(Reuters) — Citigroup Inc. will begin enforcing a previously announced “no-jab, no job” policy as of Jan. 14, according to a source familiar with the matter, making it the first major Wall Street institution to implement a strict COVID-19 vaccine mandate.

The move comes as the financial industry, which has long been keen to get back to business as usual, grapples with how to safely bring workers back to the office amid the spread of the highly infectious omicron coronavirus variant.

Other major Wall Street banks, including Goldman Sachs & Co., Morgan Stanley and JPMorgan Chase & Co., are telling unvaccinated staff to work from home, but have not yet gone as far as terminating their employment.

While Citigroup is the first Wall Street bank to enforce a vaccine mandate, a handful of other major U.S. companies have introduced “no-jab, no-job” policies, including Google and United Airlines, with varying degrees of stringency.

Citigroup said in October it would require U.S. employees to be vaccinated as a condition of their employment but did not say when it would begin enforcing the new policy.

The bank said at the time it was complying with the policy of the Biden administration requiring all workers supporting government contracts to be fully vaccinated, as the government remains a “large and important” client of Citigroup.

Citigroup will assess exemptions on religious or medical grounds, or any other accommodation by state or local law, on a case-by-case basis, the bank said at the time.

The source said the bank would begin enforcing that policy as of Jan. 14, but did not provide further details.

The U.S. Supreme Court on Friday was hearing arguments over requests by Republican state officials and business groups to block a Biden vaccine mandate for employers with more than 100 workers.

Bloomberg first reported Citigroup's Jan. 14 deadline on Friday. Citigroup will place workers who do not comply by then on unpaid leave, with their last day of employment at the end of the month, the news outlet reported.

More than 90% of Citigroup staff have so far complied with the mandate and that figure is rising rapidly, Bloomberg reported, citing a Citigroup spokeswoman.

Many financial companies have pushed back their return-to-office plans and are encouraging staff to get vaccinated and boosted but have so far avoided vaccine mandates for legal reasons.

“This is going to be a challenging and complex policy to implement. The problem here is there are a variety of different laws that weigh in on this,” said Chase Hattaway, a partner at law firm RumbergerKirk.

“Citi will have to tailor its policy to state legislation, and in many cases, cities and municipalities will have different regulations, as well, that may require even further carve-outs,” Hattaway said.

The Biden administration has used regulations to require businesses with at least 100 employees to require vaccination or weekly testing of employees.

An increasing number of U.S. companies have been using vaccine requirements to protect staff and operations from disruptions.

United Airlines CEO Scott Kirby said last month the carrier fired 200 of its 67,000 employees for failure to comply with its mandate.

Many hospitals have fired staff for failing to comply with mandates, which have been imposed on the health care industry in more than 20 states.

While some companies such as Tyson Foods Inc. have gotten more than 96% of its employees to take a vaccine, those in construction and retail have resisted vaccine mandates over fears of staff resistance amid a very tight labor market.