A university in the US state of North Carolina announced today it will switch to entirely virtual classes for its nearly 20,000 students, after dozens tested positive for Covid-19 in the first week back at school.

The move mirrors the decision in multiple US cities that opted for hybrid classes (a few days per week in the classroom) for primary and secondary schools, or entirely virtual classes in major cities including Chicago, Houston, Los Angeles and Washington.

An announcement from the University of North Carolina at Chapel Hill, one of the most popular in the country, said that 177 students who tested positive are currently in isolation, and an additional 349 were placed in quarantine.

The positive test rate on campus jumped to 13.6% last week from 2.8% the previous week, the statement said.

"As much as we believe we have worked diligently to help create a healthy and safe campus living and learning environment, we believe the current data presents an untenable situation," UNC Chapel Hill chancellor Kevin Guskiewicz and vice-chancellor Robert Blouin said in the statement.

Hotspots sprang up in several dormitories and in a fraternity house, the student newspaper The Daily Tar Heel reported.

Only 60% of the dorms were occupied, and 30% of students were attending in-person classes, but the university announced that from Wednesday, all undergraduate students - more than 19,000 people - would switch to remote learning.

The university's more than 10,000 graduate students could continue to attend classes in-person.

Of nearly 3,000 other US universities and higher education institutions, 30% chose to teach primarily or completely online, 15% opted for the hybrid model and 23% decided to resume classes primarily or completely in person. The rest had not yet announced their decision, according to the College Crisis Initiative.

Among the 151 universities that decided to hold classes entirely virtually are the prestigious Harvard University and Johns Hopkins University in Baltimore.