A precipitous decline of coronavirus cases in state prisons has transformed California’s correctional system from a cautionary tale of mass incarceration in the time of a plague to something more unexpected: an intensely monitored field study that could help scientists develop strategies to defeat the pandemic outside prison walls.

Highly effective vaccines distributed in the prisons combined with the lack of reinfections among inmates and staff previously diagnosed with COVID-19 appear to have quelled the explosive viral outbreaks that have rocked state prisons during the past year.

Active cases have dipped so low that some researchers are theorizing that California’s state prison populations, which suffered spectacularly high coronavirus infection rates through January and were among the first targets for ongoing mass vaccination campaigns, are manifesting collective resistance to the virus.

“I think what you’re seeing is herd immunity,” said Dr. George Rutherford, a professor of epidemiology and biostatistics at UC San Francisco.

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