The Centers for Disease Control and Prevention is investigating reports that a very small number of teenagers and young adults vaccinated against the coronavirus may have had heart problems, according to the agency’s vaccine safety group.

The group’s statement was sparse in detail, saying only that there were “relatively few” cases and that they may be completely independent of vaccination. The condition known as myocarditis is inflammation of the heart muscle and can occur after certain infections.

The CDC’s review of the reports is in the early stages, and the agency has yet to determine if there is evidence that the vaccines caused the heart disease. The agency has published guidelines on its website urging doctors and clinicians to look out for unusual heart symptoms in young people who have just received their scans.

“It may just be a coincidence that some people develop myocarditis after vaccination,” said Dr. Celine Gounder, an infectious disease specialist at Bellevue Hospital Center in New York. “It’s more likely that something like this happened by accident because so many people are being vaccinated.”

The cases appear to have occurred predominantly in adolescents and young adults about four days after the second dose of one of the mRNA vaccines manufactured by Moderna and Pfizer-BioNTech. And the cases were more common in men than women.

“Most of the cases appear to be mild and the case follow-up is ongoing,” the vaccine safety group said. The CDC strongly recommends Covid vaccines for Americans 12 and older.

“We look forward to more data on these cases so that we can better understand whether they are vaccine-related or if they are accidental,” said Dr. Yvonne Maldonado, Chair of the Infectious Diseases Committee of the American Academy of Pediatrics. “In the meantime, it is important for pediatricians and other clinicians to report any health concerns that arise after vaccination.”

Experts pointed out that the potentially rare side effect of myocarditis pale in comparison to the potential risks of Covid, including the persistent syndrome called “Long Covid”. Acute Covid itself can cause myocarditis.

As of May 13, the coronavirus had infected more than 3.9 million children and sent more than 16,000 to hospitals, more than were hospitalized for flu in an average year. This is evident from data collected by the AAP. Approximately 300 children have died from Covid-19 in the United States, making it one of the top 10 causes of child death since the pandemic began.

“And that is related to all mitigation measures that have been taken,” said Dr. Jeremy Faust, emergency doctor at Brigham and Women’s Hospital in Boston.

Updated

May 23, 2021, 3:32 p.m. ET

In the general population, about 10 to 20 in 100,000 people develop myocarditis each year, with symptoms ranging from fatigue and chest pain to arrhythmias and cardiac arrest. Many others are likely to have mild symptoms and, according to researchers, never get diagnosed.

Currently, the number of post-vaccination reported cases of myocarditis does not appear to be any higher than is common among young people, according to the CDC. However, the agency’s vaccine safety group members felt that information on reports of myocarditis should be provided to providers, ”the report said.

The agency did not disclose the age of the affected patients. The Pfizer BioNTech vaccine has been approved for ages 16 and over since December. Earlier this month, the Food and Drug Administration extended this approval to children ages 12-15.

On May 14, the CDC alerted doctors to the possible link between myocarditis and vaccines. On May 17, the task force reviewed the Department of Defense’s data on myocarditis, reports submitted to the Vaccine Adverse Event Reporting System, and others.

State health departments in Washington, Oregon, and California have alerted emergency providers and cardiologists to the potential problem, and a report of seven cases has been submitted to Pediatrics magazine for review.

Dr. Liam Yore, former president of the Washington State Chapter of the American College of Emergency Physicians, said in an interview that he recently saw a teenager with myocarditis after the vaccination.

The patient was treated for a slight inflammation of the lining of the heart and then sent home. But the teenager later returned to care, with a decrease in cardiac output. Still, Dr. Yore, he’s seen worse results in teens with Covid, including a 9-year-old who arrived at the hospital after suffering cardiac arrest last winter.

“The relative risk is very favorable to receiving the vaccine, especially considering how many doses of the vaccine have been given,” he said.

More than 161 million people in the United States have received at least one dose of a coronavirus vaccine. About 4.5 million of them were between 12 and 18 years old.