More than 300 cases of heart inflammation after Covid-19 vaccinations have been reported in young people, CDC Director Dr. Rochelle Walensky said Thursday. "The cases are rare," she said. "Over 20 million adolescents and young adults [have been] vaccinated in the United States." Still, the cases of either myocarditis or pericarditis, which involve inflammation of the heart or the surrounding tissue, are higher than what would be expected for this age group.