Maintaining Important Distance During Lengthy Epidemics Act of 2020 or the MIDDLE Act of 2020 This bill prohibits air carriers, during the COVID-19 (i.e., coronavirus disease 2019) public health emergency, from (1) permitting adjacent seats to another occupied seat to be filled on a passenger aircraft, (2) passing along the cost of any seat switch undertaken by such air carriers in order to comply with the prohibitions, and (3) permitting a passenger or crewmember to board a passenger aircraft if the passenger or crewmember is not wearing a mask. The Federal Aviation Administration must issue guidance for appropriate exceptions to the prohibitions based on federal public health guidance.