That is because coronavirus cases are surging, and officials around the world are bracing for a new wave stoked by the Omicron variant, which appears to be most contagious version yet and is playing a major role in the sharp increase of infections in recent days.
The threat of Omicron is particularly acute in Britain, where government officials estimate that about 200,000 people a day are being infected with the new variant.
Similar efforts to encourage boosters are lagging in the United States, where officials are preparing for a wave as infections rise and the proportion of coronavirus cases caused by the Omicron variant increases sharply.
Still, research shows that the variant s likely to infect many people who have been vaccinated or recovered from bouts with older versions of the virus.
The decision on Friday, by a split three-judge panel of the Court of Appeals for the Sixth Circuit in Cincinnati, overturned a ruling by its counterpart in New Orleans, the Fifth Circuit, that had blocked the rule last month.
The Centers for Disease Control and Prevention said on Friday that unvaccinated students exposed to the coronavirus can remain in school, as long as they are tested for the virus twice in the week after and both tests come back negative.