The nine justices of the U.S. Supreme Court on Friday heard more than 3-1/2 hours of arguments over President Joe Biden's vaccine mandates:

the first over the vaccine-or-testing requirement for businesses with 100 or more employees, and the other over the administration's vaccine requirement for healthcare workers.

But it was the first case that kicked off a heated debate, with liberal Justice Elena Kagan laying out the stakes.

"Why isn't this necessary to abate a grave risk? This is a pandemic in which nearly a million people have died. It is by far the greatest public health danger that this country has faced in the last century. More and more people are dying every day. More and more people are getting sick every day. I don't mean to be dramatic here. I'm just sort of stating facts. And this is the policy that is most geared to stopping all this."

But the conservative justices on the court, who hold a 6-3 majority, questioned the legality of the mandate, which is being challenged by the state of Ohio and a business group, and is set to go into effect on Monday.

Chief Justice John Roberts appeared skeptical that a 1970 law gave the U.S. Occupational Safety and Health Administration, or OSHA, the power to enforce the mandate.

"That was 50 years ago that you're saying Congress acted. I don't think it had COVID in mind. That was almost closer to the Spanish flu than it is to today's problem."

As for the second case, conservative justices, including Roberts and Brett Kavanaugh, appeared more open to the vaccine requirement for healthcare workers, which states led by Missouri and Louisiana are asking the court to block nationwide.

Two of the officials who presented arguments seeking to block the vaccine mandates - the solicitors general from Ohio and Louisiana - made their cases remotely on Friday, after having tested positive for COVID-19.

Decisions by the Supreme Court in both cases are expected soon.