Chloroquine/ Hydroxychloroquine Prevention of Coronavirus Disease (COVID-19) in the Healthcare Setting

Study Participants The study population is adult healthcare workers and other persons defined by the site investigator at risk of contracting COVID-19. Inclusion Criteria 1. Participant is willing and able to give informed consent for participation in the study and agrees with the study and its conduct 2. Agrees not to self-medicate with chloroquine, hydroxychloroquine or other potential antivirals 3. Adults (exact age is dependent on countries) less than 70 years old at the time of consent 4. Not previously diagnosed with COVID-19 5. Not currently symptomatic with an ARI 6. Participant is a healthcare worker or is a person at risk of contracting COVID-19. 7. Possesses an internet-enabled smartphone (Android or iOS) Exclusion Criteria: 1. Hypersensitivity reaction to chloroquine, hydroxychloroquine or 4-aminoquinolines 2. Contraindication to taking chloroquine as prophylaxis e.g. known epileptic, known creatinine clearance \< 10 ml/min 3. Already taking chloroquine, hydroxychloroquine or 4-aminoquinolines, or history of these medications within the previous 7 days 4. Taking prohibited medications 5. Known retinal disease 6. Inability to be followed up for the trial period 7. Known prolonged QT syndrome (however ECG is not required at baseline) 8. Known pregnancy or women who are actively trying to become pregnant 9. Prior diagnosis of porphyria 10. Previously received any dose of COVID-19 vaccine The investigator may consult the physician's guidance documents for any further questions regarding eligibility of potential participants. Prohibited medications for the purpose of study enrollment include: * Antiarrhythmic medications: digoxin, amiodarone, sotalol, flecainide * Antiparasitic/malarial agents: mefloquine, halofantrine, praziquantel * Antibiotics: levofloxacin, moxifloxacin, ciprofloxacin, azithromycin, clarithromycin, erythromycin * Antifungal drugs: fluconazole, ketoconazole, itraconazole, terfenadine * Psychoactive drugs: lithium, quetiapine, chlorpromazine, thioridazine, ziprasidone, haloperidol, droperidol, methadone * Migraine treatment: sumatriptan * Antihistamines: astemizole * Antiemetics: prochlorperazine, metoclopramide * Cancer treatments: abiraterone, dabrafenib, dacomitinib, enzalutamide, idelalisib, mitotane * Other specific drugs: ciclosporin, conivaptan, agalsidase alfa or beta, mifepristone, stiripentol PrincipaI Investigators will also be directed to crediblemeds.org to check other agents that may prolong QT interval