It's unclear to me (and apparently to the people doing this research - they state they haven't been able to figure that out yet). It is clear, though, that not

It's unclear to me (and apparently to the people doing this research - they state they haven't been able to figure that out yet). It is clear, though, that not everyone gets covid, but (in Israel) essentially everyone gets the vaccine (>80% of age>16 already), so a vaccine must be significantly safer than the disease it prevents. Furthermore, the most worrying signal they remark on is that covid myocarditis seems to increase with older age, whereas post-vaccine myocarditis seems to increase with lower age. I've read a hypothesis that bits of free vaccine mRNA migrate in the blood, and if taken by heart muscle cells, will cause the immune system to potentially attack those heart cells when they actually express the S protein -- especially if there is some prior immunity (previous infection or cross immunity). I don't know enough to judge its likelihood myself. Only thing I take from reading this report is that the vaccine safety is not to be taken for granted the way it has been so far with the massive deployment in Israel.