Early, heavy flu season sweeps U.S. as drifted H3N2 strain tests vaccine match

ATLANTA — A fast‑moving strain of flu is driving one of the earliest and heaviest influenza seasons the United States has seen in years, sickening millions and raising new questions about how well this year’s vaccine can keep up.

The Centers for Disease Control and Prevention estimates that, as of Dec. 20, the 2025‑26 flu season has already caused at least 7.5 million illnesses, 81,000 hospitalizations and 3,100 deaths nationwide, including eight children. At the same point last season, federal estimates put the toll at roughly 3.1 million illnesses, 37,000 hospitalizations and 1,500 deaths — less than half this winter’s burden.

A drifted H3N2 strain takes hold

Behind the surge is a drifted version of the H3N2 flu virus, known as subclade K, that has rapidly become dominant in the United States and much of the Northern Hemisphere. Laboratory data show the strain is a poor match for the H3N2 component of this year’s vaccine, fueling concern and online claims that the annual shot is “worthless.” But early real‑world data from abroad suggest the picture is more complicated: the vaccine appears to be offering substantial protection against severe disease, even if it is less effective at preventing infection.

Public health officials say that nuance is critical as flu activity climbs in most of the country and hospitals brace for January.

“Influenza A(H3N2) viruses are the most frequently reported influenza viruses so far this season,” the CDC said in its latest FluView surveillance report, released Dec. 30. Genetic testing shows that nearly 90% of H3N2 samples analyzed since late September fall into the new subclade K.

Those viruses are spreading in a landscape already primed for trouble. In the week ending Dec. 20, 6% of outpatient visits nationwide were for flu‑like illness — defined as fever plus cough or sore throat — nearly double the national baseline of 3.1%. Twenty‑two states and jurisdictions were at “very high” levels of influenza‑like illness activity, with another 10 classified as “high.”

Hospitalizations rise, records fall in some states

Hospitalizations are climbing quickly. In a network of 14 states that together cover about 10% of the U.S. population, there have been 6,349 laboratory‑confirmed flu hospitalizations since Oct. 1. The cumulative hospitalization rate — 18.2 per 100,000 people — is the third‑highest for this point in the season since 2010, behind only the 2022‑23 and 2023‑24 winters.

Some states are already setting records. New York has reported more than 70,000 lab‑confirmed flu cases in a single recent week, the highest weekly total since the state began tracking two decades ago, along with a sharp jump in hospital admissions. Health officials there say the vast majority of H3N2 viruses typed in the state are from a branch that differs from the vaccine strain — essentially the same pattern CDC is seeing nationally.

Why the vaccine appears mismatched

The mismatch traces back to the way flu vaccines are made. Each spring, experts convened by the World Health Organization recommend which viral strains should be included in the Northern Hemisphere vaccines for the coming winter, based on what has been circulating around the world. Manufacturers then need months to grow, formulate and distribute tens of millions of doses.

For the 2025‑26 season, the H3N2 component was chosen from a lineage known as 2a.3a.1 J.2, based on viruses that predominated in 2024. Subclade K, a distinct offshoot within the same broader family, only rose to prominence later.

In the CDC’s latest antigenic tests — experiments that measure how well antibodies produced by the vaccine recognize circulating viruses — just three of 35 H3N2 samples were “well recognized” by antisera raised against the vaccine reference strain. Most showed the kind of reduced response scientists associate with antigenic drift, when a virus accumulates mutations that help it partly evade existing immunity.

Historically, that kind of lab finding has often signaled lower vaccine effectiveness. But real‑world evidence from England, where subclade K is also dominant, offers a more reassuring picture.

Early UK data suggest protection against severe illness

An early analysis from the UK Health Security Agency this month found that the flu vaccine is about 70% to 75% effective at preventing flu‑related hospital attendance among children ages 2 to 17 and roughly 30% to 40% effective in adults, despite the genetic differences between the vaccine strain and circulating viruses.

“Flu vaccines are providing important protection against severe disease this winter despite the emergence of a new H3N2 subclade,” the agency said, urging people not to skip vaccination based on reports of a mismatch.

The CDC has pointed to those findings in recent weekly reports, saying vaccination “remains an effective tool in preventing influenza‑related hospitalizations this season,” especially for children and older adults. Officials also stress that the shot protects against other flu strains that are circulating at lower levels, including H1N1 and influenza B.

Vaccination lags and disparities persist

Even so, uptake has been modest. By early December, manufacturers had distributed about 130 million doses of flu vaccine in the United States, according to CDC data. National surveys suggest around 4 in 10 Americans had been vaccinated as of mid‑December, similar to last year at this time but below some pre‑pandemic seasons.

Pediatric flu vaccination has trended downward over several years, even as last winter brought a record 280 flu‑related deaths in children, the highest in a non‑pandemic season since modern tracking began.

Age and race data from this season’s hospitalization surveillance echo longstanding disparities. Adults 65 and older have by far the highest hospitalization rate, at 53.4 per 100,000 people. Children younger than 5 are next, with a rate of 21.5 per 100,000. When broken down by race and ethnicity, Black Americans currently have the highest age‑adjusted flu hospitalization rate among major groups, more than double that of Asian and Pacific Islander residents.

Global health agencies say there is no clear evidence that subclade K is inherently more virulent than earlier H3N2 strains. The Pan American Health Organization, in a December briefing, said there were “no indications at this time of increased intrinsic severity” but called for strengthened vaccination campaigns and close monitoring, especially because H3N2 seasons have historically been harder on older adults.

Antivirals move to the forefront

With a partially drifted virus and middling vaccination rates, another tool has moved to the forefront of public health messaging: antiviral drugs.

Four antivirals are currently recommended for flu treatment in the U.S. — oseltamivir, zanamivir, peramivir and baloxavir. Recent CDC testing of hundreds of circulating viruses found no resistance to any of those medicines. In guidance updated Dec. 19, the agency told clinicians that antiviral treatment “should be started as soon as possible and should not wait for laboratory confirmation of influenza” in hospitalized patients, those with severe or progressive illness, and people at higher risk of complications.

Clinical trials and past seasons have shown that antivirals work best when started within 48 hours of symptom onset, but CDC notes there can still be benefits for very sick or hospitalized patients even when treatment begins later.

What happens next

Exactly how severe this season will ultimately be is still uncertain. Flu activity typically peaks between December and February but can stretch into spring. Last season, which CDC classified as “high severity,” ultimately caused an estimated 43 million illnesses, 560,000 hospitalizations and 38,000 deaths in the United States.

For now, surveillance indicators are climbing, a drifted virus is in firm control of the H3N2 landscape, and vaccination and antiviral use are far from universal. Public health experts say what happens over the next several weeks — whether more people get vaccinated, how quickly high‑risk patients receive antivirals, and how heavily hospitals are hit — will largely determine whether this winter merely echoes last year’s toll or surpasses it.