PROS1 and infection: Previously, a study demonstrated that the infection rate of SARS-CoV-1 was increased by porcine pancreas-derived elastase [6], NE and CatG levels were increased in nasopharyngeal swabs of SARS-CoV-2 patients in comparison to the control group [16], and our in vitro data using NSPs and furin support the concept that proteolytic digestion of the S protein adjacent to the polybasic sequence play a role in priming of the S protein in an early event and might be one of several reasons for increased transmission of novel variants.