The U.S. is facing record-high flu cases and hospitalizations during the 2025-2026 season due to a mutated influenza strain.
According to the World Health Organization, flu cases are surging nationwide to levels unseen in almost three decades. These cases are predominantly driven by the currently circulating Influenza A (H3N2) variant, subclade K, nicknamed the “super flu.” This is due to its highly transmissible nature and ability to cause more severe symptoms, leading to serious illness and increased hospitalizations.
“We are starting to see increasing influenza illness and emergency department visits,” said San Mateo County (SMC) Health educator Tamta Mchedlishvili.
Elevated influenza activity
Seasonal influenza activity remains elevated nationwide, with high or very high flu activity reported in 36 states. The Centers for Disease Control and Prevention (CDC) estimates that at least 18 million illnesses, 230,000 hospitalizations, and 9,300 deaths have occurred from flu so far this season.
The U.S. is in the middle of a record-breaking flu season, with outpatient respiratory illness visits reaching an unprecedented high of 8.3% in nearly 30 years, surpassing the previous record of 7.8% set last season and well above the 3.1% baseline.
Flu activity in California is considered high, according to the latest CDC report, which aligns with what SMC Health officials are observing.
“We are still early in the flu season, so it’s hard to make a full comparison to previous years,” Mchedlishvili said. “However, early statewide data show higher flu activity than usual.”
The elevated influenza activity has left students worried.
“I’m more concerned about the flu this season because there have been more cases, and I do not want to get the flu,” said Sara Brittain, a Carlmont sophomore.
Severe outcomes for vulnerable subpopulations
The California Department of Public Health (CDPH) noted in its health advisory that 97% of influenza viruses reported by U.S. public health laboratories were Influenza A. Of those subtyped, 92% were Influenza A (H3N2), and of the viruses that underwent additional genetic characterization at the CDC, 89% belonged to subclade K.
According to the CDPH, Influenza A viruses can mutate to produce novel strains that have not previously infected humans, leaving people with little to no immunity. Subclade K is a highly contagious novel virus that is predominantly driving this season’s flu surge.
“Influenza A (H3N2), especially a newer version called subclade K, is the main strain driving this year’s flu surge because it has changed enough that people’s immune systems don’t recognize it well,” Mchedlishvili said. “These mutations make the virus spread more easily and cause stronger symptoms.”
CDC data show subclade K is disproportionately affecting young children and older adults, who have the highest influenza-related hospitalization rates.
“People at highest risk for serious flu complications this season include young children, adults 65 and older, pregnant people, and those with chronic health conditions,” Mchedlishvili said.
Students also expressed concern about the severity of this flu season.
“I’m more concerned this year, not only due to the higher case numbers, but also because I’ve heard cases have been worse for many people,” said Carlmont sophomore Hila Dremer.
Experts recommend vaccination protection
Both the CDPH and SMC Health recommend that everyone aged six months and older receive an annual flu vaccine.
“Flu season typically runs from October through May, with activity usually peaking between December and February. It’s not too late to get vaccinated,” Mchedlishvili said.
Subclade K emerged in Europe during the summer of 2025, which was after the strains used to formulate this year’s flu vaccine were selected. This reduces the vaccine’s effectiveness against the strain. Despite this, health experts say vaccination can still reduce severe illness and hospitalizations.
“Current seasonal flu vaccines remain effective at reducing severe illness and hospitalization, including the currently circulating viruses,” said Dr. Erica Pan, CDPH Director and State Public Health Officer, in a Jan. 13 news release.
Some students said they have received their flu shots, while others have not.
Brittain was encouraged by her doctor and parents to get vaccinated.
“I decided to get the flu shot this season because my doctor told me to, and my parents also made me do it,” Brittain said.
Dremer has not yet received the vaccine.
“I did not get the flu shot this season because I was overdue and didn’t have a reason to yet,” Dremer said.
According to the CDC, the 2024-2025 flu season was classified as high severity, marking the highest hospitalization rate since the 2010-2011 season. Influenza activity began increasing in mid-November, peaked in early February, then steadily declined to interseasonal levels by May 2025. The Influenza A viruses, H1N1 and H3N2, were also co-dominant.
Experts caution that the U.S. is not out of the woods despite the CDC reporting declining flu activity for two straight weeks. Second surges often occur after winter holidays, and historical CDC data show flu activity most often peaks in February.
Countries that were earlier affected by subclade K may offer insight into what the U.S. could experience. According to a Dec. 10, 2025, seasonal influenza update from the World Health Organization, some countries in the southern hemisphere experienced unusually long flu seasons compared to previous years.
While the full impact of this record flu season remains unclear, preventive measures aren’t foolproof in reducing its toll.
“Sadly, people still die due to influenza-related illness each year,” Mchedlishvili said.
