In a toughening of guidance issued earlier this month, the U.S. Centers for Disease Control and Prevention (CDC) also recommended all students, teachers and staff at schools for kindergarten through 12th grade wear masks regardless of whether they were vaccinated.
"We are dealing with an evasive type of an a virus, it evolves," the nation's top infectious disease official, Dr. Anthony Fauci, said on the PBS NewsHour after the announcement.
"So it isn't a question of the CDC flip-flopping in a vacuum. They are keeping up with the evolution of what's going on with the virus," he said.
U.S. coronavirus cases have been rising due to the highly contagious Delta variant, which emerged in India but has quickly spread and now accounts for more than 80% of U.S. coronavirus cases.