COVID-19 transmission has been widespread across the California prison system, and at least two of these outbreaks were caused by transfer of infected individuals between prisons . Risks of individual prison outbreaks due to introduction of the virus and of widespread transmission within prisons due to poor conditions have been documented . We examine the additional risk potentially posed by transfer between prisons that can lead to large-scale spread of outbreaks across the prison system if the rate of transfer is sufficiently high . We estimated the threshold number of individuals transferred per prison per month to generate supercritical transmission between prisons, a condition that could lead to large-scale spread across the prison system . We obtained numerical estimates from a range of representative quantitative assumptions, and derived the percentage of transfers that must be performed with effective quarantine measures to prevent supercritical transmission given known rates of transfers occurring between California prisons . Our mean estimate of the critical threshold rate of transfers was 14.59 individuals transferred per prison per month in the absence of quarantine measures . Available data documents transfers occurring at a rate of 60 transfers per prison per month . At that rate, estimates of the threshold rate of adherence to quarantine precautions had mean 75.68% . While the impact of vaccination and possible decarceration measures is unclear, we include estimates of the above quantities given reductions in the probability and extent of outbreaks . We conclude that the risk of supercritical transmission between California prisons has been substantial, requiring quarantine protocols to be followed rigorously to manage this risk . The rate of outbreaks occurring in California prisons suggests that supercritical transmission may have occurred . We stress that the thresholds we estimate here do not define a safe level of transfers, even if supercritical transmission between prisons is avoided, since even low rates of transfer can cause very large outbreaks . We note that risks may persist after vaccination, due for example to variant strains, and in prison systems where widespread vaccination has not occurred . Decarceration remains urgently needed as a public health measure.