A CDC safety panel on Wednesday said there's a "likely association" in adolescents and young adults between receiving mRNA Covid-19 vaccines and developing myocarditis—heart inflammation—and pericarditis—inflammation of the membrane around the heart.

Officials emphasized, however, that the cases are rare and typically mild, and they said the benefits of the vaccines far outweigh the risks.

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The association between heart inflammation and vaccines

CDC said it is currently tracking around 1,200 reports of heart inflammation following doses of mRNA vaccines, although the cases have not definitively been linked to the vaccines.

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Specifically, CDC said that as of June 11, it has received 1,226 reports of cases of myocarditis or pericarditis after receiving an mRNA vaccine. Of those, 267 reported cases followed a first dose of a vaccine, with a median patient age of 30 years, and 827 followed a second dose, with a median age of 24 years. (For the remaining 132 reports, it wasn't known which dose the case followed.)

The median time to symptom onset was three to four days, and significantly more cases occurred in males than in females.

CDC also conducted a deeper analysis of 323 cases that met its working case definition of myocarditis or pericarditis in people under the age of 29. Of those, 218 people fully recovered, nine were hospitalized, and two of those nine required intensive care, CDC said.

CDC emphasized that heart inflammation cases following vaccination are very rare, with only about 12.6 cases of heart inflammation per one million vaccine doses. CDC found that around 20 cases occurred per million doses of Moderna's Covid-19 vaccine, while about eight cases occurred per million doses of Pfizer-BioNTech's vaccine.

CDC added that the majority of heart inflammation cases have been mild, with symptoms including chest pain, fatigue, and heart rhythm disturbances that tend to resolve within a day or so.

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"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," Grace Lee, chair of CDC's vaccine safety committee, said.

According to Sarah Long, a member of CDC's panel, post-vaccine myocarditis presents itself differently than other types of the condition.

While post-vaccine myocarditis may present quickly with relatively severe symptoms, typical myocarditis is "a little more subtle in onset" and slower to resolve. "Traditional myocarditis is not (multi-inflammatory syndrome in children (MIS-C)), and it's not this," she said. As such, she added, the "[c]hances that this … is related to these vaccines … [are] very highly likely."

FDA to add warning to fact sheets on mRNA vaccines

In response to CDC's findings, an FDA official said the agency will "move rapidly" to add a warning to fact sheets on Pfizer-BioNTech's and Moderna's Covid-19 vaccines regarding the rare risk of heart inflammation.

The warning will advise that anyone experiencing symptoms related to myocarditis or pericarditis "seek medical attention" and will state that information on the long-term effects of the condition is limited.

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"Based on the available data, a warning statement in the fact sheets for both health care providers and vaccine recipients and caregivers would be warranted in this situation," Doran Fink, deputy director of FDA's vaccines division, said.

Officials and experts emphasize the benefits of Covid-19 vaccines

In response to the announcement, CDC and HHS, along with 15 medical and public health organizations—including the American Academy of Pediatrics, American College of Obstetricians and Gynecologists, the American Medical Association, and the American Nurses Association—issued a joint statement saying they "strongly encourage everyone 12 and older" to get vaccinated, as the benefits of the vaccines outweigh the risks.

"The facts are clear: This is an extremely rare side effect, and only an exceedingly small number of people will experience it after vaccination. Importantly, for the young people who do, most cases are mild, and individuals recover often on their own or with minimal treatment," the statement said.

"The choice to avoid an mRNA vaccine in order to avoid myocarditis ignores the fact that both Covid and MIS-C cause myocarditis, and far more commonly," Paul Offit, a vaccine expert at the Children's Hospital of Philadelphia and member of FDA's vaccine safety committee, said. "There are no risk-free choices."

"These events are really very rare, extremely rare," Brian Feingold, an expert on heart inflammation in children at the UPMC Children's Hospital of Pittsburgh, said. "That needs to be taken in context with illness and morbidity and mortality related to Covid."

Feingold added that more than 4,000 children with Covid-19 have developed multisystem inflammatory syndrome, which included symptoms related to the heart. "You can say no to the vaccine, but you're assuming other risks" (Weixel, The Hill, 6/23 [1]; Sun, Washington Post, 6/23; Mandavilli, New York Times, 6/23; Carbajal, Becker's Hospital Review, 6/23; Joseph, STAT News, 6/23; Fernandez, Axios, 6/23; Weise, USA Today, 6/23; Gardner, Politico, 6/23; Walker, MedPage Today, 6/23; Weixel, The Hill, 6/23 [2]).