Also note this observation from a July 2020 article in The International Journal of Clinical and Experimental Medicine:
The practice of physical activities strengthens the immune system, suggesting a benefit in the response to viral communicable diseases. Thus, regular practice of adequate intensity is suggested an as auxiliary tool in strengthening and preparing the immune system for COVID-19.
An August 2020 article in Diabetes & Metabolic Syndrome likewise remarked:
This review found that increasing the aerobic capacity could produce short-term safe improvements in the function of immune and respiratory systems, particularly those specific for COVID-19 infections.