COVID-19 Individual Liberty Act of 2021 This bill nullifies executive orders and otherwise limits the authority of the federal government and federally funded entities to mandate COVID-19 vaccines and other prevention measures. Specifically, the bill nullifies two executive orders issued on September 9, 2021, which (1) mandate COVID-19 vaccines for federal employees, and (2) require federal contractors to comply with workplace safety guidance for preventing COVID-19. In addition, the bill bars the use of federal funds for implementing or enforcing a COVID-19 vaccine mandate, including employer-based mandates. The bill also prohibits requiring, as a condition of participation in Medicare or Medicaid, that health care providers mandate COVID-19 vaccinations for their employees. Furthermore, the federal government may not (1) issue any standardized documentation that certifies an individual's COVID-19 vaccination status to a third party, or (2) condition the provision of any service or benefit on the receipt of documentation certifying an individual's COVID-19 vaccination status or post-transmission recovery. These prohibitions apply to state, tribal, and local governments (excluding schools and institutions of higher education) as a condition of receiving federal COVID-19 relief funds. The bill also requires the Department of Health and Human Services to publish guidelines on the extent to which natural immunity to COVID-19 prevents contracting or spreading the disease.