Science Photo Library / IAN HOOTON

The Centers for Disease Control (CDC) is investigating cases of heart inflammation in children and teenagers who got the Covid-19 mRNA vaccine.

Myocarditis is inflammation of the heart muscles, which affects the heart's electrical system that tells the heart to beat regularly.

Myocarditis is usually caused by a virus, such as Covid-19. The symptoms include irregular heart beat, chest pain, fatigue (extreme tiredness), fluid retention (swelling in legs), and shortness of breath.

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The CDC says doctors have not established a link between the mRNA vaccines and reports of myocarditis in children who have received the injections.

The CDC urged people to still get vaccinated despite the troubling reports.

Doctors say the condition is "mild" and often goes away on its own.

"Reported cases appear to be mild and often go away without requiring treatment," said CDC spokesman Benjamin Haynes. He said that the reports are "rare given the number of vaccine doses administered."

As of Monday, nearly half of the U.S. population received at least one dose of the mRNA vaccine.