I’m also aware that the standard approaches to verify safety and efficacy have been used and safety was never a concern. A quibble: Phase IV "post-marketing" i

I’m also aware that the standard approaches to verify safety and efficacy have been used and safety was never a concern. A quibble: Phase IV "post-marketing" is where you find the really rare adverse effects that you can't find in Phase III trials, which for the FDA authorized and approved vaccines were as big as they get, but still have a maximum of 23,000 people getting the vaccine. So that rare brain blood clotting problem with Janssen's vaccine actually had one case of it in its clinical trial, but that wasn't enough signal to detect it as a problem. But after 6 million or so people got it in the US there were enough cases reported to the CDC's Vaccine Adverse Event Reporting System (VAERS), a handful or so it could be detected and addressed. Same sort of thing happened with the heart and heart lining inflammation for the mRNA vaccines. But as you say, these are the standard approaches, no extra length pre-marketing testing would have discovered them based on how vaccines work, and for drugs roughly the same thing happens as you discover there are some people who just don't tolerate them. For more on this, look into "black box" warnings that are regularly added to the prescribing information for doctors from Phase IV surveillance.