[Gatsky]

I don't think challenge trials would have really helped much. The issue is that obtaining an accurate estimate of vaccine efficacy in the real world is a crucial piece of information. There is a very large difference between a 95% effective and a 70% effective vaccine in terms of the roll out and who/how many need to be vaccinated. Additionally, challenge trials are fundamentally contrived in terms of the innoculum size and mode of transmission. There is no substitute for a proper randomised trial under native transmission conditions. Challenge trials would lead us into a long night of uncertainty about efficacy.