WASHINGTON (NYTIMES) - The Centres for Disease Control and Prevention is looking into reports that a very small number of teenagers and young adults vaccinated against the coronavirus may have experienced heart problems, according to the American agency's vaccine safety group.

The statement was sparse in details, saying only that there were "relatively few" cases and that they may be entirely unrelated to the inoculation. The condition, called myocarditis, is an inflammation of the heart muscle that can occur following certain infections.

The CDC's review of the reports is in the early stages, and it has yet to determine whether there is any evidence that the vaccines caused the condition. It has posted some guidance on its website for doctors and clinicians to be alert to unusual heart symptoms among young people who have just received their shots.

"It may simply be a coincidence that some people are developing myocarditis after vaccination," said Dr Celine Gounder, an infectious disease specialist at Bellevue Hospital in New York.

"It's more likely for something like that to happen by chance, because so many people are getting vaccinated right now."

The cases seem to have occurred predominantly in adolescents and young adults about four days after their second dose of one of the mRNA vaccines, which are made by Moderna and Pfizer-BioNTech. And the cases were more common in males.

"Most cases appear to be mild, and follow-up of cases is ongoing," the vaccine safety group said. The CDC strongly recommends shots for Americans aged 12 and older.

"We look forward to seeing more data about these cases, so we can better understand if they are related to the vaccine or if they are coincidental," said Dr Yvonne Maldonado, chair of the American Academy of Paediatrics' committee on infectious diseases. "Meanwhile, it's important for paediatricians and other clinicians to report any health concerns that arise after vaccination."

Experts emphasised that the potentially rare side effect of myocarditis paled in comparison to the potential risks of Covid-19, including the persistent syndrome called "long Covid-19". Acute Covid-19 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 those who are hospitalised for flu in an average year, according to data collected by the American Academy of Paediatrics.

About 300 children have died of Covid-19 in the United States, making it one of the top 10 causes of death in children since the pandemic began.

"And that's in the context of all the mitigation measures taken," said Dr Jeremy Faust, an emergency medicine physician at Brigham and Women's Hospital in Boston.

In the general population, about 10 to 20 of every 100,000 people each year develop myocarditis, or an inflammation of the heart, experiencing symptoms ranging from fatigue and chest pain to arrhythmias and cardiac arrest. Many others likely have mild symptoms and are never diagnosed, according to researchers.

At the moment, the number of cases of myocarditis reported after vaccination does not appear to be greater than would normally be seen in young people, according to the CDC.

But members of the agency's vaccine safety group "felt that information about reports of myocarditis should be communicated to providers", the report said.

The agency did not specify the ages of the patients involved.

The Pfizer-BioNTech vaccine has been authorised for ages 16 and above since December. Earlier this month, the Food and Drug Administration extended that okay to children aged 12 to 15.

On May 14, the CDC alerted clinicians to the possible link between myocarditis and vaccines. And on May 17, the working group reviewed data on myocarditis from the Department of Defence, reports filed with 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 the journal Paediatrics for review.

Dr Liam Yore, past president of the Washington state chapter of the American College of Emergency Physicians, said in an interview that he had seen a teenager with myocarditis after vaccination.

The patient was treated for mild inflammation of the lining of the heart, and sent home. But the teenager later returned for care with a decrease in the heart's output. Still, Dr Yore said he had seen worse outcomes in youngsters with Covid-19, including a nine-year-old who arrived at the hospital following a cardiac arrest last winter.

"The relative risk is a lot in favour of getting the vaccine, especially considering how many doses... have been administered," he added.

More than 161 million people in the US have received at least one dose, with about 4.5 million of them aged between 12 and 18.