Contra Costa Health Services nurse Jessica Lipscomb administering a Pfizer COVID-19 vaccine to Miguel Castro, 13, at a vaccination clinic in the gymnasium of Antioch Middle School in Antioch, Calif., on May 19, 2021. (Eli Walsh/Bay City News)

COVID-19 is resurging in Contra Costa County, thanks to the delta variant and about 20 percent of the county’s population over 12 remaining unvaccinated, health officials told the county Board of Supervisors on Tuesday.

Supervisors said they want to explore making vaccination mandatory for county employees after officials said there’s been a new outbreak in county detention facilities. There were 23 infections as of Monday, which county officials expect to grow as test results come back.

“Given that we quarantine everyone who comes into the jail for 10 days, I think it’s a fair assumption that this isn’t inmate-driven,” Supervisor Karen Mitchoff said. “It’s either a visitor or, unfortunately, a deputy or other staff.

“It’s time to get beyond personal beliefs here, other than religious and medical reasons, and get vaccinated,” Mitchoff said.

Dr. Ori Tzvieli, the county’s deputy health officer, said there have been 2,800 new cases countywide in the past two weeks, and four times more hospitalizations this month than in June, when the state reopened.

“And our case rate is now 12.8 new cases per 100,000 (people), which only a week ago was 5.3,” Tzvieli said. “So you can see the rise has been very rapid. This is very different in the vaccinated versus the unvaccinated communities.”

Tzvieli said the case rate last week among unvaccinated residents was 33 per 100,000. The rate among vaccinated people was 5.9.

He said the delta variant is now the dominant strain in Contra Costa County. Of COVID-related deaths in Contra Costa since vaccines have been available, 99.5 percent were unvaccinated.

“We are in the fight against COVID for the long haul,” Tzvieli said. “Vaccines remain the best way to end the pandemic.”