Voluntary efforts, including Kelso's promise in May of a vaccine rewards program with prizes totaling $100,000, "haven’t been given enough opportunity,” union attorney Gregg Adam told the judge.
By contrast, the president of the state's largest union, Service Employees International Union Local 1000, said it will fight Newsom's order.
Vaccination rates among correctional officers vary widely, with a high of 61% at San Quentin State Prison. Prison officials are trying to avoid infection outbreaks like the one that sickened 75% of inmates at the prison north of San Francisco last summer, killing 28 inmates and a correctional officer.
But just 16% of officers were vaccinated as of mid-month at High Desert State Prison in the state's rural northeast. And fewer than half were vaccinated at the California Health Care Facility in Stockton, which has the highest concentration of medically vulnerable inmates.
Among the prison system’s 99,000 inmates, 76 inmates are infected, all confirmed in the last two weeks.
But 48 of those cases are at a single prison, Sierra Conservation Center, near Jamestown in the Sierra Nevada foothills. That is one of two training centers for inmate firefighters who are helping in several massive blazes across California.
No other prison has more than four active cases.
In all, there have been at least 49,000 COVID-19 infections since the start of the pandemic, and 232 inmates have died along with 28 employees.
Virus conditions nationwide and in California had been improving until the highly contagious delta variant took root, causing cases and hospitalizations to again increase, particularly among the unvaccinated.
Vaccines protect most people from becoming seriously ill or dying. But unlike with previous variants, the U.S. Centers for Disease Control and Prevention says vaccinated people infected with the delta variant can spread that virus to others.