“The U.K. has always, from the beginning, emphasized they do not see a place for face coverings for children if it’s avoidable,” said Dr. Shamez Ladhani, a pediatric infectious-disease specialist at St. George’s Hospital in London and an author of several government studies on the virus and schools.

The potential harms exceed the potential benefits, he said, because seeing faces is “important for the social development and interaction between people.”

The British school system is different than the American one. But with school systems all over the United States debating whether to require masking, Britain’s experience during the Delta surge does show what happened in a country that relied on another safety measure — quarantining — rather than face coverings for young children.