Pandemic Risk Insurance Act of 2020 This bill establishes the Pandemic Risk Reinsurance Program within the Department of the Treasury. The program generally provides compensation to insurers if they incur losses as a result of coverage related to pandemics and outbreaks of disease. Insurers may elect to participate in the program if they meet certain criteria. Specifically, insurers participating in the program must offer, in all business interruption insurance policies, coverage for public health emergencies related to an outbreak of infectious disease or a pandemic declared on or after January 1, 2021, and certified by the Department of Health and Human Services. The program is triggered when aggregate industry insured losses for participating insurers resulting from a covered public health emergency exceed $250 million. The bill establishes the share of insured losses the program would cover, conditions for payment to insurers, caps on annual liability, and the amount insurers must annually pay in deductibles. The bill also provides for the treatment of existing business interruption insurance policies and state residual market insurance entities. The Government Accountability Office must report on the availability and affordability of business interruption insurance.