SARS-CoV-2—the virus responsible for COVID-19—is a respiratory virus. When first discovered, the virus was thought to mostly affect the lungs. We now know that it affects many different organs in the body, including the heart.

All respiratory viruses, including the seasonal flu, lead to a higher risk of cardiovascular disease. But SARS-CoV-2 appears to be especially bad for the heart. Some studies have found that 20 to 30% of people hospitalized with COVID-19 had signs of heart damage. Others reported that almost 70% of hospitalized COVID-19 patients had abnormal heart function.

Researchers have found a few ways that SARS-CoV-2 can cause such damage to the heart. Severe pneumonia, which is common with COVID-19, forces the heart to work harder. This puts the heart muscle under stress and can lead to a heart attack, particularly if the person already has heart disease.

An overreaction of the immune system can also play a part. Despite playing an essential role in responding to a viral infection, sometimes small molecules released by the immune cells (called cytokines) get out of control and start to attack other parts of the body, including the heart. This exaggerated response—called a cytokine storm—can cause inflammation of the heart muscle (myocarditis). Myocarditis can lead to permanent damage and possibly heart failure.

Too much inflammation can also affect the blood vessels and increase the risk of developing blood clots, which can lead to heart attack. Inflammation can also worsen or trigger irregular heartbeats in some people.

The cause of myocarditis in people with COVID-19, including some who recovered at home and had no other signs of heart disease, is not known. It is possible that even though they had less severe disease, inflammation caused their heart damage. Another possibility is that the virus directly injures the heart muscle cells. Research has shown that 60% of people in one study who had COVID-19 and died of pneumonia had signs of the virus in the heart and possible signs of myocarditis. Myocarditis caused by COVID-19 has also been found in up to 15% of college football players who tested positive for the virus.

Although we still have a lot to learn about the long-term effects of COVID-19, it is becoming increasingly clear that one important part in the recovery process will be managing these heart injuries.

John Chatham, DPhil, FAPS, is a professor of pathology in the Division of Molecular and Cellular Pathology at the University of Alabama at Birmingham.