Flu Hospitalizations Surge as Drifted H3N2 Variant Spreads, Raising Questions on Vaccine Match

As Americans returned to work and school after the holidays, hospitals across much of the country remained busy with a different kind of traffic: patients with flu.

By the week ending Jan. 3, federal health officials estimate the 2025–26 influenza season had already caused about 15 million illnesses, 180,000 hospitalizations and 7,400 deaths in the United States. At least 17 children have died.

Those figures, released in the Centers for Disease Control and Prevention’s weekly FluView report on Jan. 9, put this winter on track to be one of the heaviest flu seasons in years. Yet in its in‑season framework, the agency currently labels the season only “moderately severe.”

“We’re seeing a lot of influenza activity. This is going to be a long, hard flu season,” New York State Health Commissioner Dr. James McDonald said in an interview published this month.

Driving the wave is a drifted form of influenza A(H3N2), an emergent “subclade K” that now dominates lab‑confirmed cases and is poorly matched by this year’s vaccine. The viral shift has raised questions about how well the shot is working and, for parents, what to do amid reports that federal officials are moving away from a long‑standing recommendation that every child get an annual flu vaccination.

A season that feels worse than “moderate”

CDC surveillance shows influenza activity elevated in every region of the country. In the last week of December, 7.2% of outpatient visits were for influenza‑like illness, well above the national baseline in all 10 Health and Human Services regions. About 6.3% of emergency department visits carried a flu diagnosis.

Geographic maps classify 27 states and jurisdictions as having “very high” levels of respiratory illness and 17 as “high.” Only two were listed at “minimal” levels.

Hospitalizations are climbing faster than in most recent seasons. In FluSurv‑NET, a network covering roughly 10% of the U.S. population, there were 14,153 lab‑confirmed flu hospitalizations between Oct. 1, 2025, and Jan. 3, 2026. That translates to a cumulative rate of 40.6 hospitalizations per 100,000 people — the second‑highest at this point in the season since at least 2010–11.

The burden is not evenly shared. Adults 65 and older have the highest cumulative hospitalization rate, at 130.7 per 100,000. Children under 5, especially infants, are also being admitted at high rates: 46.0 per 100,000 in the 0‑4 age group, including 73.7 per 100,000 among babies younger than 1.

For all pediatric age groups combined, CDC says the cumulative hospitalization rate is the second‑highest seen at this point in the season in more than a decade.

Last winter was already unusually deadly for children. During the 2024–25 flu season, CDC recorded roughly 280 pediatric deaths, the highest toll in any non‑pandemic year since the agency began tracking pediatric flu deaths in 2004. Many of those children were not fully vaccinated.

By comparison, 17 pediatric flu deaths have been reported so far this season, a number that typically rises as reports are finalized.

A new H3N2 strain takes over

Laboratory data show this year’s epidemic is overwhelmingly powered by influenza A(H3N2). In specimens subtyped during the first week of January, 92.3% of influenza A viruses were H3N2 rather than H1N1.

Among H3N2 viruses that CDC scientists have genetically sequenced since late September, 91.5% fall into a newly emergent branch of the virus that the agency and global partners refer to as “H3N2 subclade K.” In technical terms, it is part of the J.2.4.1 lineage.

The subclade was first identified by CDC in August 2025, months after experts from the World Health Organization, the Food and Drug Administration and other groups met to select strains for the Northern Hemisphere’s 2025–26 vaccines.

Genetic changes in the virus’s hemagglutinin, the surface protein targeted by most antibodies, have made subclade K “antigenically drifted” from earlier H3N2 strains and from the H3N2 component used in this year’s vaccines, the CDC says.

There is no clear evidence so far that subclade K infections are more severe on a case‑by‑case basis than other recent H3N2 viruses. But like past H3N2‑dominated seasons, the current wave is falling hardest on older adults and young children, who are more likely to develop pneumonia or other complications.

International health agencies have also flagged the strain. The Pan American Health Organization, in a December alert, warned of a “rapidly increasing circulation” of H3N2 subclade K in parts of Europe and Asia and urged countries in the Americas to strengthen vaccination and genomic surveillance. In Europe, the European Centre for Disease Prevention and Control and the World Health Organization’s regional office have reported early and intense flu waves linked to the same variant, with pressure on hospitals in the United Kingdom, Germany and Spain.

