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.

Experts in infectious disease and public health warn that the Covid-19 pandemic will be with us until there is an effective vaccine, possibly 12 to 18 months in the future. This situation has given rise to calls for human challenge studies, in which healthy volunteers are injected with an experimental vaccine and then infected with the disease to test the vaccine’s efficacy. Is this ethically justifiable?