https://www.bbc.co.uk/news/health-52737169Chloroquine, hydroxychloroquine or a placebo will be given to more than 40,000 healthcare workers from Europe, Africa, Asia and South America. All the participants are staff who are in contact with Covid-19 patients [...] It will test whether the drugs can prevent healthcare workers exposed to the virus from contracting it [...] Hydroxychloroquine regulates the body's immune response and is also used in the treatment of rheumatoid arthritis and Lupus [...]
It has not yet been shown to be safe and effective in the prevention or treatment of coronavirus and can cause dangerous heart arrhythmias.
Could someone who understands this better than me please explain how you can prove a negative in this case? Take this and you won't get Covid. Even with a control group, aren't there many other factors that could skew the results?