But in a separate challenge, some justices seemed more open to a vaccine mandate aimed at certain health care workers.

The court heard arguments for almost four hours as the number of infections is soaring, and 40 million adults in the US are still declining to get vaccinated.

The three liberal justices on the court expressed clear approval for the administration's rules in both areas.

For the first time, seven of the justices appeared in the majestic chamber wearing masks, though Justice Neil Gorsuch chose not to. Before arguments began, the court announced that Justice Sonia Sotomayor, who has diabetes, would hear arguments remotely from her chambers even though a spokeswoman said she was not ill. Sotomayor sits next to Gorsuch on the bench under normal circumstances.