When the number of COVID-19 cases began to rise again in Manaus, Brazil, in December 2020, Nuno Faria was stunned. The virologist at Imperial College London and associate professor at the University of Oxford had just co-authored a paper in Science estimating that three-quarters of the city’s inhabitants had already been infected with SARS-CoV-2, the pandemic coronavirus—more than enough, it seemed, for herd immunity to develop. The virus should be done with Manaus. Yet hospitals were filling up again. “It was hard to reconcile these two things,” Faria says. He started to hunt for samples he could sequence to find...