Not long after severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) was identified as the cause of coronavirus disease 2019 (Covid-19), in vitro experiments revealed that SARS-CoV-2 infection of human cells depended on the binding of the viral spike protein to the human cell-surface receptor angiotensin-converting enzyme 2 (ACE-2).1 Additional experiments demonstrated that infection could be blocked by inhibiting transmembrane protease, serine 2 (TMPRSS2), which is a host enzyme that cleaves the viral spike protein after binding to ACE-2 and facilitates entry of the virus into the host cell.