Nepal, July 6 -- I was recently stunned to learn of the serious consideration being given to deliberately infecting human volunteers with the SARS-CoV-2 virus in order to assess the effectiveness of potential Covid-19 vaccines.

My first reaction was that the advocates of such 'human challenge studies' had gone so mad with panic that they had forgotten the history and horrors of medical experimentation on humans. But on closer inspection, I saw that they included some of the world's most respected vaccine researchers and medical ethicists, and even the World Health Organisation.

As far as I can tell, their principal argument is that waiting for an answer from naturally occurring infections will take too long. The new coronavirus has alre...