If you were dealing with a disease that is rare so that it might take a long time for enough people in phase II or III to get exposed to it naturally, an HCT might be able to cut the amount of time you need for phase II and maybe phase III.

With COVID, there should be enough people getting exposed naturally to make that unnecessary in phase III and probably in phase II.

What an HCT might do for a COVID vaccine is let you figure out early that your vaccine does not work. That might let you abandon an ineffective vaccine during phase II instead of not finding out it doesn't work until after you have spent a lot of time and money on phase III.