Last week, the European Medicines Agency declared the AstraZeneca COVID-19 vaccine safe and effective, after several European Union member states had suspended its use because of blood clot concerns. Will the public trust this message? This week's news could help—a U.S. phase 3 clinical trial of the vaccine shows promising efficacy in preventing symptomatic COVID-19. But sentiments toward vaccines are volatile and reflect external events—such as recent concern about AstraZeneca's efficacy data—as well as internal emotions.