The 200 mg twice a week is the malaria prophylactic level, and as a consequence of vast numbers of people taking hat dose and having Covid infection rate and deaths near zero, one can reasonably conclude it's effective. I don't know of the demanded-when-convenient, unnecessary-when-not double-blind, pair-matched, randomized peer-reviewed study on it. Of course Remdesivir to be lauded to the skies didn't need any such study turning out positive for the primary outcome measure. But that's different don'cha know.