SAN FRANCISCO (NYTIMES) - Seventy people at San Francisco's largest homeless shelter have tested positive for the coronavirus, Mayor London Breed said on Friday (April 10).

The outbreak, which included two shelter employees, is the largest reported at a single shelter in the United States. It reinforces a major fear that homeless people, many whom have preexisting respiratory illnesses, are especially vulnerable to the pandemic.

Advocates in San Francisco, where there are more than 8,000 homeless people, had expressed concern in recent weeks that the city had not moved quickly enough to use empty hotel rooms to thin out the shelter system.

California has procured more than 8,000 hotel rooms for homeless people and those who need to quarantine themselves, far short of the more than 100,000 people in the state who sleep on the streets.

The shelter where the outbreak occurred, Multi-Service Centre South, normally houses around 400 people. In recent weeks, the city had reduced that number of occupants to 144, all of whom were tested on Friday.

The outbreak underlined the breathtaking speed at which the virus can spread in a congregate setting.

Dr Grant Colfax, the director of the San Francisco Department of Public Health, said the city tested residents of the shelter on Wednesday and again on Friday.

"On Wednesday, there were five positive cases," Dr Colfax said. "Today, there are 70."

New York City, which has the country's largest homeless population, has identified coronavirus cases in dozens of shelters. In Silicon Valley, a homeless person living in an encampment died of the disease. At least a dozen homeless people have tested positive for the virus in Los Angeles County, according to the authorities.

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San Francisco has deployed a dual strategy in trying to protect its homeless population, spacing out beds in homeless shelters and lifting its ban on tent encampments. Many streets, largely empty of other residents, are now lined with camping tents that city workers make sure are kept at least 2m apart.

"We are no longer trying to break up encampments, whether it's one tent or 15 tents," said Mr Jeff Kositsky, a city official charged with managing the coronavirus response for the roughly 5,000 people in San Francisco who sleep on the streets.