So what is the reason given for it being unethical? Of course I realize there is a tradeoff involved in delaying the rollout of a vaccine but I don't see how that is related to the way vaccines are made, specifically.
It's not been done once by modern western medicine because it would be unethical by modern medical practices to intentionally infect a person.
If you give 40,000 people a random mix of placebo and vaccines and then monitor whether or not they contract the target disease, you get an indication of efficacy by comparing the vaccinated group to the placebo group.
Please explain how would your proposal be any more valid than a proper controlled study where you do the treatment on portion of your participants and wait how it works out for them.