Myocarditis and pericarditis, which mysteriously skew young and male, can arise from an array of triggers, including bacteria and fungi as well as medications and autoimmune disease, but many cases go entirely unexplained. There’s no curative, or even standard, treatment for either condition; doctors try to manage symptoms and tamp down inflammation, said John Jarcho, a cardiovascular-medicine specialist at Brigham and Women’s Hospital, in Boston. Cases related to vaccines are more puzzling still. Only a few immunizations have previously been linked to heart inflammation, among them the smallpox vaccine, which looks nothing like the jabs we’re doling out now. Researchers...