A number of countries have recently dropped mask requirements, including the United States, where a federal mask mandate for public transportation was struck down by a judge on Monday.

“Regarding travelers and travel by plane, we know that the use of masks has reduced dramatically the transmission during traveling,” Dr. Ugarte said when asked about the ruling that voided the mandate in the United States. He expressed concern that people may be stigmatized for choosing to wear masks when they are no longer required.

Mask wearing should be made voluntary only in countries where there is virtually no community transmission, and only when accompanied by increased testing and vaccination, he said.

“We’ve seen that in some countries, the use of masks has been reduced to voluntary levels, and at the same time they’ve reduced the detection of cases,” Dr. Ugarte said. “We can’t let our guard down.”