Already, several Republican governors have filed suits challenging the employer vaccine mandate, as have the Republican National Committee, the small-business group Job Creators Network and several individual firms. On Friday, a federal appeals court temporarily suspended the policy, which was set to take effect in January, in response to a suit spearheaded by Texas’ Republican attorney general.

The vaccine mandate “is a federal overstep that will only create more confusion and legal challenges,” Sen. Richard Burr (R-N.C.), the ranking member of the Senate health committee, said in a statement to POLITICO. “The move proves the administration continues to be out of touch with working Americans and risks exacerbating labor and testing shortages. Heavy-handed mandates will not solve this problem. Instead, the administration should foster cooperation with businesses and workers, with clear communication, transparency, and trust.”

And some worker advocates who generally support the Biden plan are concerned about the administration’s decision to make unvaccinated people pay for their own masks and tests while on the job, given that OSHA typically requires employers to pay any work-related safety costs.

“It’s very unfortunate that this new rule does not require employers to pay for face masks, or for the cost of testing for workers who choose not to get vaccinated,” said Jessica Martinez, co-executive director of the National Council for Occupational Safety and Health. “Pushing these costs onto workers is wrong-headed and an unprecedented departure from all previous OSHA standards.”

A Department of Labor spokesperson said that “payment for the costs associated with testing pursuant to other laws or regulations not associated with the OSH Act is beyond OSHA’s authority and jurisdiction,” in response to questions about employers’ concerns.

But the spokesperson also noted that some companies might choose to pay for testing, in part or in full, “as an inducement to keep employees in a tight labor market.” Others may try to make affected employees pay for the screening, the spokesperson added.

The Biden policy will require businesses with more than 100 employees to verify their staff is vaccinated against Covid-19 or submit to weekly tests. The Occupational Safety and Health Administration estimates that more than 75 million workers — nearly 90 percent of those covered by the regulation — will be vaccinated by the time the requirement is set to take effect. That would leave roughly 9 million holdouts, most of whom the government predicts will work in person rather than remotely.

The Equal Employment Opportunity Commission has already clarified that employers can mandate vaccinations in their workplace, so long as they provide accommodations for workers who say they can’t get the shot because of their religious beliefs or a disability.

Those accommodations are required by the Americans with Disabilities Act and Title VII of the Civil Rights Act of 1964, which allow employers to refuse only if they have an “undue hardship” or if providing an accommodation would present a “significant difficulty or expense.”

But meeting that threshold is difficult, and attorneys say they are skeptical that the cost of testing would qualify.

Employers usually bear the cost of accommodations, explained said Ian Carleton Schaefer, chair of Loeb and Loeb’s employment and labor practice in New York, because an employee could argue that they are being financially penalized for their disability or religious belief, by being required to pay for their own reasonable accommodation.

“I think employers will be hard pressed to push the cost on to employees who fall properly within those exemptions,” Schaefer said of the Covid-19 testing costs.

Ed Egee of the National Retail Federation said he expects some of its members will still cover the costs of testing, given the lengthy process it takes to verify whether a worker’s request for an accommodation is valid.

Employers will also likely have to cover the cost of testing if they require a worker to get tested during the workday, under current overtime and minimum wage laws.

And it is not clear whether insurance companies will begin to cover workplace Covid-19 testing, adding another layer of complexity.

America's Health Insurance Plans, a payer trade lobby, said it is “still evaluating” the OSHA regulation in response to questions about if insurers would cover the costs of testing for the unvaccinated. Federal law requires insurers to cover Covid-19 testing without cost-sharing requirements “when the purpose of the testing is for individualized diagnosis or treatment,” rather than workplace use.

But several public health experts told POLITICO it is unlikely insurers will be keen to cover weekly Covid-19 testing for unvaccinated employees due in part to tri-agency guidance issued by the Departments of Labor, Treasury, and Health and Human Services. The regulations, updated earlier this year, state that “issuers are not required to provide coverage of testing such as for public health surveillance or employment purposes” under the Families First Coronavirus Response Act.

