A promising vaccine candidate, the Moderna mRNA vaccine, was ready in a lab in January 2020. The hundred or so doses that would have been required to get very solid evidence of effectiveness could have been made at lab scale in a few days. We could have known by February, or March at the latest, whether the vaccines worked. Yes, we’d still have had to scale up production, and that would have taken months. But the whole process would have started earlier. Instead, Moderna’s vaccine was not given emergency use approval in the US until December.

As the philosopher Richard Yetter Chappell points out, there are obvious-seeming objections to human challenge trials. You have to give people a potentially dangerous disease: what if it kills one of them?

But one plausible estimate is that roughly 18 million people have died of Covid during the pandemic – that’s an average of about 28,000 a day. Bringing the end of the pandemic forward by even a single day could easily save thousands of lives. A small risk to a small number of young, healthy volunteers was hugely outweighed by a very likely large reduction in risk to many thousands of old, vulnerable people.