The study will start with a 10-patient prospective observational pilot study. All these patients will be subject to the same entry and exclusion criteria for the randomized trial, and undergo the same procedures. They will all receive chloroquine at the doses used in the trial (see sections below); they will not be randomized. The purpose of the pilot is to develop the study procedures for the randomized controlled trial, including the safe monitoring of patients, to refine the CRF, and to acquire some preliminary data on the safety of chloroquine in those with COVID-19.

Once the pilot study has been completed, and the data reviewed by the TSC and DMC, and the MOH ethics committee, we will then proceed to the trial. We will aim for minimum delay between completing the pilot study and starting the randomized trial.

The main study is an open label, randomised, controlled trial that will be conducted in 240 in-patients in Ho Chi Minh City. Viet Nam.

Patients will have daily assessment as per standard of care while in-patients by the hospital staff. While in-patients the study will collect the following data: peripheral oxygen saturation (pulse oximeter), respiratory rate, and FiO2. These will be recorded between 2 and 4 times per day depending on the practice of the treating site. Where recording is twice daily, one record will be made from the time period of 00:00 until 12:00, and the second recording between 12:01 and 23:59. Where the parameters are recorded four times/day they will be recorded in each of the time periods 00:00 - 06:00, 06:01 - 12:00, 12:01 - 18:00 and 18:01 - 23:59. Vitals recorded will include: FiO2, SpO2, Temp, RR HR BP. The use of ventilator or other assisted breathing device will be recorded each day.

Patients will have clinical assessment recorded as per the study schedule.

The decision to discharge patients will be at the discretion of the attending physician and depend upon the clinical status of the patient. According to current standard of care recovery and hospital discharge is dependent upon the patient having had 2 daily consecutive negative PCR throat/nose swabs. Following discharge patients will be seen on days 14, 28, 42 and 56 post-randomization.

In a subset of patients admitted to HTD we will look for ECG changes, using real-time monitoring.

Patients will have up to 1 hour ECG continuous recordings daily. The ECG recording will be downloaded from standard monitor (GE Careview) and stored electronically. ECG changes (including QT interval) will then be analyzed by machine learning.