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That’s because myocarditis can lead to heart failure. When that happens, some people don’t get any advance warnings, which include chest discomfort, heart palpitations, shortness of breath, lightheadedness, swelling and fatigue. And the most serious outcome — sudden cardiac arrest — can also occur without symptoms. Before the pandemic, some 400,000 adults in the U.S. died of sudden cardiac arrest each year. It’s unclear how often myocarditis is the cause; estimates range from 1 percent over all to as many as 20 percent in young adults.

Even if myocarditis turns out to be a common feature of Covid, we won’t know how much it increases the total number of those affected by the condition. The definitive way to diagnose it is through a biopsy of heart tissue, but unless people show symptoms, they aren’t usually screened for myocarditis, which is typically caused by viruses, including influenza. When detected cases result in reduced cardiac function, about half the time the heart returns to normal on its own, even if scarring remains, according to Clyde Yancy, chief of the division of cardiology at the Northwestern University Feinberg School of Medicine and deputy editor of JAMA Cardiology. When it comes to cases caused by Covid, he says, “the hope is that it resolves spontaneously, which happens in many other circumstances where a virus has affected the heart.”

To find out if it does, researchers must follow people who get myocarditis diagnoses to see if they suffer complications. Baggish, Drezner and others have formed a national registry to track Covid cases and heart-related impacts in N.C.A.A. athletes to gather that kind of data; this effort might lead to improvements in prevention and screening for myocarditis in college sports. (When the Big Ten announced on Sept. 16 that its football teams would return to play, it noted that players who test positive for the virus would have to undergo “comprehensive cardiac testing.”) And Eike Nagel, a senior author of the July JAMA study and director of the Institute for Experimental and Translational Cardiovascular Imaging at University Hospital Frankfurt, in Germany, says the group will be assessing the study’s patients for several years. Even if the observed abnormalities increase an individual’s risk of heart failure only slightly, Nagel says, that outcome could forecast a significant cumulative impact on public health: “On the population level, I would think we’d see an increased incidence of heart failure in a couple of years. But we don’t have the data.”

It’s also possible that fear — of getting Covid, of exercise after recovering from it — could cause its own widespread harm. In July, in a letter to The New England Journal of Medicine, Italian researchers reported a 60 percent increase in out-of-hospital cardiac deaths compared with the previous year in four provinces hit with coronavirus outbreaks. It’s impossible to say, though, whether that change resulted from Covid infections or because people avoided getting medical care because they feared contracting the virus (or whether both — or other — factors played a part).



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