Another important observation of the latter studies is the fact that despite co-expression of ACE2 and TMPRSS2 within small bronchioles and sporadic alveolar pneumocytes in the majority of the species evaluated, aside from Syrian hamsters, SARS-CoV-2 infection did not result in the development of pneumonia in the experimental infections undertaken by our laboratory in the above-described species. Here, TMPRSS2 is linked to susceptibility to pneumonia measurement.