I’m curious if we have data wrt to safety and approval process length. How fast (in terms of trial - size and duration) - can we go on approvals without compro

I’m curious if we have data wrt to safety and approval process length. How fast (in terms of trial - size and duration) - can we go on approvals without compromising safety? Maybe a progressive approval is best. Approve for x% of population for next n months then study the data and lift limits. What if it was dangerous that we went from 0-100 with the current round of vaccines?