PAPPA and COVID-19: The SARS-CoV-2 S protein interaction process is mediated by proteases such as transmembrane protease serine 2 (TMPRSS2), cellular protease furin and cathepsin L. Interestingly, we measured PAPP-A expression in the nasopharyngeal samples of 90 COVID-19 patients but did not observe any change between the control group and patients (data not shown), so this does not seem to be the case.