The Occupational Safety and Health Administration (OSHA) has issued its ruling on when large U.S. employers will have to start enforcing the COVID-19 vaccine mandate announced by Pres. Joe Biden in September. Barring court or legislative action stopping the order, Jan. 4 is the day employees who work at companies with 100 or more employees will need to be fully vaccinated against COVID-19 or undergo weekly testing for the virus.

Biden’s mandate will apply to about 84 million workers at medium and large businesses, although it is not clear how many of those employees are unvaccinated.

OSHA regulations will force the companies to require that unvaccinated workers test negative for COVID-19 at least once a week and wear a mask while in the workplace. The administration tasked OSHA with issuing the rules because it has virtual free reign in mandating safety in the workplace. By assigning OSHA to administer the program, the administration bypassed any legislative approval process. OSHA drafted the rules under emergency authority meant to protect workers from an imminent health hazard, the agency said.

Under the Biden mandates, tougher rules will apply to another 17 million people who work in nursing homes, hospitals and other facilities that receive money from Medicare and Medicaid. Those workers will not have an option for testing. They will be required to be vaccinated.

The ruling issued by OSHA will allow workers to ask for exemptions on medical or religious grounds. It remains unclear what conditions would qualify employees for those exemptions.

OSHA said companies won’t be required to provide or pay for the tests, but they must give paid time off for employees to get vaccines and sick leave to recover from side effects that prevent them from working. The requirements for masks and paid time off for shots will take effect Dec. 5.

The OSHA announcement said companies that fail to comply with the regulations could face penalties of nearly $14,000 per violation, although it is unclear how OSHA plans to enforce the rules.

Senior administration officials said the rules preempt any contrary state laws or orders, including those that ban employers from requiring vaccinations, testing or the wearing of face masks. The administration will face immediate challenges from Republicans who are eager to fight Biden in court and in Congress over the mandates.