Sure they do.
”We did a systematic review and meta-analysis, searching four international databases from Jan 1, 1947, to Dec 31, 2021, for studies in English reporting on the incidence of myopericarditis following vaccination (the primary outcome). We included studies reporting on people in the general population who had myopericarditis in temporal relation to receiving vaccines, and excluded studies on a specific subpopulation of patients, non-human studies, and studies in which the number of doses was not reported.”
There are links to all of the studies at the end of the paper.
Yes, I read that part and it certainly explains how they got the rate for previous vaccinations. It doesn’t specifically say that is how they got their rate for COVID vaccinations though.
If that is the only clue they give, then I wonder, how many studies exactly were published before December 2021 on the rate of myo and pericarditis post-COVID vaccination, and which met their criteria?