In new rules likely to be tested in court, U.S. workers must be vaccinated by Jan. 4 or undergo weekly COVID-19 testing under emergency safety rules from the Biden administration, U.S. Labor Department officials said Thursday.

Employers who don’t enforce the vaccine mandate could be fined $13,600 per violation, according to the Occupational Safety and Health Administration. The agency estimated that the vaccine mandate will save more than 6,500 worker lives nationwide and prevent more than 250,000 hospitalizations over the next six months.

The 490-page OSHA rules apply to medium- and large-size businesses with at least 100 workers and aim to protect the 84 million workers employed at such firms. Federal officials first previewed the new rules in September as the delta variant was fueling a surge in coronavirus infections. OSHA has asked for public comment on whether smaller employers could shoulder the new rules, leaving open the possibility of extending the regulations further to companies with fewer than 100 workers.

At present, 62.4% of these workers are vaccinated, OSHA said. That leaves 37.6% unvaccinated, or about 31 million workers.

OSHA expects these numbers to improve quickly. “We know that the vast majority of workplaces will be in compliance,” Jim Frederick, the top OSHA official, said on a teleconference call on Thursday.

Almost 80% of Philadelphia-area private sector workers, or 1.8 million employees, would fall under the Biden mandate, based on an Inquirer analysis of 2018 U.S. Census data on workplaces.

With many employers scrambling to fill open jobs, business groups have expressed concern that employees could quit instead of complying with a vaccination mandate. In September, Arizona Attorney General Mark Brnovich sued Biden and other officials in his administration over the vaccination mandate, claiming that it was unconstitutional. Other legal challenges are expected.

David N. Taylor, president of the Pennsylvania Manufacturers’ Association, said that manufacturers were against Biden’s national vaccine mandates and, on the other hand, were also against state governors prohibiting employers from mandating vaccinations for their workers. “Everybody needs to take a deep breathe and let people adjust,” said Taylor, adding that he is pro-vaccine but he fears that the mandate could lead to worse labor shortages and supply-chain bottlenecks.

Senior Biden administration officials said Thursday the new rules preempt conflicting state laws or orders, including those that ban employers from requiring vaccinations, testing or the wearing of face masks.

More than 70 business groups, individuals and other interested parties met with administration officials in the final days before the rule was released to ask questions and express concerns such as how the requirement would affect staffing with labor shortages if people refused to comply and quit their jobs. A number of industries are already struggling to find employees, particularly in sectors like construction and trucking.

The rule is slated to last for just six months. OSHA has posted online a webinar, answers to frequently asked questions and compliance materials.

Still, the new policy “is likely to increase compliance costs and cause regulatory burdens that will exacerbate several headwinds facing the construction industry,” Ben Brubeck, a vice president at Associated Builders and Contractors, said in a statement. OSHA estimated the private sector could spend close to $3 billion doing the paperwork.

The White House will face an immediate challenge from Republican state officials who are eager to fight Biden in court and in Congress. Senate Republicans immediately launched a petition to force a vote to overturn the vaccine mandate, but with Democrats controlling the chamber, the effort is nearly certain to fail.

Last week, 19 states sued to stop Biden’s narrower mandate that employees of federal contractors be vaccinated. That requirement was scheduled to take effect Dec. 8, but the administration said Thursday it will be delayed until Jan. 4 to match the requirements on other large employers and health care providers.

Workplaces could be inspected by OSHA through complaints or spot checks, federal officials said. It is not clear how effective the agency will be with only about 1,850 inspectors at eight million workplaces. The mandate rules will be officially published on Friday. Within 60 days, they will take full effect, federal officials said.

Workers can claim exemptions for religious or medical reasons, federal officials said. The requirements will not apply to people who work at home or outdoors. Companies won’t be required to provide or pay for COVID-19 tests, but they must give paid time off for employees to get vaccines and sick leave to recover from side effects.

James J. Sullivan Jr., co-chair of the OSHA workplace safety practice at the Cozen & O’Connor law firm, said the new rules discourage the testing option because they don’t require companies to pay for the tests. They also don’t require employers to pay for masks. These and other factors will discourage testing and lead more people to get vaccinated, he said.

Sullivan was also concerned that employers who don’t comply could incur large penalties because they could be cited for multiple violations.

In a separate announcement Thursday, the Centers for Medicare and Medicaid Services said it is requiring vaccinations for eligible staff at health care facilities that receive funding from the programs and not providing the testing option. This requirement applies to about 17 million health care workers nationwide, according to a media statement.

This article contains information from the Associated Press and Washington Post.