Occupational Safety and Health Administration (OSHA): COVID-19 Emergency Temporary Standards (ETS) on Health Care Employment and Vaccinations and Testing for Large Employers [Updated December 10, 2021] [open pdf - 2MB]
From the Summary: "On June 21, 2021, the Occupational Safety and Health Administration (OSHA) promulgated an Emergency Temporary Standard (ETS) for the prevention of the transmission of SARS-CoV-2 [severe acute respiratory syndrome coronavirus 2], the virus that causes COVID-19 [coronavirus disease 2019] in health care employment settings. On November 5, 2021, OSHA promulgated an additional ETS that requires employers with 100 or more employees to require that all employees either be fully vaccinated against COVID-19 by January 4, 2022, or test negative for COVID-19 weekly in order to work onsite. On November 6, 2021, the U.S. Court of Appeals for the Fifth Circuit ordered a stay on the OSHA COVID-19 vaccination and testing ETS pending additional judicial review and reaffirmed this stay on November 12, 2021. The Occupational Safety and Health Act of 1970 (OSH Act) gives OSHA the ability to promulgate an ETS that would remain in effect for up to six months without going through the normal review and comment process of rulemaking. OSHA, however, has rarely used this authority in the past--not since the courts struck down its ETS on asbestos in 1983."
CRS Report for Congress, R46288
Congressional Research Service: https://crsreports.congress.gov/