Hydroxychloroquine is an effective drug for the treatment of diseases like lupus and malaria and because it is used off-label for maladies that act in a biologically similar way to how COVID-19, caused by the SARS-CoV-2 virus, acts in the body, there has been discussion that clinical trials might show it having a positive therapeutic impact.

Could it prevent COVID-19, though? A clinical trial of health care workers finds that it does not, though the good news is that because the trial was health care workers they had low rates of infection anyway, likely due to other prevention measures. Masks, hand-washing, and social distancing work.

The trial was undertaken because in lab studies hydroxychloroquine could prevent SARS-CoV-2 from entering cells in tissue culture. So the team wanted to test whether taking a substantial dose of hydroxychloroquine - 600 milligrams daily for two months- would have an effect on infection rates. They conducted this study among hospital workers that regularly came into contact with COVID-19 patients.

The researchers were able to analyze a pool of 125 physicians, nurses, certified nursing assistants, emergency technicians, and respiratory therapists that they recruited for the study. This population worked in several different areas of the two University hospitals, including the emergency departments and COVID-19 units. Roughly half of the participants in the study took hydroxychloroquine while the other half took a matching placebo (a cellulose pill). The study was double-blinded, meaning neither the researchers, nor the participants knew which drug they were assigned.

Extensive testing was used to rigorously prove who did or did not contract the virus. Each person received swab and antibody testing for COVID-19 at the start of their participation in the study, halfway through, and at the end--an eight-week span during the study period that began April 9 and ended July 14, 2020. Participants also had electrocardiogram (ECG) tests because of concerns about hydroxychloroquine causing heart rhythm problems in severe cases of COVID-19.

Given all that, it is not recommend for routine use of hydroxychloroquine among health care workers to prevent COVID-19.