The agency is investigating after more than a dozen people who were inoculated developed myocarditis -- an inflammation of the heart muscle. The condition often goes away on its own and can be caused by a variety of viruses, the CDC said.
"We do know that myocarditis can occur, and it can occur following a viral infection," said Dr. Richard Besser, president and CEO of Robert Wood Johnson Foundation. "So the question they want to see is are they certain there's no connection here. And so far, there's no signals that they're saying that raise that alarm."
The number of cases isn't more than what health officials would expect to see among the same millions of people who haven't been vaccinated, but scientists at the CDC said the issue needs to be investigated.
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"The CDC issued this notice out of an abundance of caution. They're trying to investigate to see if they're related at all to the vaccine," said Dr. Yvonne Maldonado, chair of the American Academy of Pediatric's Committee on Infectious Diseases. "At this point, there is no relationship proven."
The CDC reports myocarditis seems to occur more often in teens, young adults, and men more than women.
The U.S. Department of Defense is looking into 14 cases of the condition in service members who took the Pfizer or Moderna vaccines.
The cases are minimal compared to the nearly 128 million people who are fully vaccinated in the U.S. and not reported any issues.
So far, about 1 in 5.3 million people who have received the vaccine reported heart inflammation. Meanwhile, the odds of being struck by lightning are 1 in 700,000.
Doctors underlined that the benefits of immunization outweigh the small risks.