heck, I haven’t even seen short term comparisons on antibody response to 3 vs 4th shots. this info should be out there.
personally i worked around trial and pdufa fda data (non-medical professional) for years. you get a feel for how the fda is on trial design, adverse event follow-up, etc. it is quite clear to me the agency is making sure nothing adverse ever gets into the data on these (what little data we have).
as far as the usual animal then human trials over many years, “what could possibly go wrong?” is the famous quote, isn’t it?