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US President Joe Biden will enforce a mandate that workers at companies with at least 100 employees be vaccinated against COVID-19 or be tested weekly starting January 4.
The move has spurred legal challenges from Republican governors who say Biden is overstepping his authority.
Within hours governors from Florida, Iowa and Indiana had vowed to fight the new rule, arguing it infringes on individual freedom.
Despite growing political opposition, the delayed roll-out of the mandate offered a reprieve to businesses facing labour shortages during the holiday season.
Biden's related decision to push back a deadline for federal contractors to the same date suggested the White House accommodated requests from companies and industry groups.
The administration also said millions of workers in healthcare facilities and nursing homes participating in the Medicare and Medicaid government healthcare programs will need to get their shots by the same date.
The action on the private-sector vaccinations was taken under the US Occupational Safety and Health Administration's (OSHA) emergency authority over workplace safety, officials said.
The mandate applies to 84.2 million workers at 1.9 million private-sector employers. Another 18.5 million workers for those employers are exempt because they either work remotely or outside all the time, OSHA said.
"While I would have much preferred that requirements not become necessary, too many people remain unvaccinated for us to get out of this pandemic for good. So I instituted requirements - and they are working," Biden said in a statement.
An average of about 1100 Americans are dying daily from COVID-19, most of them unvaccinated. It has killed more than 745,000 people in the United States.
OSHA estimates that 31.7 million of covered workers are unvaccinated and 60 per cent of employers will require vaccinations, up from 25 per cent today, resulting in another 22.7 million employees getting vaccinated. The healthcare industry rule will lead to 2.4 million worker vaccinations during the first year.
The administration's various vaccine rules cover 100 million employees, about two-thirds of the US workforce, the White House said.
OSHA will consider during a 30-day public comment on the private-sector rule expanding the mandate to cover businesses with fewer than 100 workers, officials said.
The private-sector mandate is likely to trigger a legal battle hinging upon the rarely used law on which the action was based and questions over the constitutional limits of federal power and authority over healthcare practices.
The administration said the action falls well within OSHA's authority.
Biden in September unveiled plans for the mandate, seeking to increase vaccination rates amid a dangerous surge in COVID-19 cases and get more people back to work.
Employers will not be required to provide or pay for tests. The administration estimates about five per cent of employees covered by the rule will seek and receive religious or medical accommodations.
Failure to comply with the mandate would trigger fines of about $US14,000 ($A18,900) per violation, which would increase with several violations, officials said.
They did not specify whether workers would be fired for refusal to be vaccinated or tested.
A company can have all its workers unvaccinated under the rule as long as they get tested regularly and wear masks, officials said.
A minority of Americans has refused to be vaccinated. About 70 per cent of US adults have been fully vaccinated and 80 per cent have received at least one shot, according to the latest data.