It is one of three (two 'drugs' and one supplement) used in a cocktail with success.
Also important are the Azithromycin and Zinc Sulfate.
Hydroxychloroquine provides the pathway for the Zinc ions to transit the cell membrane, the zinc stops viral replication and takeover of the cell. Azithromycin provides some immunosuppression to avoid cytokine storm and lung damage from that, as well as acting to safeguard against opportunistic bacterial infection.