Still, the benefits of immunization greatly outweighed the risks, and advisers to the C.D.C. strongly recommended vaccination for all Americans 12 and older. The heart problems reported are myocarditis, an inflammation of the heart muscle; and pericarditis, inflammation of the lining around the heart. The risk is higher after the second dose of an mRNA vaccine than after the first, the researchers reported, and much higher in men than in women. But overall, the side effect is very uncommon — just 12.6 cases per million second doses administered. The researchers estimated that out of a million second doses given to boys ages 12 to 17, the vaccines might cause a maximum of 70 myocarditis cases, but would prevent 5,700 infections, 2,215 hospitalizations and two deaths.