Given that it is well documented that the vaccine reduces the severity of the disease I would also expect it to modify the risk of myocarditus in some way (I assume that it reduces the risk)... That further complicates the picture...

P(covid | no_vaccine) * covid_myocarditus_risk(no_vaccine)

vs

P(covid | vaccine) * covid_myocarditus_risk(vaccine) + vaccine_myocarditus_risk