MARY LOUISE KELLY, HOST:

President Biden's coronavirus vaccine requirements covering 100 million workers will have a big day in court tomorrow. The Supreme Court is holding a special session to hear arguments for and against the Biden administration's vaccine mandate for health care workers; also its vaccine-or-test rule for private sector workers. NPR's Andrea Hsu has been following the legal battles. She's here now.

Hey there.

ANDREA HSU, BYLINE: Hi, Mary Louise.

KELLY: So I want to get to why the Supreme Court is weighing in here in a second. First, walk us through the details of what these rules actually are.

HSU: Yeah. I'll start with the vaccine-or-test rule for companies. That rule was issued by OSHA, the federal workplace safety agency, and it covers some 84 million workers, which, Mary Louise, is a majority of the workers in this country. The rule requires companies to either mandate vaccines for their workers or make unvaccinated workers test at least once a week. And then separately, there's this other rule covering 17 million health care workers. These are people who work for hospitals, clinics, nursing homes, home health agencies that get funding through Medicare or Medicaid, and most of them do. And these workers have to get vaccinated. They do not have a testing option. Originally, both these rules were supposed to be enforced starting January 4, which was two days ago, but now the deadlines have been pushed back a few weeks or more.

KELLY: Yeah, so how did we get here? How did these cases wind up before the Supreme Court?

HSU: Well, there were legal challenges almost immediately. The OSHA rule, which applies to companies, was blocked a day after it was issued. Then a week - about a week before Christmas, another court unblocked it. And now dozens of businesses and religious organizations and Republican-led states have asked the Supreme Court for an emergency stay. Now, the health care rule, that has also been stuck in the courts. And right now, confusingly, it's blocked in half the states. So the Biden administration wants the Supreme Court to unblock that rule so it can go into effect nationwide. So that's what tomorrow's special session at the court is all about.

KELLY: So let's see if we can get a little preview. Given that more than 800,000 people in the U.S. have died from COVID, given we know vaccines work to prevent severe illness and death, given those things, what are opponents to the vaccine rules - what are they going to argue tomorrow?

HSU: Well, their main argument is that the government overstepped its authority in issuing these rules. They say mandating vaccines or tests for 100 million people is not something that federal agencies can do. They argue that something that significant must be authorized by Congress, and they've also taken issue with how the rules were rolled out without public input.

KELLY: And what about the other side? What's the government likely to argue?

HSU: Well, the Biden administration has said repeatedly that this is an emergency, that it has the authority and also the responsibility to act swiftly when workers' health is at risk. After all, there have been numerous COVID outbreaks in workplaces - remember the meatpacking plants. And OSHA has estimated that the vaccine-or-test rule would save more than 6,000 lives and prevent 250,000 hospitalizations over a six-month period. But here's what the challengers will say to that. They'll point out that COVID is not a workplace-specific threat. You know, people can be exposed to it wherever they go. And besides, we're already two years into this pandemic, so they argue that the government cannot use a workplace-specific rule as a way to increase the country's overall vaccination rate and bring an end to this pandemic, even if that is a desirable goal.

KELLY: Well - and I wonder if it's worth just injecting one other possible factor here. There's concern that critical workers will quit over these rules.

HSU: For sure. And this is something the challengers and also some lower court judges have cited. They are especially worried that the vaccine mandate for health care workers could lead to staffing crises. Now, in hospitals that have already imposed their own vaccine mandates, there has not been a mass exodus of workers. But we do know especially with omicron that health care facilities need every worker they can get right now. And some hospitals have even paused their vaccine requirements to ensure they have enough staff.

KELLY: NPR's Andrea Hsu.

Thanks, Andrea.

HSU: Thank you. Transcript provided by NPR, Copyright NPR.