COVID-19 and SARS virus both bind to the angiotensin converting enzyme 2 (ACE2) receptor of host cells through the receptor binding domain on the surface of the key component S protein; however, the affinity of COVID-19 surface protein to ACE2 of host cells is 10 to 20 times higher than that of the SARS virus [9]. Here, PROS1 is linked to COVID-19.