WASHINGTON — More than six weeks after promising a new vaccination-or-testing rule covering the millions of Americans at companies with 100 or more workers, President Joe Biden’s most aggressive move yet to combat the COVID-19 pandemic is almost ready.

An obscure White House office is expected to give the green light any day to the rule’s fine print detailing how and when companies will have to require their employees to be vaccinated or undergo weekly testing.

The full enforcement deadline, which could carry penalties of about $14,000 per violation, may not take effect until after the new year. That’s why Biden and his aides have for weeks encouraged businesses to act as though the rule was already in effect and start imposing vaccination requirements.

The regulation, to be published in the Federal Register, was drafted by the Occupational Safety and Health Administration and will cover an estimated 80 million U.S. workers.

Unlike healthcare providers or federal employees, who may not have a testing alternative to vaccination, private sector workers won’t necessarily face termination if they don’t get vaccinated.

But some businesses may choose to impose their own more stringent vaccination mandate, and it’s possible that businesses may be allowed to pass on the cost of weekly COVID-19 testing to their unvaccinated employees.

The U.S. Chamber of Commerce and other groups that represent large employers are worried that the proposal -– applying to companies with 100 or more employees — could cause workers to migrate to jobs at smaller employers.

“We really stressed the concern about employers losing employees, and what that would mean in the context of current supply-chain challenges and the upcoming holiday season,” said Marc Freedman, vice president for employment policy at the Chamber of Commerce. “You could start to see some very serious disruptions.”