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The drugs used in this study, chloroquine and hydroxychloroquine, have been shown in the laboratory to kill the novel coronavirus (SARS-CoV-2), the virus that causes COVID-19. The drugs have been in the news a lot because they have been tested in trials for the treatment of COVID-19. The two largest trials (the WHO SOLIDARITY trial and the RECOVERY trial run by the University of Oxford, UK) concluded that hydroxychloroquine is not effective for the treatment of individuals hospitalised with COVID-19, but no antiviral has proven effective at this late stage when inflammation predominates, and experts believe antivirals have a better chance if given earlier. Other trials evaluating whether the drugs may provide benefit for individuals with mild or moderate symptoms are ongoing.

The drugs have also been studied for their ability to prevent COVID-19 but most randomised controlled trials only included people already exposed to an infected contact (post-exposure prophylaxis) and all the studies were too small to give a definite answer, but have been suggestive of modest benefits in preventing COVID-19. However, currently we do not know definitively if these drugs can prevent COVID-19.

COPCOV is a very large study and the only remaining trial trying to answer the question of whether these drugs can prevent COVID-19 if given before an individual is exposed (pre-exposure prophylaxis). COPCOV aims to enrol 40,000 participants and provide a definite answer as to whether the drugs are effective and safe. Hydroxychloroquine and chloroquine are affordable and globally accessible and so if shown to work the potential for public health impact would be large, particularly in regions where vaccine deployment is likely to take many months or years.

COPCOV studies chloroquine and hydroxychloroquine for the prevention of COVID-19 before an individual is exposed or infected, which is different from the treatment of patients that already have COVID-19 or treatment of individuals who have already been exposed to an infected contact, both of whom carry a much higher load of the virus.