The use of hydroxychloroquine does not appear to prevent coronavirus disease 2019 (COVID) after moderate- to high-risk exposure, according to findings of a study published in the New England Journal of Medicine.

The double-blind, placebo-controlled prevention study included individuals who had either known household or occupational exposure to a person with laboratory-confirmed COVID-19. “Because of limited access to prompt testing, healthcare workers could initially be enrolled on the basis of presumptive high-risk exposure to patients with pending tests; however, on March 23, eligibility was changed to exposure to a person with a positive polymerase-chain-reaction (PCR) assay for [severe acute respiratory syndrome coronavirus 2], with the eligibility window extended to within 4 days after exposure,” the authors reported.

Patients were then assigned to receive either placebo (n= 354) or hydroxychloroquine 800mg once, followed in 6-8 hours by 600mg, and then 600mg once daily for 4 consecutive days (n=365). The primary end point of the study was the incidence of COVID-19 disease among those who were asymptomatic at baseline (number of participants at 14 days post enrollment with active COVID-19 disease).

Among the 821 asymptomatic participants, 87.6% reported high-risk exposure (ie, distance of <6 feet for >10 minutes with no face covering). Results showed that the incidence of COVID-19 illness was not observed to be significantly different between the groups (11.8% for the hydroxychloroquine group vs 14.3% for the placebo group; absolute difference: -2.4 percentage points [95% CI, -7.0 to 2.2] P =.35). While no serious adverse reactions were reported, side effects were more frequent with hydroxychloroquine and included nausea, loose stools, and abdominal discomfort.

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“This randomized trial did not demonstrate a significant benefit of hydroxychloroquine as postexposure prophylaxis for COVID-19,” the authors concluded. It is yet to be determined whether high-risk populations may benefit from preexposure prophylaxis (clinical trials are ongoing), however, the study authors noted, the risks associated with hydroxychloroquine may negate any benefits.


Boulware DR, Pullen MF, Bangdiwala AS, et al. A randomized trial of hydroxychloroquine as postexposure prophylaxis for Covid-19 [published online June 3, 2020]. New England Journal of Medicine. doi: 10.1056/NEJMoa2016638.