A mismatched vaccine that still seems to help

All flu vaccines distributed in the United States this season are trivalent, designed to protect against one H1N1 strain, one H3N2 strain and one influenza B virus from the B/Victoria lineage.

For H3N2, federal regulators chose a virus similar to A/District of Columbia/27/2023, part of the J.2 clade. When that decision was made in February and March 2025, subclade K had not yet emerged.

CDC’s own testing now suggests a significant mismatch. Of 59 H3N2 viruses collected since Sept. 28 and tested in the agency’s antigenic assays, only three — about 5% — were “well recognized” by ferret antisera raised against the vaccine strain. The rest showed reduced reactivity, a laboratory sign that antibodies trained by the shot are less likely to attach effectively to the circulating viruses.

Despite that, early field data from abroad indicate the vaccine is still offering meaningful protection, especially against severe disease. Preliminary estimates from a multinational European network for the first part of the season found overall vaccine effectiveness of about 52% against H3N2 infections that led to primary care visits.

England’s surveillance has reported even stronger protection against serious outcomes, with flu vaccination estimated to reduce the risk of flu‑related hospitalization in children by roughly 70% to 75%, and in adults by around 30% to 40%.

Those numbers are likely to vary by age group and region, and U.S.‑specific estimates are not yet available. But they suggest that even in a drifted year, vaccination remains an important tool for reducing the risk of ending up in the hospital.

CDC continues to recommend flu shots for everyone 6 months and older, stressing that vaccines work alongside antiviral drugs such as oseltamivir and baloxavir, which remain effective against the viruses tested this season.

Mixed messages on childhood shots

While CDC’s public guidance has not changed, recent reporting has pointed to a shift in how federal officials frame flu vaccination for children.

In coverage carried by several outlets, federal health officials were quoted this month saying they will no longer recommend flu vaccination for all U.S. children as a blanket policy, instead describing it as a decision to be made case by case between families and pediatricians. That represents a departure from the long‑standing universal recommendation for children 6 months through 18 years old.

The apparent pivot has unsettled some advocacy groups, especially in light of last season’s record pediatric deaths and this year’s high hospitalization rates in young children.

“I can’t begin to express how concerned we are about the future health of the children in this country,” Michele Slafkosky, executive director of Families Fighting Flu, said in a statement. “Now, with added confusion for parents and healthcare providers about childhood vaccines, I fear that flu seasons to come could be even more deadly for our youngest and most vulnerable.”

Any change in federal recommendations could have ripple effects on state school‑entry requirements and insurance coverage, many of which rely on guidance from CDC’s Advisory Committee on Immunization Practices. Federal agencies have not published a revised childhood flu schedule, and the disconnect between public web guidance and officials’ remarks has added to confusion for some clinicians.

Unequal burden and what lies ahead

The burden of this season’s flu is also falling disproportionately on communities of color. In FluSurv‑NET data, age‑adjusted hospitalization rates are highest among non‑Hispanic Black Americans, at 72.2 per 100,000, more than double the rate seen in Asian and Pacific Islander populations. American Indian and Alaska Native and Hispanic communities also have higher hospitalization rates than non‑Hispanic white populations.

Public health researchers say those patterns mirror inequities seen in past flu seasons and during the COVID‑19 pandemic, reflecting differences in underlying health conditions, access to care, occupational exposure and vaccination coverage.

With January under way, it is not yet clear whether flu activity has peaked nationally. Some indicators dipped slightly in the last week of December, but CDC analysts caution that holiday disruptions in doctor visits and reporting can create artificial lulls.

What is evident is that the combination of a fast‑spreading H3N2 variant, a partially mismatched vaccine and mixed public messaging has produced a season that, while officially “moderately severe,” looks and feels severe to many hospitals and families. Health officials say that existing tools — vaccination, prompt antiviral treatment for those at highest risk, and basic measures such as staying home when sick — can still blunt the impact.

How widely those tools are used over the next several weeks, especially for children and older adults, may help determine whether this winter’s flu wave remains a grueling season or grows into something closer to last year’s record‑setting epidemic.