Federal health officials said Wednesday there is a “likely association” between two coronavirus vaccines and increased risk of a rare heart condition in adolescents and young adults, the strongest assertion so far on the link between the two.

Data presented to advisers to the Centers for Disease Control and Prevention adds to recent findings, most notably from Israel, of rare cases of myocarditis — inflammation of the heart muscle — predominantly in males ages 12 to 39, who experience symptoms after the second dose of the Moderna or Pfizer-BioNTech vaccine.

Most cases have been mild and have taken place several days to a week after the second shot, officials said. Chest pain is the most common symptom. Patients generally recover from symptoms and do well.

There have been 1,226 reports of myocarditis out of about 300 million mRNA doses administered in the United States, as of June 11, according to Tom Shimabukuro, a CDC vaccine safety official. Of those, 267 were reported after the first dose and 827 after the second, and 132 reports did not indicate which dose.

Experts and health officials said the additional data needs to be understood in the broader context of risk: With virus variants increasing, and adolescents and young adults making up a greater percentage of covid-19 cases, unvaccinated teens and young adults are far more likely to contract the disease. Getting covid-19 puts someone at far greater risk of heart inflammation and other serious medical problems than the risk of getting myocarditis from vaccination, they said.

On May 10, the Food and Drug Administration authorized the Pfizer coronavirus vaccine for 12-to-15-year-olds. Here's what parents should know. (Joy Yi/The Washington Post)

The CDC and the Department of Health and Human Services, together with 15 of the country’s leading medical and public health organizations — including the American Academy of Pediatrics, the American College of Obstetricians and Gynecologists, the American Medical Association and the American Nurses Association — issued a joint statement after the meeting saying they “strongly encourage everyone 12 and older” to get the shots because the benefits far outweigh any potential harms.

“Especially with the troubling Delta variant increasingly circulating, and more readily impacting younger people, the risks of being unvaccinated are far greater than any rare side effects from the vaccines,” the statement said.

The CDC and the Food and Drug Administration plan to do a three-month follow-up of these cases, officials said. Both agencies are also updating their fact sheets for providers and patients to reflect the additional data about the condition.

“The choice to avoid an mRNA vaccine in order to avoid myocarditis ignores the fact that both covid and MIS-C [a rare inflammatory condition diagnosed in some children after covid infections] cause myocarditis, and far more commonly,” said Paul A. Offit, a vaccine expert at Children’s Hospital of Philadelphia. “There are no risk-free choices.”

The additional data on myocarditis is part of continued safety monitoring by federal health agencies as they consider recommending the coronavirus vaccines for younger children in coming months.

A presentation from the vaccine safety work group of the Advisory Committee on Immunization Practices noted the “likely association” of myocarditis with mRNA vaccination in adolescents and young adults.

In males 12 to 39, the risk of myocarditis after the second dose of any mRNA vaccine was 32 cases per million, or about 1 in 31,000, according to a CDC analysis of data from one of several vaccine safety monitoring systems. For females in that age group, there were 4.7 cases per million, or about 1 in 212,000.

By comparison, the estimated incidence of the rare inflammatory syndrome in children is about 1 in every 3,200 covid-19 infections — with 36 percent of cases reported in those ages 12 to 20, according to CDC data. More than 4,000 cases of MIS-C have been reported since the pandemic began.

Treatment for myocarditis is largely supportive care. CDC officials said individuals should follow the guidelines of the American Heart Association and the American College of Cardiology, which recommend “restriction from competitive sports for about three to six months or until you can show documentation that the heart has recovered from this acute process,” said Matthew Oster, a CDC physician.

Some experts noted that the evolving data and unknowns make it harder to answer questions from anxious parents.

We worry a little bit about, are we going to make the community nervous, or have them be more hesitant to vaccinate?” said Patricia Stinchfield, director of infectious-disease control at Children’s Minnesota, a liaison from the National Association of Pediatric Nurse Practitioners.

But, she added, the discussion “allows us to have conversations. … And the parents that I have talked to, which are numerous about this, are very appreciative of that. And [they] do go ahead and vaccinate and are very, very happy that we’re doing this kind of deep analysis, even on rare events.”

Now that many older adults have been vaccinated, adolescents and young adults — those 12 to 29 — have the highest incidence of covid-19, according to CDC data. Since the beginning of the pandemic, at least 7.7 million covid cases have been reported in this age group — with 2,767 covid-19 deaths.

Of those deaths, 316 have been reported since April 1.

In a risk-benefit analysis, CDC officials found that for every million second doses of mRNA vaccinations given to females 18 to 24, vaccinations would prevent:

º 14,000 covid-19 cases.

º 1,127 hospitalizations.

º 93 ICU admissions.

º 13 deaths.

They also would result in four to five myocarditis cases.

For every million second doses given to males 18 to 24, vaccinations would prevent:

º 12,000 covid-19 cases.

º 530 hospitalizations.

º 127 ICU admissions.

º Three deaths.

They also would result in 45 to 56 myocarditis cases.