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.

“Clinical presentation of myocarditis cases following vaccination has been distinct, occurring most often within one week after dose two, with chest pain as the most common presentation,” said Dr. Grace Lee, who chairs the committee’s safety group. CDC officials are gathering more data to fully understand the potential risks, how to manage it and whether there are any long-term issues, she said.

The agency said there have been 267 cases of myocarditis or pericarditis reported after receiving one dose of the mRNA vaccines and 827 reported cases after two doses through June 11. There are 132 additional cases where the number of doses received is unknown, the CDC said.

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

“This is still a rare event,” Dr. Tom Shimabukuro said at the meeting. For both vaccines combined, there were 12.6 heart inflammation cases per million doses. The cases were more frequent among Moderna’s vaccine recipients at 19.8 cases per million versus eight cases per million for Pfizer’s, he said.