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ObjectiveTo determine whether hydroxychloroquine (HCQ) is safe and effective at preventing COVID-19 infections among health care workers (HCW).DesignMulticenter, 1:1 randomized, placebo-controlled, double-blind, parallel-group, superiority trial.Setting34 clinical centers in the United States.Participants1360 HCW at risk for COVID-19 infection enrolled between April and November 2020.InterventionsA loading dose of HCQ 600 mg twice on Day 1 followed by 400 mg daily for 29 days or matching placebo taken orally.Main Outcome MeasureComposite of confirmed or suspected COVID-19 clinical infection by Day 30 defined as new onset fever, cough, or dyspnea and either a positive SARS-CoV-2 PCR test (confirmed) or a lack of confirmatory testing due to local restrictions (suspected).ResultsEnrollment for the study was closed before full accrual due to difficulties recruiting additional participants. The primary composite endpoint occurred in 41 (6.0%) participants receiving HCQ and 53 (7.8%) participants receiving placebo. No statistically significant difference in the proportion of participants experiencing clinical infection (estimated difference of -1.8%, 95% confidence interval -4.6% to 0.9%, p=0.20). We identified no significant safety issues.ConclusionOral HCQ taken as prescribed appeared to be safe in a group of HCW. No significant clinical benefits were observed. The study was underpowered to rule out a small but potentially important reduction in COVID-19 infections.Trial RegistrationNCT04334148