TMPRSS2 and susceptibility to pneumonia measurement: SARS-CoV-2 is an enveloped RNA virus and uses the spike protein (S) on viral envelope to attach to cellular receptors such as angiotensin-converting enzyme 2 (ACE2) and Transmembrane serine protease 2 (TMPRSS2) to enter human cells (especially lung cells), leading to severe pneumonia [4].