HOW DO you catch or spread the coronavirus? Much has been said about keeping six feet away and avoiding someone’s cough or sneeze. That’s fine, but additional evidence is surfacing about how the virus is transmitted, a key question for reopening decisions. Studies drawn from completed contact tracing — tracking down all those an infected person came into contact with — are showing that sustained, close contact and close quarters are the biggest risks.

Muge Cevik, a physician and scientist at the School of Medicine, University of St. Andrews, who examined a series of contact tracing studies, found increased rates of infection in enclosed and connected environments, including households, long-term-care facilities, churches, public transit and homeless shelters, among others. She said casual, short interactions appear not to be main drivers of the pandemic, but family and friends are.

For example, 2,147 close contacts of 157 coronavirus cases were examined in Ningbo, Zhejiang province, south of Shanghai. The researchers found higher risks of infection for friends, family and relatives; also, higher rates of infection among those living with the infected person or taking the same transit or dining together. In another China study, based in Shenzhen, 1,286 close contacts of 391 virus cases were tracked; again the finding was that “household contacts and those traveling with a case were at higher risk of infection.”

In Chicago, public health authorities in February and March investigated a large cluster of illness among multiple families. The spread of the virus began at a funeral. A close friend of the deceased, who was infected and experiencing “mild respiratory symptoms,” spent hours with the family, sharing a “potluck-style” meal and embracing family members. Three days later, still feeling the mild symptoms, the close friend went to a birthday party attended by nine other family members. There were hugs and shared food. Researchers concluded the initial case “likely triggered a chain of transmission that included 15 other confirmed and probable cases” after the funeral and birthday party, and “ultimately resulted in three deaths.”

Illustrating the risks of tightly enclosed spaces, researchers examined the case of an infected Englishman, who had recently been in Singapore, and in January stayed at a chalet in the French Alps with 10 other British tourists, and a British family of five who lived in France. Of these 16 people, 12 were later found to be infected, for an overall virus attack rate of 75 percent in the chalet. In Boston, when 408 residents of a homeless shelter were checked from March 28 to April 1, 147 were positive for the coronavirus.

All of these examples suggest that the virus spreads when and where people are congregating, and among households, family and close contacts. This insight must be taken into account for every decision about opening up the country. The problem of breaking the virus chains of transmission remains urgent and incomplete. Until a vaccine or therapy is ready, social distancing, isolating the sick, wearing face masks and constant hand-washing are absolutely necessary. Such are the sad lessons of a Chicago funeral and birthday party.

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