A Centers for Disease Control and Prevention (CDC) safety panel said there is a "likely association" of mild heart inflammation in adolescents and young adults after they were vaccinated with an mRNA COVID-19 vaccine.
The initial cases of myocarditis, inflammation of the heart muscle, and pericarditis, inflammation of the membrane surrounding the heart, reported on the federal government's tracking system were generally mild, especially compared to traditional myocarditis, scientists said.
Most cases have been mild, with symptoms like fatigue, chest pain and disturbances in heart rhythm that quickly clear up within a day or so. CDC scientists said they will need to follow up with patients in the months ahead in order to get a complete picture of the impact.
"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 Grace Lee, chairwoman of the CDC's vaccine safety committee.
Officials said they are tracking about 1,200 initial reports of the rare heart inflammation following doses of mRNA coronavirus vaccines have been filed with the federal government's Vaccine Adverse Event Reporting System (VAERS), though they have not yet been definitively linked to the vaccines.
Most reports came from people in their late teens and early 20s, and many more occurred after the second dose than the first.
There were more cases in males than females, and the cases essentially disappeared in older age groups.
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.
But the reports are preliminary, and do not mean the health issues have been linked to the vaccine. The database is meant as a repository of all events observed after vaccination.
There were 323 confirmed reports of myocarditis and pericarditis for people under the age of 29, which is the group CDC is investigating. Among those confirmed, 218 people have fully recovered. Nine people were hospitalized, with two in intensive care as of June 11, according to the CDC.
There have been about 300 million vaccine doses administered nationwide.
Scientists have emphasized this occurrence is rate — for both mRNA vaccines combined, there were 12.6 heart inflammation cases per million doses.
The highest confirmed rate of myocarditis and pericarditis was about 20 cases per 1 million doses with Moderna's vaccine, compared to 8 cases per million for Pfizer’s.
Officials emphasized that the benefits of vaccines outweigh the risks, and noted that for every million doses of mRNA vaccine given, there are far more COVID-19 cases and hospitalizations prevented compared to the number of potential myocarditis cases.