The Covid-19 pandemic is unlikely to end until there is a safe, effective, and widely distributed vaccine. How soon can researchers achieve this goal? The answer largely depends on which strategies researchers are willing to adopt. One potential strategy is to conduct human challenge studies, in which researchers give an experimental vaccine to healthy volunteers and then test—or “challenge”—the vaccine by purposely exposing volunteers to the virus. Although a growing number of voices are calling on researchers to employ this strategy, the proposal is generating a heated debate about the ethics of such research.