Small outbreaks and universal genomic sequencing provides unique insights into how coronavirus spreads

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Jenene Crossan doesn’t know where she got it. “I caught it in London, have no idea where or from who, in March 2020,” she says. “I’ve been sick ever since.” Crossan used to worry about it – going back over possible infection scenarios, exchanging theories with a friend who got ill at the same time. These days, though, she’s come to terms with not knowing. “The reality is it doesn’t matter,” she says. “London was awash.”

Like many of the vast majority of people unlucky enough to receive a positive Covid test result, the precise moment of infection remains a mystery. Some might narrow it down to a likely household member, friend or workmate who began showing symptoms too. Others trace it to a gathering – a wedding, funeral, or dinner party, where several attendees subsequently came down sick. But most are left wondering. As a New York Times headline put it last year: “How Are Americans Catching the Virus? Increasingly, They Have No Idea”.