As coronavirus cases have increased across the United States, some universities and public school systems have reimposed indoor mask mandates on their campuses, a sign that while the academic year may be coming to a close, the pandemic is still not.

The University of Hawaii’s mandate was expanded Wednesday on its 10 campuses, requiring masks in all indoor spaces except when working alone or where social distancing is possible. The university said it took the action because nearly the whole state was now in the high-risk category for community transmission under Centers for Disease Control and Prevention guidance.

The University of Delaware cited rising new-case reports and hospitalizations both in its home state and across the nation when it announced its mask mandate would once again include all indoor spaces, effective Tuesday. President Biden, an alumnus, is scheduled to give a commencement address at the university on Saturday.

Some public school systems have taken similar steps this week to reintroduce universal indoor masking, including two in Rhode Island, in Providence and Central Falls. Both are in a county that was recently classified as high risk, officials from each system said.

The schools in Philadelphia restored their mask mandate on Monday. When city officials there tried to do the same for other indoor public settings in April, they reversed course after four days.

Another Omicron subvariant, known as BA.2.12.1, which spreads more rapidly than previous versions, has become the dominant form of the virus among new U.S. cases, according to federal estimates. For the first time since February, the country is now averaging more than 100,000 new confirmed cases a day, according to a New York Times database, and the widening use of at-home testing means the true number of infections is probably higher.

Federal health officials have warned that a large share of Americans were living in areas with medium or high levels of virus transmission. The White House’s Covid-19 coordinator, Dr. Ashish K. Jha, is also urging Americans to mask up. “I feel that very strongly, that in crowded indoor spaces, in places with high transmission, people should be doing that,” Dr. Jha said Sunday on the ABC program “This Week.”

Still, conditions appear to be stabilizing in some Northeastern states that were among the first to see a spring surge in cases. Though still high, case rates have started to level off or decline in New Jersey, New York and Rhode Island.