A small number of U.S. colleges and universities are reinstating mask policies, citing coronavirus outbreaks on campus. The moves come at a time when the highly transmissible Omicron subvariant, known as BA.2, has become the dominant version among new U.S. cases, which are ticking up.

The nation was averaging more than 31,000 new coronavirus cases a day as of Sunday, according to a New York Times database —  a 3 percent increase from two weeks ago. New York City and Washington, D.C., have seen sharper increases, though the figures there remain well below earlier peaks. Several prominent officials and some Broadway stars have recently tested positive. With new cases low but rising sharply in recent days, the city of Philadelphia announced on Monday that it will reinstate an indoor mask mandate a little more than a month after lifting it, becoming the first major U.S. city to do so.

During the winter surge driven by another form of Omicron, the BA.1 subvariant, some universities shifted to pandemic policies aimed at living with the virus rather than trying to keep it off campus completely — policies like using quarantines and social distancing when outbreaks occur, rather than canceling in-person instruction altogether. As the numbers of new cases decreased, some colleges and universities further loosened precautions like surveillance testing programs, despite criticism from public health experts.

Now, while some universities like Harvard are letting professors decide for themselves which rules to follow in their classrooms, others are imposing new campuswide mask rules:

• American University said 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.” Last week, Barnard College, a women’s undergraduate college affiliated with Columbia, reinstated its indoor mask rule.

• 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.”

• George Washington University said Monday that its mask mandate would return and stay in place through commencement, the school’s student newspaper reported. Officials quoted in the report cited the university’s seven-day positivity rate, which is nearing 2 percent after remaining close to 1 percent for much of the semester.

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

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

Adeel Hassan contributed reporting.