Roughly 300 million of the shots had been administered as of June 11, the agency said.

For both vaccines combined, there were 12.6 heart inflammation cases per million doses.

A CDC safety group said there's a "likely association" between a rare heart inflammatory condition in adolescents and young adults mostly after they've received their second Covid-19 vaccine shot, citing the most recent data available.

There have been more than 1,200 cases of a myocarditis or pericarditis mostly in people 30 and under who received Pfizer's or Moderna's Covid vaccine, according to a series of slide presentations published Wednesday for a meeting of the Centers for Disease Control and Prevention's Advisory Committee on Immunization Practices.

Myocarditis is the inflammation of the heart muscle, while pericarditis is the inflammation of the membrane surrounding the heart.