Leana Wen, an emergency physician and public health professor at The George Washington University, argued insurers should not be required to cover testing of unvaccinated workers unless they have Covid-19 symptoms or have a known exposure to someone infected with the virus.

“It is completely unreasonable to require insurers to incur that cost, the cost of testing should be entirely borne by the individual who chooses to remain unvaccinated,” Wen said. “Individuals have a choice; they could get a free, safe and effective vaccine, or they need to incur the financial penalties accordingly.”

In the meantime, there are also questions about whether the plan will squeeze the nation’s testing system.

The federal government has scrambled in recent months to shore up the U.S. supply of rapid tests, including at-home tests. But the vaccine mandate does not allow at-home tests to be used to satisfy the weekly testing requirement for unvaccinated workers unless employers or authorized telehealth proctors observe workers taking the tests.

That requirement appears to be an attempt to ensure that unvaccinated workers are submitting results from Covid-19 tests that they themselves took, said Harvard University epidemiologist Michael Mina. But it is unclear if the OSHA regulation will be effective at preventing bad actors from mailing in fraudulent samples for lab-based testing.

“Nobody is actually validating whose nose that swab went into,” Mina said. “The chain of custody for all these tests is really being broken all the time, so the verified piece maybe doesn’t go far enough if our real goal is to say we’re trying to ensure the authenticity of a test.”

Mayo Clinic Laboratories President William Morice, who also serves as president of the American Clinical Laboratory Association board of directors, said he is concerned about the potential for testing delays and backlogs — especially if false positive results necessitate further confirmatory testing.

“It's going to be a burden on the testing system, we're going to run into test shortages,” said Georges Benjamin, executive director of the American Public Health Association. “It's going to be an administrative burden that will foster more people getting vaccinated just because of the hassle.”

Others argue that weekly testing may inconvenience unvaccinated people and provide a behavior nudge to seek out Covid-19 shots, but it won’t help stop the coronavirus’ spread.

The testing requirement is aimed at “making it more difficult for people who choose not to get vaccinated,” said Céline Gounder, an infectious disease specialist and a former Covid-19 adviser to the Biden transition.

“With Delta, you would really need to be testing every other day at a minimum if you’re really trying to prevent transmission in the workplace,” she said.

Many at-home tests come with instructions directing asymptomatic people to take two tests over three days to ensure accurate results, for instance. Still, the Biden plan's weekly testing requirement is in line with current guidance from the Centers for Disease Control and Prevention.

In the meantime, trade associations for the laboratory and diagnostics industries say they are ready to meet any new demand for testing driven by the Biden mandate.

"We have the supply to be able to meet this additional level of testing," said Susan Van Meter, executive director of AdvaMedDx, a medical-device trade group.

And American Clinical Laboratory Association President Julie Khani said big testing laboratories — which include Quest Diagnostics and LabCorp — “have ample testing capacity and stand ready to meet the nation’s testing needs.”

It’s unclear whether the vaccine or test mandate will actually be enforced starting Jan. 4, given the current stay on the policy issued by the 5th Circuit U.S. Court of Appeals. Many legal experts predict the dispute could ultimately reach the Supreme Court.

While the Department of Justice has vowed to “vigorously defend” the rule in court, at least a dozen legal challenges have been filed in the 5th, 6th, 7th, 11th, and D.C. Circuits, triggering a lottery over which circuit will ultimately hear the challenge.

According to a DOJ court filing, the lottery to decide which court will hear the case is scheduled for Nov. 16.

Historically, OSHA emergency standards have had a roughly 50/50 chance of survival in the courts. The agency has issued 10 emergency temporary standards in its five-decade history. Of those, five were blocked either completely or partially.

“In the end, this is a heavily politicized fundamental rights kind of argument” against the rulemaking, said David Miller of Bryant Miller Olive P.A. He added that he expects the Supreme Court is the venue where “it's got to be settled.”