maybe this will answer your questions
https://clinicaltrials.gov/ct2/show/NCT04308668?cond=Covid-19&cntry=US&draw=2&rank=5
there are currently several studies underway or just finished that are testing the prophylactic efficacy
many front line workers are in these studies and either get HCQ or a placebo to make the study accurate as to HCQ's efficacy