Millions of U.S. workers now have a Jan 4. deadline to get a COVID vaccine.

The federal government on Thursday announced new vaccine requirements for workers at companies with more than 100 employees as well as for workers at health care facilities that treat Medicare and Medicaid patients. It also extended a deadline for federal contractors.

The Biden administration says 70% of all adult Americans are now fully vaccinated. The new rules are aimed at the tens of millions who remain unvaccinated.

Here are the key takeaways:

Companies with 100 or more employees must require those workers to get fully vaccinated - with two shots of the Moderna or Pfizer vaccines, or one shot of Johnson & Johnson - by Jan. 4. After that date, any employee who remains unvaccinated must provide a verified negative COVID test weekly. Companies aren’t required to pay for those tests. Unvaccinated employees also must wear masks.

The requirements don't apply to people who work at home or who work outdoors.

Starting Dec. 5, employers must offer paid time off for workers to get vaccinated and sick leave if workers experience vaccine side effects.

The Occupational Safety and Health Administration will assist companies with vaccination plans. It will also enforce the rules. Fines for non-compliance will vary based on a company’s size and other factors, but companies might have to pay up to $13,653 per individual violator. But enforcement will be difficult. OSHA has only around 1,850 inspectors to oversee 130 million workers at 8 million companies.

OSHA says it's on sound legal footing, and has the emergency authority to issue rules that protect workers from an imminent health hazard. It also says the rules pre-empt all state law. Several states and Republican governors have threatened to sue, contending the government doesn’t have the power to make such rules.

OSHA said Thursday that the mandate could be expanded to companies with fewer than 100 workers. The agency said it will make a decision after collecting public comment on that proposal for the next 30 days.

Workers at health care facilities that treat Medicaid and Medicare patients must be fully vaccinated by Jan. 4. There is no weekly testing option for those workers, but they can ask for religious or medical exemptions. The mandate will cover about 17 million health care workers at hospitals, nursing homes, surgical centers and other facilities.

In September, the Biden administration said it would require employees at federal contractors to get vaccinated by Dec. 8. On Thursday, the government pushed back that deadline to Jan. 4.