Stanford Medicine investigators have unearthed the biological process by which mRNA-based vaccines for COVID-19 can cause heart damage in some young men and adolescents — and they’ve shown a possible route to reducing its likelihood. Using advanced but now common lab technologies, along with published data from vaccinated individuals, the researchers identified a two-step sequence in which these vaccines activate a certain type of immune cell, in turn riling up another type of immune cell. The resulting inflammatory activity directly injures heart muscle cells, while triggering further inflammatory damage. The mRNA vaccines for COVID-19, which have now been administered several...