Incarcerated population density is reported as percentage of facility design capacity. In the counties in which state prisons are located, the COVID-19 incidence was, on average, less than 50 cases per 100 000 person-weeks from June 2020 to September 2020. COVID-19 incidence in these counties was, on average, greater than 250 cases per 100 000 person-weeks from December 2020 to January 2021. Error bars indicate 95% CIs.