I disagree with that. It is very possible that there will not be enough randomness in the vaccinated and non-vaccinated groups to positively identify the vaccine as a reason.
As far as ethics or morality, I have read stories of parents who child has a disease, such as a unique form of leukemia. When told that their child is certain to die in a short time span, they search for new or alternative treatments. I remember at least a couple of those in which a treatment was being studied for potential FDA approval. (Some of the treatments are just total bunk) The parents appeal to doctors and to the FDA to allow their child to be treated with said treatment. When refused, they take their kids to Mexico to get the treatments. I don't remember any of those stories in which the child survives, but if you are thinking about the ethics: Is it more unethical to allow a person in their 30s who is unlikely to suffer significant harm from COVID to remain unvaccinated or is it more unethical to tell a parent whose child is certain to die that they cannot use a medicine because the FDA has not yet reviewed the paperwork from the manufacturer? It is my opinion that IF the science of clinical trials matters, then it should be followed EVERY time. If the science of clinical trials doesn't really matter, then there is no reason to prohibit a person who has a terminal illness from doing whatever they want to do.
It might work this time for COVID, but mixing science up for political or societal pressure will not end well. The next time there is some concern, people will push to use the same shortcuts. Several times from now, the shortcuts will be the norm. If the normal FDA approval process is necessary, it should be followed EVERY time. If it isn't absolutely necessary, then it should be modified for all approvals.