2025‑26 season one of the most punishing in years. The surge is testing clinics and hospitals that never fully recovered from the pandemic and unfolding amid new political fights over childhood immunizations.

A “moderately severe” season, on paper

The CDC’s in‑season severity framework classifies influenza seasons as low, moderate, high or very high based on three indicators: outpatient visits for influenza‑like illness, flu‑associated hospitalizations and the percentage of deaths due to pneumonia, influenza or COVID‑19.

“As of Week 52, CDC’s in‑season severity assessment framework classified the season as a moderately severe season for the first time,” the agency wrote in its latest summary. Officials noted that flu activity was “elevated and continues to increase across the country.”

The numbers suggest a faster and heavier start than last winter. At the same point in the 2024‑25 season, federal estimates put the toll at about 5.3 million illnesses, 63,000 hospitalizations and 2,700 deaths—roughly half the burden now.

Hospitalization data show the same pattern. By late December, the cumulative rate of lab‑confirmed flu hospitalizations reached 28.1 per 100,000 people, the third‑highest level for Week 52 since the 2010‑11 season. Hospitals reported admitting 33,301 flu patients that week alone. Older adults 65 and over and children younger than 5 are being hospitalized at the highest rates.

Outpatient clinics and urgent cares are seeing unprecedented demand. All 10 federal health regions are above their regional baselines for influenza‑like illness, and state health departments say waiting rooms are crowded with children and working‑age adults alike.

“This is one of the busiest respiratory seasons our outpatient network has ever recorded,” a CDC spokesperson said in an email. “Influenza is the primary driver of that activity right now, although COVID‑19 and RSV are also contributing.”

The rise of subclade K, the so‑called “super flu”

Behind the numbers is a heavily mutated version of influenza A(H3N2) that has swept through the United States and much of the Northern Hemisphere.

So far this season, more than 90% of subtyped influenza A viruses detected in U.S. laboratories have been H3N2. Genetic sequencing shows that about 90% of those H3N2 viruses fall into a lineage scientists call subclade K.

Subclade K is part of a broader family of H3N2 viruses that have circulated for years. But it has accumulated changes in the gene for hemagglutinin—the protein spikes on the virus’s surface—that help it partly evade existing immunity.

Local news outlets and some clinicians have dubbed it the “super flu” or “Super‑K,” a label that has also appeared in Australian and European coverage after off‑season outbreaks there.

Experts caution that the nickname can be misleading.

“It’s not that this virus is killing people at a dramatically higher rate per infection,” said Dr. Emily Martin, an epidemiologist at the University of Michigan who studies influenza but is not involved in the CDC’s surveillance reports. “What we’re seeing is a virus that’s more transmissible and better at infecting people despite previous vaccination or infection, which means more people are sick at the same time.”

That sheer volume is enough to strain health systems, she said, even if the risk for any single infected person is similar to recent H3N2 seasons.

A vaccine that doesn’t perfectly match—but still helps

The 2025‑26 Northern Hemisphere flu vaccine was designed early last year using an H3N2 component based on an A/Croatia/10136/RV/2023‑like virus. At the time global health authorities made that choice, subclade K had not yet come to dominate.

Since then, laboratory tests at the CDC have found that most of the H3N2 viruses circulating in the United States are “antigenically drifted” from that vaccine strain. In simple terms, antibodies generated by the shot do not recognize the new viruses as well.

In its Week 52 report, the CDC said only a small fraction of recently tested H3N2 viruses were well inhibited by ferret antisera raised against the vaccine strain, one of the standard methods for assessing match.

Those findings have fueled headlines about a “vaccine mismatch” and raised public questions about whether getting a flu shot still makes sense this season.

Early real‑world data from abroad, however, suggest the vaccine is offering meaningful protection—especially against severe outcomes—even in the face of drift.

A preliminary analysis by the United Kingdom Health Security Agency for this season estimated that the flu vaccine was around 70% to 75% effective at preventing hospital or emergency department attendance for H3N2 in children and adolescents, and roughly 30% to 40% effective in adults. A multicountry study coordinated by the European Centre for Disease Prevention and Control reported vaccine effectiveness of about 50% to 60% against medically attended H3N2 infection in primary care.

