US CDC is investigating reports on a few American teenagers appearing cardiovascular problems after vaccination of Covid-19 vaccines.

The US Department of Disease Control and Prevention (CDC) is not detailed, but only a few cases of muscleitis in teenagers have been injected with Covid-19 vaccines.

"Maybe this is just a coincidence, when some people are developing the heart of myocarditis after vaccination, Celine Gounder, infectious disease expert in the center of Bellevue Hospital in New York, said.

A teenager was vaccinated with Vaccine Covid-19 in Philadelphia, Pennsylvania in May 18.

Cases mainly occur in teenagers four days after they inject the second vaccine dose.

CDC has posted instructions on the agency's website so that doctors wary of abnormal heart symptoms in young people who have been vaccinated.

As of May 13, more than 3.9 million children in the US infected with Covid-19, of which more than 16,000 cases were hospitalized.

More than 160 million Americans have injected at least one dose of Vaccine Covid-19, of which about 4.5 million people aged 12-18 years old.