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Cases of Covid 19 began to soar in the US in the autumn. By early January there were around 300,000 new cases a day. But since then the numbers have fallen steeply. What caused this dramatic drop? From herd immunity to the weather, Tim Harford explores some of the theories with Derek Thompson of The Atlantic magazine and Professor Jennifer Dowd, deputy director of the Lever Hume Centre for Demographic Science at the University of Oxford.