Interesting read. From a layman's point of view the main issue I have with this study is that they have a selection bias to select those who have had severe COV

Interesting read. From a layman's point of view the main issue I have with this study is that they have a selection bias to select those who have had severe COVID symptoms. "Subjects were chosen from applicants who responded to online advertisements seeking recovered COVID-19 patients for this study." Those who had asymptomatic/mild COVID symptoms might not even know they had COVID and they therefore would not apply to participate in the study. This biases the applicant pool to those who had severe enough COVID symptoms to either go to a hospital or get tested. For the duration of the pandemic I have always advocated that COVID related studies should take random samples from the general population instead of selecting patients which are known to have COVID. Of course it would be much more work to pick a random 10000 people from the general population to participate in your study, but I think it would give better insight into the asymptomatic or mild COVID cases which would not be selected for a trial since the applicant themselves doesn't know they have COVID. Generally, studies with the selection bias make COVID look worse since the mild/asymptomatic cases are ignored.