Tens of millions of Americans who work at companies with 100 or more employees will need to be vaccinated against COVID-19 by Jan. 4 or get tested for the virus weekly.
The new government rules issued by the Department of Labor's Occupational Safety and Health Administration (OSHA) take effect Thursday.
OSHA says companies that fail to comply could face penalties of nearly $14,000 per violation.
The new requirements were first previewed by President Joe Biden in September.
They will apply to about 84 million workers at medium and large businesses. It is not clear how many of those employees are unvaccinated.
A senior administration official says the agency would target companies if it gets complaints.
This story is breaking and will be updated.