I only have time to checkout out your first article. It seems plausible the KPN folks might have caught a tail-end effect of myocarditis in Covid vaccination. However the Kaiser numbers don’t appear that different or concerning to me as an educated lay person. They’re not an order of magnitude higher AFAICT without re-normalizing the units used in the TFA and the KPN paper. AFAICT from the error bars on table 2 in the KPN paper they have pretty wide error bars and CI. That makes sense given that even with on the order of a 100k+ vaccinations you only get 16 cases occurring.

TFA presents their data in a form seemingly normalized to 100,000 person-days not just events-per dose. This would potentially also capture much of the long tail effect. Though I’m not well versed in the odds ratio statics most common in these sort of medical studies.

Now that being said I am actually somewhat concerned of the lifetime cumulative risk of specifically Covid-19 vaccinations. Particularly if it becomes standard practice to get a new booster every 6 months for the rest of your life. Luckily that practice seems to be ebbing as Covid-19 has become endemic. TFA reports higher rates after the second dose, and excluded booster shots AFAICT. It’s possible booster doses could present the same or higher risk of myocarditis in particular if you’re targeting the exact same spike protein. Some quick back of the napkin calculations show a lifetime risk approaching the fatality risk of say omicron of you did get a dose every 6 months, and the risk was constant. Both of those conditions seem unlikely. For example susceptible people would likely be “weeded out” and the myocarditis risk would drop, and it’s doesn’t seem likely people will gets bi-annual boosters. Also the dosing of mRNA vaccines will likely get better and methods for treating rare events like myocarditis will improve.