An initial Chinese study, published in Clinical Infectious Diseases:
The Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) first broke out in Wuhan (China) and subsequently spread worldwide. Chloroquine has been sporadically used in treating SARS-CoV-2 infection. Hydroxychloroquine shares the same mechanism of action as chloroquine, but its more tolerable safety profile makes it the preferred drug to treat malaria and autoimmune conditions. We propose that the immunomodulatory effect of hydroxychloroquine also may be useful in controlling the cytokine storm that occurs late-phase in critically ill SARS-CoV-2 infected patients.
Hydroxychloroquine was found to be more potent than chloroquine to inhibit SARS-CoV-2 in vitro.
Read the original study report here: https://academic.oup.com/cid/article/doi/10.1093/cid/ciaa237/5801998?searchresult=1