Current participant exclusion criteria as of 03/06/2021: 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 any of the following prohibited medications: 4.1. Antiarrhythmic medications: digoxin, amiodarone, sotalol, flecainide 4.2. Antiparasitic/malarial agents: mefloquine, halofantrine, praziquantel 4.3. Antibiotics: levofloxacin, moxifloxacin, ciprofloxacin, azithromycin, clarithromycin, erythromycin 4.4. Antifungal drugs: fluconazole, ketoconazole, itraconazole, terfenadine 4.5. Psychoactive drugs: lithium, quetiapine, chlorpromazine, thioridazine, ziprasidone, haloperidol, droperidol, methadone 4.6. Migraine treatment: sumatriptan 4.7. Antihistamines: astemizole 4.8. Antiemetics: prochlorperazine, metoclopramide 4.9. Cancer treatments: abiraterone, dabrafenib, dacomitinib, enzalutamide, idelalisib, mitotane 4.10. Other specific drugs: ciclosporin, conivaptan, agalsidase alfa or beta, mifepristone, stiripentol 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 11. The investigator may consult the physician’s guidance documents for any further questions regarding the eligibility of potential participants. Previous participant 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 4. Taking a concomitant medication 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

Current participant exclusion criteria as of 03/06/2021: 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 any of the following prohibited medications: 4.1. Antiarrhythmic medications: digoxin, amiodarone, sotalol, flecainide 4.2. Antiparasitic/malarial agents: mefloquine, halofantrine, praziquantel 4.3. Antibiotics: levofloxacin, moxifloxacin, ciprofloxacin, azithromycin, clarithromycin, erythromycin 4.4. Antifungal drugs: fluconazole, ketoconazole, itraconazole, terfenadine 4.5. Psychoactive drugs: lithium, quetiapine, chlorpromazine, thioridazine, ziprasidone, haloperidol, droperidol, methadone 4.6. Migraine treatment: sumatriptan 4.7. Antihistamines: astemizole 4.8. Antiemetics: prochlorperazine, metoclopramide 4.9. Cancer treatments: abiraterone, dabrafenib, dacomitinib, enzalutamide, idelalisib, mitotane 4.10. Other specific drugs: ciclosporin, conivaptan, agalsidase alfa or beta, mifepristone, stiripentol 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 11. The investigator may consult the physician’s guidance documents for any further questions regarding the eligibility of potential participants. Previous participant 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 4. Taking a concomitant medication 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

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 4. Taking a concomitant medication 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

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 4. Taking a concomitant medication 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