New Flu Strain, H3N2 K, Fuels Rising Cases and Hospitalizations Across California and the U.S.
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California health officials are warning residents of a surge in influenza cases driven by a newly emerged strain, as a particularly severe flu season grips the nation.
California is experiencing an “elevated” level of seasonal flu activity, with a new influenza A strain – H3N2 subclade K – contributing to increased cases and hospitalizations statewide. Public health experts nationwide are echoing these concerns, as the virus has already caused significant disruption globally.
Flu Cases Rising, But Still Below Last Year’s Peak
Data indicates that flu test positivity rates – the percentage of patients presenting with flu-like symptoms who test positive for influenza – have been climbing in recent weeks. However, current rates remain comparatively lower than those observed during last year’s flu season.
“Flu started to rise, in earnest, by mid-December and rates are still up,” stated a regional physician chief of infectious diseases for Kaiser Permanente. While experts are hopeful for a plateau in the coming weeks, data delays due to the recent holidays mean a clearer picture will emerge within the next seven to ten days.
Vulnerable Populations at Highest Risk
While most flu-related cases are being managed without hospitalization, those aged 65 and older, as well as individuals with underlying health conditions, are disproportionately represented among those requiring hospital care. Data from the state’s public health agency reveals a high concentration of positive flu cases in Central California and the Bay Area, with moderate rates around Sacramento and Southern California. Rural areas in the northern part of the state currently report lower case rates.
In Los Angeles County, between the end of last year and the beginning of 2026, 162 flu-related hospitalizations were recorded, with an additional 18 patients admitted to intensive care units.
National Flu Season Reaches 30-Year High
The situation nationally is far more acute. The Centers for Disease Control and Prevention (CDC) reports that this flu season has resulted in the highest number of cases in the U.S. in over three decades. Since late fall, the CDC estimates at least 15 million infections, 180,000 hospitalizations, and 7,400 deaths. Tragically, at least two of those fatalities have been children, confirmed by the state’s Department of Public Health.
Vaccine Effectiveness and Emerging Concerns
Last year, infectious disease specialists predicted a challenging flu season, particularly for children, due to declining vaccination rates and the emergence of a highly mutated strain. “Last year’s flu season was particularly bad, but little did we know what was in store for us this year,” noted a medical director of antimicrobial stewardship with Keck Medicine of USC.
Experts believe the current H3N2 subclade K strain has mutated in a way that allows it to “more likely evade” immunity conferred by the current flu vaccine, as it emerged after the vaccine formula had already been determined. Despite this, Dr. Erica Pan, state public health officer, emphasized that “current seasonal flu vaccines remain effective at reducing severe illness and hospitalization, including the currently circulating viruses.”
Another expert added that even if the flu shot doesn’t prevent infection entirely, “it lessens your odds of having a severe case, keeps you out of the hospital and shortens the duration of the illness.”
Health officials are strongly urging the public, especially high-risk individuals, to get vaccinated or seek immediate antiviral treatment, such as Tamiflu. The flu can manifest with debilitating symptoms – fatigue, fever, cough, and body aches – described by one physician as feeling like being “hit by a Mack truck.” Children and other vulnerable populations may experience more severe complications, including dehydration, pneumonia, and, in rare cases, inflammation of the brain and heart.
The impact of this strain extends beyond the U.S., with severe flu seasons reported in Australia, Japan, the United Kingdom, and other parts of Europe and Asia.
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