Rice University in Houston has reinstated its mask policy for classrooms, regardless of vaccination status.Credit...Annie Mulligan for The New York Times

As of Sunday, there were an average of more than 31,000 new cases across the country, according to a New York Times database, a 3 percent increase over the last two weeks. New York City and Washington, D.C., have seen sharper increases, though the average number of cases remains a fraction of each city’s peak. Several prominent officials and some Broadway stars have recently tested positive.

Amid the winter surge driven by another form of Omicron, some universities shifted to enacting school policies aimed at living with the virus, such as quarantining and social distancing when outbreaks occur, rather than canceling instruction altogether. As new cases decreased, some colleges and universities further loosened precautions, such as widespread surveillance testing programs, despite criticism from public health experts.

But now, while some universities, such as Harvard, are letting professors decide what protocol to follow in the classroom, others are imposing new mask rules:

• American University announced on Monday that it would start requiring masks in all campus buildings, including medical facilities and the school’s shuttles, beginning on Tuesday. Professors will be able to remove their masks in classrooms while teaching.

• Columbia University said on Sunday that students would have to wear non-cloth masks in classrooms for the remainder of the spring semester, through May 2, “based on the current situation and in an abundance of caution.”

• Georgetown University announced last week that masks would be required inside any campus buildings until further notice because of campus Covid outbreaks, “particularly among undergraduate students.”

• Johns Hopkins University said last week that it would require masks indoors, along with twice-weekly testing of students, citing that nearly 100 of its more than 5,000 undergraduate students had tested positive since April 1.

• Rice University reinstated its mask policy for classrooms, regardless of vaccination status. Those who are unvaccinated also must continue to wear a mask in other indoor areas.