“These are in the ballpark of what we expect in H3N2 seasons,” said Dr. John Brownstein, a Harvard Medical School professor who tracks influenza trends. “The vaccine is clearly not a perfect match, but it still substantially reduces the risk of ending up in the hospital.”

CDC officials have not yet released U.S.‑specific effectiveness estimates for this season but continue to recommend vaccination for everyone 6 months and older, noting that most people in the country remain unvaccinated.

Falling flu‑shot uptake and a new political fight

Flu vaccination coverage in the United States has been edging downward.

During the 2023‑24 season, about 44.9% of adults received a flu shot, according to CDC survey data, down slightly from the year before. Preliminary estimates for 2024‑25 suggested coverage ranged from roughly 35% among adults 18 to 49 to about 71% among people 65 and older. A national poll last spring found only about 4 in 10 adults reported getting a flu shot that season.

Within‑season data from state immunization registries suggest even lower uptake in some regions, with adult coverage stuck in the 15% to 25% range in late fall.

Health officials and outside experts have linked that slide to pandemic fatigue, vaccine misinformation and growing political polarization over public health.

Those tensions intensified this month when the Department of Health and Human Services, led by Secretary Robert F. Kennedy Jr., moved to revise the federal childhood immunization schedule.

In a decision announced in early January, a senior HHS official directed the CDC to remove universal recommendations for several pediatric vaccines, including the seasonal flu shot, from the schedule used by pediatricians and schools. Under the revision, flu, rotavirus, hepatitis A and some meningococcal and RSV vaccines are now framed primarily for children with high‑risk conditions or as options to be decided jointly by families and clinicians.

The change bypassed the CDC’s Advisory Committee on Immunization Practices, the expert panel that typically reviews evidence and votes on vaccine recommendations in public meetings.

Pediatric infectious‑disease specialists and several former CDC officials criticized the move.

“This change endangers children by ignoring decades of public‑health progress,” Dr. Lisa Patel, a Stanford pediatrician and past president of the American Academy of Pediatrics’ California chapter, said in a statement. “The vast majority of hospitalizations we see for flu and RSV are in previously healthy kids. Universal vaccination recommendations are how we prevent that.”

Governors and health departments in California, Oregon, Washington and Hawaii said they would continue to follow the prior CDC schedule and the AAP’s guidance, preserving universal flu vaccination recommendations for children in their states.

Equity, antivirals and what comes next

As with COVID‑19, the burden of this flu season is not falling evenly.

Federal surveillance data show that flu‑associated hospitalization rates so far are highest among non‑Hispanic Black people, followed by American Indian and Alaska Native and Hispanic populations, with lower rates among non‑Hispanic white and Asian or Pacific Islander groups. Experts point to lower vaccination coverage, greater exposure in essential jobs, crowding and barriers to timely primary care as likely contributors.

Clinicians stress that there are still tools beyond vaccination to blunt the impact of the season.

Antiviral medications such as oseltamivir (Tamiflu), zanamivir and baloxavir remain effective against the H3N2 viruses circulating this year, including subclade K. Federal laboratories have not detected evidence of widespread resistance.

Health authorities urge people at higher risk of complications—including young children, adults 65 and older, pregnant people and those with chronic health conditions—to seek care early if they develop flu symptoms.

“Even if you didn’t get vaccinated, there is a window where treatment can make a big difference,” Martin said. “The key is not waiting until you are severely ill to reach out to a doctor.”

The CDC’s “moderately severe” label is not fixed. The agency can revise its assessment as more weeks of data come in, particularly if hospitalizations and deaths continue to climb.

For now, doctors and public‑health officials are watching closely to see whether influenza activity peaks in the coming weeks or continues to spread alongside COVID‑19 and RSV.

In the Atlanta emergency department, the pediatrician trying to keep up with the wave of patients said the question she hears most from parents is the hardest to answer definitively: Will next winter be just as bad?

“I tell them we don’t know what the virus will look like a year from now,” she said. “What we do know is that vaccination and early treatment still work, and right now a lot of people who could benefit from those protections don’t have them.”