1. Prove it works (is there data that provides lasting immunity to all other strains?)
2. Prove it is safe (e.g. is it mild/asymptomatic across all populations?)
Both of those things take time to fully vet. What's the difference between rolling the dice with a live strain vs. one of the half dozen vaccines already in development?
If you are stuck in the “every approach has to be tested to conventional FDA standards” then yes. If you are able to look outside the box for a little then you will realise that you don’t. This is not a problem that is going to be solved by old thinking.