Just two months ago, the incredible performance of new vaccines from Moderna and Pfizer had people cheering for an imminent end to the pandemic. But an onslaught of fast-spreading and potentially dangerous mutations of the virus changed that. So now, even as pharma companies ramp up production in the early stages of a massive rollout, they are racing to retool their vaccine strategies. Robert Langreth reports that booster shots could give drugmakers a lucrative new revenue stream.