ACE2 and infection: However, a large proportion of patients were immunocompromised, samples at diagnosis and follow-up were combined (skewing the results), a high proportion of participants were healthcare workers (reinfection may have been a confounder), and, as the authors suggested, a limitation was that the Vero cell line used was overtly permissive to infection compared with the human lung carcinoma cell line H1299 ACE2, which is a biologically representative cell line (and one that we used).