On Thursday, senior Biden administration officials announced details of a highly anticipated mandate requiring U.S. employers with 100 or more workers to ensure employees are either fully vaccinated for COVID-19 or tested each week for the virus.

The officials also unveiled details of a separate mandate that will require employers participating in Medicare or Medicaid to have a fully vaccinated workforce, with no testing alternative.

The first rule covers 84 million U.S. workers, or roughly half of the U.S. workforce, officials said. That rule, which will be administered by the Labor Department's Occupational Safety and Health Administration, will require employees to be fully vaccinated by Jan. 4.

Employees who aren't fully vaccinated by Jan. 4 have to produce a verified negative test to their employers every week and begin wearing a mask at work as of Dec. 5. Workers who test positive for COVID-19 or receive a diagnosis from a licensed health care provider have to be removed from the workplace. The rule doesn't require employers to pay for the tests, though agreements with unions might require employers to cover the bill.

Starting Dec. 5, the OSHA rule will require covered employers to provide workers with paid time off to receive vaccinations, as well as sick leave to recover from vaccination side effects that preclude working.

Employer penalties for noncompliance with OSHA's rule could include fines based on the number of violations and range up to approximately $13,653 for a single violation, officials said. Fines are significantly higher, up to $136,532, for employers who willfully violate standards. White House officials said OSHA plans to carry out inspections.

The second rule, administered by the Medicare & Medicaid Services, requires covered health care providers to ensure all workers are fully vaccinated by Jan. 4.

The rule applies to more than 17 million workers at approximately 76,000 health care facilities, officials said.

While covered health care employees don't have a testing option, Biden administration officials said the CMS rule will offer religious and medical exemptions — and facilities must have a plan to ensure they comply with those exemptions, the officials said.

All workers, including contractors, within covered health care facilities are subject to the CMS rule, regardless of their job responsibilities. Covered facilities include hospitals, ambulatory surgery centers, dialysis facilities, home health agencies, and long-term care facilities.

To be considered fully vaccinated, workers must have two doses of Pfizer's or Moderna's COVID-19 vaccine, or one dose of Johnson & Johnson's.

Facilities that don't comply with CMS's rule may have to pay civil penalties or may be denied funding. As a last resort, officials said, facilities can be terminated from Medicare and Medicaid programs.

Twenty-one states that adopt their own occupational standards in place of OSHA's will have a 30-day window to implement their respective rules. They must be as effective or more effective than OSHA's.

Critics of the mandate say employers shouldn’t be responsible for policing their workforce. They also question how the Biden administration can justify an emergency rule that fails to provide equal protections for workers in firms with fewer than 100 employees.

For those and other reasons, experts anticipate the mandate to draw a variety of court challenges. Workers who wish to remain unvaccinated are expected to push back against OSHA's mandate. State and local government officials have already filed preemptive lawsuits, arguing that the Biden administration is overstepping its executive authority.

This story was updated to reflect that 84 million workers is roughly half of November's U.S. workforce.

Alexis Keenan is a legal reporter for Yahoo Finance. Follow Alexis on Twitter @alexiskweed.

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