Jon CohenScience’s COVID-19 reporting is supported by the Pulitzer Center and the Heising-Simons Foundation.Now that regulators around the world have begun to issue emergency use authorizations (EUAs) for COVID-19 vaccines—the United States authorized a candidate vaccine from the biotech Moderna on Friday—a theoretical debate that has simmered for months has become a pressing reality: Should ongoing vaccine efficacy studies inform their tens of thousands of volunteers whether they were injected with a placebo or the vaccine, and also offer an already authorized vaccine to those who got the placebo?Vaccinemakers must now quickly decide how to handle this issue, called unblinding.