The race for a COVID-19 vaccine entered a new phase recently. Four different vaccine candidates, developed by Moderna, Pfizer, BionTech and Johnson & Johnson, entered final-stage trials, with two others close behind. But we won’t know exactly how these four vaccines work for months. Robert Langreth explains what we can, and, more importantly, can’t know about a vaccine developed at breakneck speed.See omnystudio.com/listener for privacy information.