Let's just say, for the sake of argument, that it was not a metaphysical impossibility that some unknown issue with the novel technology applied in these vaccines was missed in the severely abbreviated clinical trials. The control groups were all vaccinated the second the EUAs were granted, so we have no controls against which we can check our hypotheses. In the (I know, completely impossible) hypothetical I am putting forth, wouldn't the first hints of the problem be something like unexplained excess deaths coinciding with vaccine rollout?