Controlling the transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has been challenging in the community and prison systems. Where herd immunity lies for this virus is unknown, although estimates have ranged from 60% to 80%. Since the start of the pandemic, there have been multiple SARS-CoV-2 outbreaks within U.S. prison systems, which may provide more insight on where true herd immunity lies. We reviewed data from the California Department of Corrections and Rehabilitation to investigate the cumulative incidence of infection and found levels in 14 (40%) of 35 prisons were >60%. These data and existing literature suggest that in prison environments, prevalence of immunity often needs to reach >70% before transmission slows. Similar levels may be needed in the general population before transmission is suppressed.
Keywords: SARS-CoV-2; herd immunity; incarcerated populations; vaccination.