Delta made up 83% of California’s recently sampled COVID cases, matching U.S. rate
The highly infectious Delta variant of COVID-19 has made up more than 80% of California samples sequenced for July, the same proportion top U.S. health officials have recently cited as the national rate.
Delta increased from 53% of sequenced cases in June to 83% the first three weeks of this month, the California Department of Public Health reported Thursday.
Dr. Rochelle Walensky, director of the Center for Disease Control and Prevention, during Senate testimony Tuesday gave nearly identical figures at the national level: Delta recently made up 83% of genetically sequenced cases, up from about 50% in early July.
The variant made up just 2% of sampled cases from April and 6% from May in California.
The explosive increase in prevalence adds to concerns that Delta might ravage poorly vaccinated areas this summer, after COVID-19 numbers had been improving all spring.
As Delta has grown dominant, there has been a corresponding, exponential increase in total infections. Statewide test positivity has surged from 1% to 4.9% in the past month. Fifteen counties’ positivity rates are at least 6%, including Sacramento, Merced, San Diego and Riverside, according to state health data updated Thursday.
Sacramento County’s case rate has almost quadrupled in the past four weeks, from about 60 to 230 as a seven-day average, according to the local health office’s data dashboard.
The county has now detected 133 cumulative cases of Delta in residents, 32 of them reported by the local health office in the past week.
Sacramento’s true number of Delta cases is likely much higher.
Tracking variants has been a slow, difficult process because genome lab resources are limited in California, as is the case across most of the U.S.
Local health officials have said the state labs that test for variants don’t take random samples but prioritize certain categories of cases, such as hospitalized and vaccine breakthrough infections, due to these limited resources. This means Sacramento’s confirmed total of 133 doesn’t come from a representative sample of the general population.
It can also take weeks after collection for those lab results to come back, Placer County health director Dr. Robert Oldham told The Sacramento Bee last week. The state sequences only about 15% of positive cases.
But in Yolo County, which partners with the UC Davis Genome Center, all positive cases among residents are sampled for variants. Yolo in a statement last week said Delta made up 76% of cases from June 27 through July 7.
Sacramento County health officer Dr. Olivia Kasirye said last week that the recent numbers from nearby Yolo proved Delta had become the dominant variant in the capital region.
Yolo’s positivity rate, once among the lowest of any California county and still well below the state average, has jumped from 0.2% to 2.1% in about six weeks.
Sacramento County’s hospitalized COVID-19 patient total has doubled since July 4, from 65 to 131. Yolo’s count has jumped from zero to 10 in the past month.
Officials say full vaccination is still highly effective and gives substantial protection against severe COVID-19, even with the Delta variant. A study from the New England Journal of Medicine published Wednesday found the Pfizer vaccine to be about 88% effective against Delta, only a slight decrease from 94% efficacy against Alpha, California’s dominant variant during April and May.
State and local health leaders have said a large majority of hospitalized virus patients are unvaccinated. Kasirye said last week that only 19 of more than 725,000 fully vaccinated residents had been hospitalized with COVID-19 after full vaccination up to that point.
About 53% of all Californians, and 48% of all Sacramento County residents, are fully vaccinated, according to CDPH.
This story was originally published July 22, 2021 at 12:00 PM.
