> You can still measure: the immune response, antibodies, white blood cells, scenarios in which the contagion can spread, minimal social distance needed, length

> You can still measure: the immune response, antibodies, white blood cells, scenarios in which the contagion can spread, minimal social distance needed, length of exposure required for transmission, how much masks stop transmission, safety (risk reduction), and peak efficacy. [...] Of course. These are all great uses for challenge studies, I was responding specifically to testing the vaccine using a challenge study on healthy young adults. > If it causes problems in young adults, you quit because it would probably cause even more problems in riskier groups. If it doesn't, you move forward into older groups. It turns out that both Pfizer and Moderna trials have shown much more adverse reactions in the young than the old. Testing on the wrong population may result in rejecting a vaccine that could be perfectly safe on the at-risk population, but inappropriate for the general population. > Let's not pretend you would learn nothing that would speed things along Of course not. You could learn a lot, just not about the safety and efficacy profile of the vaccine on the at-risk population.