The New Yorker – “For the majority of the nearly five million COVID-19 cases across the United States, the point of infection is unknown. “We don’t know how their exposure occurred or what kind of environment they were in when it happened,” Crystal Watson, an expert on contact tracing at the Johns Hopkins Center for Health Security, told me. When an outbreak is under control, this would not be the case. Test-and-trace teams would track the percentage of cases that can be linked to another person—or, more specifically, the percentage of cases that were already on the health agency’s radar, as potentially exposed contacts. “If your contact-tracing program is doing well, the number is going to be high,” Emily Gurley, an epidemiologist who teaches a popular online course on contact tracing, told me. With cases surging across the U.S., that number isn’t high in any of the existing state and city contact-tracing teams. “One type of data that we’re not doing a good job collecting and making public,” Watson said, “is the contribution of different modes of transmission to virus spread.”…