Judges appointed by Donald Trump and Ronald Reagan declined on Friday to lift a stay on the Biden administration’s Covid-19 vaccine requirement for businesses with 100 or more workers.

One law professor said the move showed the court was “radical and anti-science”.

Under the federal Occupational Safety and Health Administration (Osha) requirement, such workers must be vaccinated by 4 January or use masks and weekly tests.

The measure is softer than those implemented by many private businesses and state or local governments, and the Biden administration has expressed confidence in its legality.

Nonetheless, the fifth US circuit court of appeals, based in New Orleans and one of the most conservative federal panels, granted an emergency stay last Saturday.

Justice and labor department lawyers filed a response on Monday in which they said stopping the requirement would prolong the Covid-19 pandemic and “cost dozens or even hundreds of lives per day”.

On Friday, a three-judge panel rejected that argument. In his ruling, Judge Kurt D Engelhardt wrote that the stay was “firmly in the public interest” and referred to the Osha requirement as a “Mandate”, with a capital “m”.

More than 762,000 people have died from Covid-19 in the US, from a caseload of nearly 47m. Nonetheless, vaccine mandates, rules and requirements and other public health measures are the focus of concentrated opposition among Republican voters and politicians, including many jockeying for the presidential nomination in 2024.

More than 434m doses of vaccines have been administered and more than 194 million Americans, or 58.5% of the population, are fully vaccinated.

Resistance to vaccine mandates has produced protests and fears of staff shortages. At the same time, the Biden administration has heralded strong jobs numbers and what it says is an economy rebounding from its Covid battering.

Engelhardt said “the mere specter of the Mandate” had stoked “workplace strife” and “contributed to untold economic upheaval in recent months”.

“Rather than a delicately handled scalpel, the Mandate is a one-size fits-all sledgehammer that makes hardly any attempt to account for differences in workplaces (and workers) that have more than a little bearing on workers’ varying degrees of susceptibility to the supposedly ‘grave danger’ the Mandate purports to address.”

Experts say Covid remains a grave danger. Writing for the Guardian, Eric Topol, a professor of molecular medicine and executive vice-president of Scripps Research, said the US was “sitting in the zone of denial”.

“We are already seeing signs that the US is destined to succumb to more Covid spread,” Topol said, “with more than three weeks sitting at a plateau of ~75,000 new cases per day, now there’s been a 10% rise in the past week.

“We are miles from any semblance of Covid containment, facing winter and the increased reliance of being indoors with inadequate ventilation and air filtration, along with the imminent holiday gatherings.

“Now is the time for the US to … pull out all the stops. Promote primary vaccination and boosters like there’s no tomorrow. Aggressively counter the pervasive misinformation and disinformation. Accelerate and expand the vaccine mandates that unfortunately became necessary and have been proven effective, and mass distribute medical quality masks and rapid home testing kits at no cost.”

Judge Engelhardt said the Biden business requirement potentially violated the commerce clause of the US constitution.

“The Mandate imposes a financial burden upon [businesses] by deputising their participation in Osha’s regulatory scheme,” he wrote, “exposes them to severe financial risk if they refuse or fail to comply, and threatens to decimate their workforces (and business prospects) by forcing unwilling employees to take their shots, take their tests, or hit the road.”

Federal judges routinely claim to be above politics. But the system for nominating and appointing them runs through the Senate, where Republicans are aggressive in seeking to tilt the bench their way.

Engelhardt was nominated by Trump and confirmed in 2018, as was Stephen Kyle Duncan, who joined Engelhardt’s opinion on Friday. So did Edith H Jones, a Reagan appointee in 1985.

All three have links to the Federalist Society, a conservative group which has worked with Republicans in Congress to install judges including Neil Gorsuch, Brett Kavanaugh and Amy Coney Barrett, supreme court justices nominated by Trump.

At least 27 states have filed challenges to the Osha vaccine rule in federal appeals courts. The federal government said in court filings the cases should be consolidated and one circuit court should be chosen at random on 16 November to hear it. Administration lawyers also say there is no reason to keep the requirement on hold while the court where the cases ultimately land remains undetermined.

In an email, Carl Tobias, a law professor at the University of Richmond in Virginia, said the fifth circuit was “acting politically”.

“But it also seems to be attempting to dictate the terms of the debate and to be violating the intent of Congress … to have an orderly system to resolve appeals when there are multiple challenges to an agency’s action and to prevent litigants from ‘forum shopping’ by racing to the courthouse to secure a decision from the court that it believes most favors the litigant’s position.

“The appeals court which ‘wins’ the lottery and receives all of the challenges is free to ignore what the fifth circuit did or other courts may do.”

On Twitter, Rick Hasen, a law professor at the University of California, Irvine, said: “One factor federal courts must consider in granting or denying emergency motions is the ‘public interest’. [It is] ASTONISHING that the fifth circuit does not even MENTION prevention of death/disease from Covid as a public interest to justify the vaccine mandate”.

He also said he did not “even think conservative is the right word” to describe the fifth circuit court.

“It’s pretty radical and anti-science,” he said.