Sedentary lifestyle increases the risk of hospitalization for COVID-19 independently of other factors. There is enough statistics to show that exercise prevents severe forms of COVID-19, but current recommendations do not set an upper limit for exercise intensity. The hypothesis presented in the paper states that intense exercise, through blood hypoxia, increases the expression of transmembrane angiotensin-converting enzyme 2 (tACE2) in the vascular endothelium, increasing the risk of developing serious forms of disease, especially in the untrained. On the other hand, moderate-intensity exercise increases the blood concentration of soluble angiotensin-converting enzyme 2 (ACE2) which has a protective role for SARS-CoV-2 infection and may prevent complications. The importance of this hypothesis consists in the revision of COVID-19 prophylaxis programs through physical exercises, with the possibility of administration of antioxidants to speed up the adaptation of vascular endothelial cells to exertion.