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Hydroxychloroquine not effective in preventing the development of COVID-19 when used as post-exposure prophylaxis

June 3, 2020 — 

A new study suggests that hydroxychloroquine is not effective in preventing the development of COVID-19 in individuals exposed to SARS-CoV-2, the virus responsible for the disease. This is the main conclusion of the first double-blind, randomized, placebo-controlled trial of hydroxychloroquine for disease prevention to be completed. Coordinated with a large study led by Dr. David Boulware at the University of Minnesota, this clinical trial was led in Canada by Drs. Todd Lee and Emily G. McDonald at the Research Institute of the McGill University Health Centre (RI-MUHC), in collaboration with partners at the Universities of Manitoba (led by Dr. Ryan Zarychanski) and Alberta (led Dr. Ilan Schwartz). The results are published today in the prestigious New England Journal of Medicine.

“While we had hope this drug would work in this context, our study demonstrates that hydroxychloroquine is no better than placebo when used as post-exposure prophylaxis within 4 days of exposure to someone infected with the new coronavirus,” said Lee, Scientist at the RI-MUHC and Associate Professor of Medicine, Division of Infectious Diseases at McGill University and one of the lead authors of the study.

This trial included 821 asymptomatic adults with household or healthcare exposure to someone with confirmed COVID-19, who were enrolled nationwide in the United States and in the Canadian provinces of Quebec, Manitoba, and Alberta. Among them, 719 participants reported a high-risk exposure, of less than six feet (2 metres) for more than 10 minutes without one of the components of personal protective equipment (e.g. mask or face shield), to a confirmed COVID-19contact. For the most part, they were healthy younger community dwelling adults (average age 40 years).

Within four days of exposure, the participants received either placebo or hydroxychloroquine by commercial courier which were to be taken over five days, starting with a stronger dose on day 1. Investigators and participants were blinded to the treatment assignments, and an independent data and safety monitoring board (DSMB) externally reviewed the data.

“We launched the first COVID-19 clinical trial in Manitoba, and played a key role in promoting this important randomized clinical trial across Canada,” said Zarychanski, Manitoba lead and associate professor of internal medicine, Max Rady College of Medicine, University of Manitoba and senior scientist, Research Institute in Oncology and Hematology.

“This is the gold standard method for this type of intervention,” added McDonald, Investigator at the RI-MUHC, Director of the MUHC Clinical Practice Assessment Unit and co-author of the study. “It is incredibly important that we complete randomized controlled trials so that we have the best available evidence for how to prevent the spread of COVID-19.”

Overall, 107 of 821 of participants developed COVID-19 (either confirmed with a test or symptomatically compatible disease) over the 14 days of follow-up. Both confirmed cases and probable cases were included, due to some lack of availability of diagnostic testing in the United States. Amongst those who received hydroxychloroquine, 49 developed the disease (or compatible symptoms such as fever or cough), vs 58 in the group that received the placebo. Two patients were hospitalized, one in each group. No deaths occurred.

Medication side effects like nausea and abdominal discomfort were more common for patients taking hydroxychloroquine compared to placebo (40% vs. 17%), but no serious treatment-related adverse reactions were reported, including any heart arrhythmia.

‘’Our study’s results set politics aside and provide unbiased evidence to guide practice in the prevention of COVID-19 and reinforce the importance of randomized clinical trials as we work together nationally and internationally to combat the novel coronavirus,’’ said Zarychanski, also a clinician-scientist who recruited many junior UM investigators to join the clinical trial team including Lauren MacKenzie, Lauren Kelly, and Sylvain Lother in addition to a senior clinician, Dr. Glen Drobot.

Canadian co-investigators also include Matthew Cheng, Ilan Schwartz.

Ongoing hydroxychloroquine clinical trials

More research is needed to determine whether hydroxychloroquine can be effective for the early treatment of COVID-19, and whether pre-exposure prophylaxis could be effective in high-risk populations. Various trials are ongoing worldwide, including in Canada ( At the MUHC, a trial looking at early treatment in the community is underway.


This work was made possible through funding from the MUHC Clinical Practice Assessment Unit (CPAU) and is one of the critical research programs being supported by the McGill Interdisciplinary Initiative in Infection and Immunity (MI4) with seed funding from the MUHC Foundation.

In Manitoba, this trial was supported by the Manitoba Medical Services Foundation (MMSF) and Research Manitoba.

Research at the University of Manitoba is partially supported by funding from the Government of Canada Research Support Fund.

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One comment on “Hydroxychloroquine not effective in preventing the development of COVID-19 when used as post-exposure prophylaxis

  1. Dwayne Hildebrandt

    Did the study also look into treating Covid positive patients with Hydroxychloroquine to see if it had any impact in reducing symptoms or severity of Covid?

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