The return of students to some college and university campuses has led to outbreaks of the coronavirus in Philadelphia and across the country.

Now, the city's health department says some students who tested positive for the virus reported attending small social gatherings that did not exceed the current legal limit of 25 people.

“We have been monitoring the outbreaks in colleges across the country, and are trying to limit the spread of COVID-19 in colleges here,” Health Commissioner Dr. Thomas Farley said in a news release. “It does not require large social gatherings for this virus to spread. Any time two or more people are near each other without wearing masks, there is a risk.”

If a student is coming in close contact with people outside their household, the department recommends wearing masks. The department says colleges and universities had to submit safety plans and are largely following them.

"However, interviews suggest that risk continues in small social gatherings that are not organized or sanctioned by colleges," the Saturday evening news release reads.

The city guidance says colleges and universities "should create a plan to enforce mask wearing and social distancing at on-campus events - both those that are school-sponsored and those organized by students."

The update in guidance comes more than a week after Penn State President Eric Barron warned that student gatherings could result in "sending everyone home" after the university had opened for some in-person learning, with students living in dorms.

La Salle University President Colleen Hanycz cited concerns about student gatherings as part of the reason La Salle shifted to virtual classes this fall.

At University of North Carolina-Chapel Hill, coronavirus clusters led to a shutdown of in-person learning and a return to virtual classes. The university later told students it wanted them to report any parties going on.