EXPLAINER: What vaccine mandate means for firms and workers

The Biden administration's sweeping new COVID-19 mandate will apply to 84 million workers at mid-size and large companies.

President Joe Biden described the rules issued Thursday as urgently needed to get more Americans inoculated. Though confirmed viral cases and deaths have fallen sharply since the start of the year, they remain dangerously high, especially in some areas and industries. Average case numbers have leveled off at about 70,000 new infections a day and confirmed viral deaths at more than 1,200 a day.

“Too many people," Biden said, “remain unvaccinated for us to get out of this pandemic for good.”

Yet several states with Republican governors have vowed to challenge the Occupational Health and Safety Administration regulations in court, calling them an unconstitutional power grab by the federal government.

Here is a closer look at the new COVID rules and what they mean for companies and their employees:

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WHAT DO THE NEW RULES DO?

Private-sector companies with 100 or more workers must require their employees to be fully vaccinated against COVID-19 or be tested for the virus weekly and wear masks on the job. These requirements will take effect Jan. 4 under an emergency temporary standard from the Occupational Safety and Health Administration.

A separate rule requires workers at health care facilities that receive federal Medicare and Medicaid payments to be vaccinated. This rule, from the Centers for Medicare & Medicaid Services, will apply to 17 million workers at 76,000 health care facilities, including hospitals and long-term care facilities. But doctor’s offices with fewer than 100 employees will be exempt because they don’t fall under the center's health and safety regulations.

Employers must give workers paid time off to be vaccinated and to...