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U.S. scientists said Wednesday that there was a “likely association” between mRNA Covid-19 vaccines and an elevated risk of heart issues in adolescents and young adults, the strongest statement yet on the link between the two.

The evidence presented Wednesday echoes what other experts and health officials in other countries have identified: that younger groups, particularly men under 30, have higher rates of myocarditis (inflammation of the heart muscle) and pericarditis (inflammation of the lining around the heart) following vaccination with the shots from Moderna or Pfizer-BioNTech. Most cases have occurred soon after the second shot of the two-dose regimens.

Overall, cases of both conditions appear to be a rare side effect of the vaccines, though the rates vary depending on sex and age, health officials said in presentations to the Centers for Disease Control and Prevention’s Advisory Committee on Immunization Practices. Most of the patients with confirmed cases were hospitalized, though most did not need serious care and have subsequently been discharged.


Officials said they are planning to add warnings about myocarditis and details about symptoms to Covid-19 vaccine fact sheets provided to administrators of the shots and people who get them. They also said they are still collecting and analyzing data on cases.

The focus on myocarditis comes as federal health agencies and their outside expert advisers continue to monitor for side effects of vaccines, particularly as they consider recommending the shots for younger populations in the coming months. Federal health agencies currently recommend Covid-19 vaccines for people 12 and older, and Sara Oliver of the CDC’s National Center for Immunization and Respiratory Diseases reiterated Wednesday that even with the risk of myocarditis, “the benefits still clearly outweigh the risks for Covid-19 vaccination in adolescents and young adults.”


As they weigh their vaccine recommendations, experts are also grappling with changing levels of Covid-19 risk depending on the amount of virus in communities, and with major differences geographically.

On top of protecting people from Covid-19, the vaccines have greatly suppressed transmission in the United States. The parts of the country that are experiencing concerning outbreaks at this point are generally those with higher rates of unvaccinated people.

Myocarditis has many causes, including, most commonly, viral infection — including from the SARS-CoV-2 coronavirus. Symptoms include chest pain and labored breathing. It generally resolves on its own, though people are often told not to exercise for a few months to ensure full recovery.

During the ACIP meeting Wednesday, Matthew Oster of the CDC’s Covid-19 Vaccine Task Force said early evidence indicates that the cases of myocarditis reported after Covid-19 vaccination appear to be milder generally than those that occur from other causes, though experts don’t have long-term data yet.

Federal health officials are using different surveillance systems that track vaccine side effects to try to establish how frequently the cases might be occurring, though by one measure, there are about a dozen reports of myocarditis and pericarditis per million shots of the second dose among people 12 to 39 years old, with higher rates among younger men in those age groups.

Health officials are still investigating to confirm reported cases, but the available data show clear patterns. Essentially, there are many more reported cases in male adolescents and younger men than older men and than women overall. There is also some sign of more cases than expected in female adolescents and young women, though not to the extent as in young males. At baseline, men, as well as adolescents and younger adults, have higher rates of myocarditis overall than other groups, and Oster said experts are trying to determine whether those differences in rates by age and sex are even more pronounced among the post-vaccination cases. 

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