Tens of millions of Americans who work at companies with 100 or more employees will need to be vaccinated against Covid-19 by January 4 or get tested for the virus weekly under government rules that took effect on Thursday.
The Occupational Safety and Health Administration (OSHA) said companies that fail to comply could face penalties of nearly 14,000 dollars (£10,300) per violation.
The new requirements, which were first previewed by President Joe Biden in September, will apply to about 84 million workers at medium and large businesses, although it is not clear how many of those employees are unvaccinated.
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 and will need to be vaccinated.
It was unclear how the OSHA planned to enforce the rules.
A senior administration official said the agency would target companies if it gets complaints.
Workers will be able to ask for exemptions on medical or religious grounds.
The release of the rules followed weeks of regulatory review and meetings with business groups, labour unions and others.
The regulations form the cornerstone of Mr Biden’s most aggressive effort yet to combat the spread of Covid-19, which has killed more than 740,000 people in the US.
The OSHA drafted the rules under emergency authority meant to protect workers from an imminent health hazard.
Senior administration officials said the rules pre-empt conflicting state laws or orders, including those that ban employers from requiring vaccinations, testing or the wearing of face masks.
The administration will face an immediate challenge from Republican state officials who are eager to fight Mr Biden in court.
More than two dozen Republicans serving as state attorneys general have indicated they plan to sue, arguing that only Congress can enact such sweeping requirements under emergency authority.
Last week, 19 states sued to stop Mr Biden’s narrower mandate that employees of federal contractors be vaccinated.
That requirement was scheduled to take effect from December 8, but the administration said on Thursday it will be delayed until January 4 to match the requirements on other large employers and healthcare providers.
Workers will need to receive either two doses of the Pfizer or Moderna vaccine or one dose of the Johnson & Johnson vaccine by January 4.
Unvaccinated employees must submit a verified negative test to the company at least once a week and wear face masks on the job.
Employees who test positive must be removed from the workplace.
The OSHA will require businesses to provide 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 from December 5.
Because vaccines are free, the OSHA said, companies do not have to provide or pay for tests.
The Centres for Medicare & Medicaid Services issued a rule requiring vaccination for workers in 76,000 health facilities and home healthcare providers that get funding from the government health programmes.
A senior administration official said that several large private healthcare organisations imposed their own mandates and achieved high vaccination rates – 96% or higher – without widespread resignations.
The White House sees the new requirements as a potent tool to winnow down the ranks of the tens of millions of Americans who have thus far refused to get a shot.
For weeks, Mr Biden has encouraged businesses not to wait for the OSHA rule to take effect.
He has touted businesses that have already announced their own vaccine mandates and urged other companies to follow their lead.
Administration officials say those efforts are paying off, with about 70% of the nation’s adults now fully vaccinated.
However, some companies have expressed fear that some vaccine-hesitant workers might quit, leaving their workforces even thinner in what is already a tight labour market.
Several large business groups complained about the timing of the mandate.
Retail groups worried that the requirement could disrupt their operations during the critical Christmas shopping period.
Retailers and others also said it could worsen supply chain disruptions.
The mandate on federal contractors led to demonstrations by opponents, including workers at a Nasa rocket engine test site in Mississippi.
Some said they are immune because they contracted Covid-19.
Others said vaccines violated their religious beliefs and constitutional rights.
“No-one should be forced to take a medical treatment just to keep their job,” said Nyla Trumbach, an engineer at the site.
“There’s years and years of experience and skill out here, and I just want anyone who’s watching to see what we stand to lose here if these people don’t keep their jobs.”
Dozens of groups sought meetings with administration officials to air their concerns and objections to likely provisions of the OSHA rule, including the US Chamber of Commerce, the National Association of Manufacturers, the AFL-CIO and anti-vaccine organisations.
