I do not have the citation handy, but in one study, they recruited individuals by enticing them with the results; e.g. if you take take part in the study, we wi

I do not have the citation handy, but in one study, they recruited individuals by enticing them with the results; e.g. if you take take part in the study, we will give you the antibody result. This would obviously bias the results toward people that believed they had the virus, but didn't have a test when sick.