This is a case study of myocarditis after an influenza vaccine. This single data point, or even the fact that this is a possible immune reaction doesn't really

This is a case study of myocarditis after an influenza vaccine. This single data point, or even the fact that this is a possible immune reaction doesn't really speak to the issue I'm asking about. In the last few years a slew of COVID vaccines were widely deployed without myocarditis as a complication. The Pfizer and Moderna vaccines made headline news for their elevated risk of myocarditis, and resulted in changes in vaccine recommendations for certain demographics. Both were mRNA based. Do they have other things in common, dosage, proteins, preservatives, shelf lives? What else is similar between them and different from the other vaccines?