Each virus has its unique pattern of spread, and scientists are starting get a handle on how the novel coronavirus behaves. This understanding is making it possible to rank the risks of different activities from high to low to trivial.

The most informative studies show how the disease is spreading in the real world — a big advance over the various simulations and models that, early on, showed only hypothetical scenarios.

The two drivers of the spread of the disease are close contact and crowding in closed spaces, said Muge Cevik, a virologist at the University of St. Andrews in the U.K. It spread through homeless shelters and  nursing and care homes, where people were crowded with many others. It spread through people’s households, and through meat packing plants.

Cevik has been collecting and reviewing papers from around the world on disease transmission. “There are some trends emerging,” she says. “Spending time dining together, being in public transport,” might risk spreading the disease, but “going to a market briefly, for five minutes or a transient encounter while you walk or run past someone, those are low risks.”

The studies come from China, Singapore, Taiwan, and to a lesser extent the U.S. They were all done through contact tracing, which may turn out to be humanity’s greatest strategy for fighting the Covid-19 pandemic. Contact tracing can stop chains of transmission, even after a disease is widespread, as physician and former World Bank president Jim Yong Kim explained in The New Yorker. Another major benefit is that it offers clues as to how the disease spreads. Each virus has its unique pattern.

The U.S. has done almost no contact tracing yet. A survey of people coming to hospitals in New York City  in May revealed that most of them had been home, and were not working or taking public transport. But why isn’t everyone admitted to the hospital being asked about this? Why aren’t we finding out who they live with, or who visited them, and tracking down where they’ve been? The lost opportunities are staggering.

Cevik said people often ask her how the disease could be so transmissible if it takes closed environments or close contact to spread. The first part of the answer is that after much speculation of extreme transmissibility, the data show something intermediate, with each infected individual transmitting the virus to between two and three others on average. But the important point, which is often missed, is that this is just an average. In the real world, most people transmit the disease to nobody, or one person, and a minority infect many others in so-called super-spreading events. It's those we must learn how to avoid.

The data show that nine percent of infected people are responsible for 80% of the transmissions, she says. Why? For one thing, the disease is apparently very infectious but only for a short window, and perhaps only in some cases. Contact tracing studies show people are most infectious right around the onset of symptoms, as well as a couple of days before and after. If someone in that stage goes to a party, or church service, or to work in a meat packing plant or nursing home, many other people will probably get sick.

One study in China showed how the virus spread at a business meeting and a restaurant. A contact tracing effort in Singapore revealed big clusters of cases stemmed from a business meeting, a church, and a visit to a shop. Another study, one of the few from the U.S., showed how one infected person in Chicago spread the disease to multiple people at a funeral and later at a birthday party, and one of those infected at the party then spread the disease to others at a church service that lasted more than two hours. Other studies connected outbreaks to crowded offices.

People who eventually developed severe symptoms were more likely to transmit the disease to others than were those who had mild symptoms, Cevik says. While it’s clear the disease can be spread by people before they have symptoms, it’s still an open question how many people have no symptoms and whether they are driving much of the spread.

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