Several COVID-19 vaccine candidates are being tested now. But why does it take 30,000 volunteers to know if one is safe and effective? And what does it mean to say a vaccine candidate is working?
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Joe Palca is a science correspondent for NPR. Since joining NPR in 1992, Palca has covered a range of science topics — everything from biomedical research to astronomy. He is currently focused on the eponymous series, "Joe's Big Idea." Stories in the series explore the minds and motivations of scientists and inventors. Palca is also the founder of NPR Scicommers – A science communication collective.