The FDA has updated labels on the top two COVID-19 vaccines to warn of a rare heart side effect, mostly seen in young men. Here's what to know.

Myocarditis, when the heart muscle gets inflamed, can make it harder for the heart to pump blood possibly causing chest pain, shortness of breath and irregular heartbeats. Pericarditis, when the thin lining around the heart (called the pericardium) becomes inflamed, can also cause sharp chest pain that feels worse when lying down.

U.S. health officials have expanded warnings about a rare heart side effect associated with the two leading COVID-19 vaccines.