The first research in the U.S. is underway, tracking adverse health effects if any " that may appear in the years following a diagnosis of vaccine-associated heart problems. Early findings from the research could be published as early as next year, sources told NBC News. In October 2021, Da'Vion Miller was found unconscious in the bathroom of his home in Detroit a week after receiving his first dose of Pfizer's Covid vaccine. Miller was rushed to Henry Ford West Bloomfield Hospital, where he was diagnosed with myocarditis, an inflammation of the heart muscle, and pericarditis, an inflammation of the outer lining of the heart. His doctor advised him not to receive a second dose of either the Pfizer or the Moderna vaccines. In some cases, people who've developed myocarditis after a viral infection can suffer scarring along the heart's tissue, reducing its ability to pump blood and circulate oxygen around the body, said Dr. Leslie Cooper, the chair of the department of cardiology at the Mayo Clinic. "It could be 2%. It could be 0%. It could be 20%," he said, referring to the percentage of people with vaccine-associated myocarditis who could experience long-term heart consequences. "We don't know the answer." Scientists still don't have a clear explanation yet for why the vaccines cause the condition, according to Dr. Paul Burton, Moderna's chief medical officer. He expects the virus's spike protein, once produced in the cell after vaccination, may generate a reaction in the body that can cause inflammation in the heart.