San Diego County’s public-health laboratory has picked up a new H3N2 offshoot, known as subclade K, in local patient samples, putting the region on the front line of what is shaping up to be an early and fast-moving flu wave. Sequencing flagged the strain’s genetic fingerprint in four specimens, including one collected in late September and three from December, just as flu activity started ramping up across the county. The finding lands at the same time local flu cases and emergency-department visits for flu-like illness are rising, which is exactly why county health leaders are turning up the volume on flu-shot reminders.
According to reporting from the San Diego Union-Tribune, the county’s new public-health lab confirmed subclade K through genetic analysis of four local patient samples and traced the earliest one back to late September. By Dec. 27 the county had logged about 4,231 confirmed flu cases, and emergency-department visits for flu symptoms sat at about 2.9% as of Dec. 20. The Union-Tribune also notes that the county’s roughly $96 million public-health lab has boosted local sequencing capacity, which lets scientists spot new variants and subclades more quickly instead of waiting on state or federal labs.
What researchers are seeing
The U.S. Centers for Disease Control and Prevention reports that nearly all of the H3N2 viruses it has characterized since late September fall into the new subclade K, a sign this variant now dominates H3N2 detections nationwide, according to CDC surveillance data. Early-season numbers from the UK Health Security Agency, published in Eurosurveillance, suggest the 2025–26 flu vaccine is still doing important work. The report estimates roughly 72% to 75% effectiveness for children and about 32% to 39% for adults in preventing emergency visits and hospital admissions, which is not perfect but is far better than going into this wave with no protection at all.
Where the numbers stand nationally
Across the country, lab test positivity has climbed quickly in recent weeks, jumping from the mid-teens to about one quarter of clinical specimens in the week ending Dec. 20, according to The Wall Street Journal, which cites CDC data. Public-health experts say severity indicators are still relatively low for now, but they warn that the rapid spread of subclade K could drive higher case counts and more hospitalizations if vaccination coverage does not improve.
What public health officials recommend
San Diego County Public Health Officer Dr. Sayone Thihalolipavan stressed that there is still time to act, telling reporters that “this is still a moment where it’s not too late to vaccinate,” as quoted by the San Diego Union-Tribune. Officials continue to urge flu vaccination for everyone 6 months and older and point out that antiviral medications can lower the risk of severe illness for people at higher risk, guidance that is in line with federal recommendations from the Centers for Disease Control and Prevention.
How to protect yourself this season
The county and community clinics are offering seasonal flu shots and COVID boosters at multiple sites across the region, and local outlets have posted clinic lists and vaccine-finder tools. For specific locations and schedules, see coverage from 10News. Health experts are sticking with the basics that work: stay home when you are sick, consider masking in crowded indoor settings if you or someone in your household is high risk, and seek testing or prompt antiviral care if you develop severe symptoms or belong to a high-risk group.
