COMMENT | Last month, the vaccine advocacy group 1Day Sooner published an open letter to Francis Collins, director of the US National Institutes of Health, urging regulators to permit and begin to prepare for “human challenge” vaccine trials in order to end the Covid-19 pandemic as soon as possible.

In these trials, fully informed volunteers would be injected with potential vaccines (or with a placebo) and then would be “challenged” by being intentionally exposed to Sars-CoV-2, the virus that causes Covid-19.