The CDC is reviewing a few cases of myocarditis, or heart muscle inflammation, among teenagers and young adults who were vaccinated against COVID-19, according to a May 17 update from the agency's Advisory Committee on Immunization Practices.
The group said there's been "relatively few" reports of the heart condition, but did not provide an exact number. Cases seem to occur predominantly in adolescents and young adults, mostly in males, typically about four days after vaccination, and seem to be more common after the second dose than the first, according to the statement.
Most myocarditis cases have been mild, and follow-up is ongoing, the CDC said, adding that the number of reports has not been more than would be expected among the general population.
The CDC has not determined whether there is a causal relationship between the heart condition and the vaccine, and further study is needed, the statement said.
"It may simply be a coincidence that some people are developing myocarditis after vaccination," Celine Gounder, MD, infectious disease specialist at New York City-based Bellevue Hospital Center, told The New York Times. "It's more likely for something like that to happen by chance, because so many people are getting vaccinated right now."
While there's only been a few cases, the agency's vaccine safety committee "felt that information about reports of myocarditis should be communicated to providers."
The cases occurred following mRNA vaccines, which are made by Pfizer-BioNTech and Moderna, though the committee did not specify the ages of those who experienced myocarditis. Pfizer's vaccine is approved for use among those aged 12 and older, and Moderna's can be administered to people aged 18 and older.
In response to the reports, the American Heart Association said it continues to recommend the vaccine among everyone ages 12 and older in a May 23 statement.
"Myocarditis is usually the result of a viral infection, and it is yet to be determined if these cases have any correlation to receiving a COVID-19 vaccine, especially since the COVID-19 vaccines authorized in the U.S. do not contain any live virus," AHA leaders wrote. "We remain confident that the benefits of vaccination far exceed the very small, rare risks. The risks of vaccination are also far smaller than the risks of COVID-19 infection itself."
Deirdre Gifford, MD, Connecticut's acting health commissioner, said 18 teens and young adults in the state had been hospitalized with symptoms after being vaccinated, NBC Connecticut reported May 24. Most of the patients were hospitalized for a few days and have since been released, Dr. Gifford said, adding that one remained hospitalized as of May 24. Four of these patients were seen at Yale New Haven Health, local CBS affiliate WFSB reports.
"We haven't seen anybody who required a longer-term hospitalization or any sort of critical care, like what you might think of as things like ventilators and blood pressure support and heart medications,” Jeremy Asnes, MD, co-director of Yale New Haven Children’s Hospital Heart Center, told the station.