In a news briefing on Tuesday, Dr. Thomas Farley said there is "definitely an outbreak occurring among Temple students."
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Over the weekend, Temple announced all classes would be virtual for two weeks after the number of positive cases on campus jumped from 58 on Friday to 103 on Sunday.
Farley said contact tracing suggests the outbreak is primarily coming from off-campus apartments with multiple residents and small social gatherings around campus.
In response, Farley said the Health Department has revised its guidance for colleges to recommend against gatherings of any size.
"We understood that some students felt that any gathering that had fewer than 25 persons was okay. Clearly it's not," Farley said.
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The health commissioner then delivered a message specifically for Temple students.
"So for any Temple student who is listening to this today I want to be really clear, and we are asking you to follow this guidance: You should assume that everyone around you is infected," he said.
He also urged students to help prevent the spread of COVID-19 by not going out unless it is essential and wearing a mask when they do.
"Otherwise this outbreak could grow quickly and threaten the rest of the campus and the rest of the city," he said.
Farley said the city would work with Temple on a long-term plan as they address this outbreak.