Pandemic Treatment Access and Affordability Act of 2021 This bill imposes licensing and price restrictions on federally supported drugs and other medicines intended to address COVID-19 (i.e., coronavirus disease 2019) or other public health emergencies. With respect to a COVID-19 drug patent developed using certain types of federal support, any license granted by the federal government shall be open and nonexclusive. Similarly, a licensee or assignee for such a patent shall grant open nonexclusive licenses for the patent. An entity that receives a license shall pay reasonable royalties to (1) the patent holder, or (2) the owner of marketing exclusivity rights granted by the Food and Drug Administration (FDA) that were terminated under this bill. The bill requires that such drugs be offered at a fair and reasonable price based on certain considerations, such as access. If the Department of Health and Human Services (HHS) finds that the price of any drug to address a public health emergency is excessive, HHS must void any FDA-granted exclusivity for the drug and grant open and nonexclusive licenses to other manufacturers. Such licenses are subject to reasonable royalty requirements.