An interesting hint could come from this pre-print study: https://www.biorxiv.org/content/10.1101/2020.03.29.014407v1 They tried different administration modalities of HCQ to in-vitro infected cells: some cells got nothing (control), some got HCQ only before infection, some got HCQ only after infection, some got HCQ both before and after infection. THe cells that got HCQ only before or only after infection had just slightly better outcome than control (they appear as heavily infected), while cells that received HCQ both before and after contagion were in much healthier conditions. If that would be the same in vivo as in vitro, that would entail that it would be possible to use HCQ as prophylaxis, given that the patients never stop taking the drug. Of course the caveat is "IF that would be the same in vivo as in vitro", that they didn't (yet) tested.