The group’s statement was sparse in details, saying only that there were “relatively few” cases and that they may be entirely unrelated to vaccination. The condition, called myocarditis, is an inflammation of the heart muscle, and can occur following certain infections. The C.D.C.’s review of the reports is in the early stages, and the agency has yet to determine whether there is any evidence that the vaccines caused the heart condition. It has posted some guidance on its website for doctors and clinicians to be alert to unusual heart symptoms among young people who had just received their shots. “It may simply be a coincidence that some people are developing myocarditis after vaccination,” said Dr. Celine Gounder, an infectious disease specialist at Bellevue Hospital Center in New York. “It’s more likely for something like that to happen by chance, because so many people are getting vaccinated right now.”