There have been a lot of athletes having weird cardiac issues ever since last spring, long before the vaccines were available. For one of many easy examples, Eduardo Rodriguez missed almost the entirety of the 2020 season (i.e., pre-vaccines) with myocarditis that developed right after he had COVID. Seeing as this thread was apparently started by sombody who follows MMA, Khamzat Chimaev was hospitalized with severe COVID last winter, also before vaccines were available, and was out of commission for several months. Unlike the subject of the original discussion, Chimaev is 24 years old and is clearly one of the fittest humans on earth.