June 24 (UPI) -- The U.S. Centers for Disease Control and Prevention said more than 1,200 Americans, mostly under the age of 30, have developed a rare heart inflammation after receiving either the Pfizer-BioNTech or Moderna COVID-19 vaccine.
During a meeting of the CDC's Advisory Committee on Immunization Particles on Wednesday, health officials presented slides that said as of June 11 a total of 1,226 cases of myocarditis or pericarditis have been reported, with the CDC confirming 323 cases among those below the age of 29.
Of those cases, 309 were hospitalized, with 295 having been discharged as of June 11, it said.
Myocarditis is inflammation of the heart muscles while pericarditis is inflammation of the heart's outer lining, the CDC said, adding that both conditions are caused by the body's immune system reacting to an infection or another trigger.
Risk factors for the disease in those who have received either of the mRNA vaccines include being younger, generally below the age of 16, and being male, it said, with the highest frequency of the disease developing a few days after receiving the second dose of an mRNA vaccine.
"Early data of acute outcomes of myocarditis after mRNA vaccines have been good," a second set of slides from the meeting said.
According to the CDC website, the new mRNA vaccines "teach our cells how to make a protein ... that triggers an immune response," which is different from traditional vaccines that put a weakened or inactive germ into the body to bring about the immune response.
The CDC said of all reported cases, 267 instances of the rare heart condition occurred after the first dose, 827 after the second dose and 132 were unknown.
Leading healthcare professionals emphasized on Wednesday, however, that the chances of contracting the disease after receiving either shot is rare, and the benefits outweigh the risks as most cases are mild and heart inflammation symptoms are more severe in those who contract COVID-19.
"The facts are clear: this is an extremely rare side effect, and only an exceedingly small number of people will experience it after vaccination," a joint statement from the CDC, the Department of Health and Human Services and several other leading doctors, nurses and public healthcare organizations said.
"Importantly, for young people who do, most cases are mild and individuals recover often on their own or with minimal treatment. In addition, we know that myocarditis and pericarditis are much more common if you get COVID-19, and the risks to the heart from COVID-19 infection can be more severe."