I think an obvious first-pass at a threshold would be a global pandemic with a clearly established non-trivial mortality rate and in which it is clear to epidem

I think an obvious first-pass at a threshold would be a global pandemic with a clearly established non-trivial mortality rate and in which it is clear to epidemiologists that -- unless a vaccine is developed -- essentially everyone is going to get the virus at some point. It's much more murky territory for more containable viruses such as ebola. But a deadly pandemic that spreads across the globe in weeks? Seems like it would be hard to argue against challenge trials with informed consent. I think the strongest argument against it is that setting any such bar only makes it possible to lower the bar in the future.