[tzs]

I doubt that early human challenge trials would have speed up vaccine development much, if at all. You still need the non-challenge trials to determine that a proposed vaccine is safe and effective in a large diverse population, and those are what took most of the time.

HCT trials involve people who we are either pretty sure won't get seriously ill from the virus being tested or pretty sure we can successfully treat if they do get seriously ill.

That generally means subjects who are young adults in good health with no known conditions that might put them at extra risk (and early into a new disease, we don't know what those conditions might be, so you really want to limit your early HCT to young adults in near perfect health).

The general population is full of people from children to the elderly, ranging in health from Captain America territory to Mr. Burns territory, with a plethora of already existing diseases and conditions.

Testing in that limited demographic that qualifies for an early HCT just doesn't tell you much about the safety or effectiveness of your vaccine for everyone else.