The study is the first randomized clinical trial that tested the antimalarial drug, touted by President Trump, as a preventive measure. It showed that hydroxychloroquine was no more effective than a placebo — in this case, a vitamin — in protecting people exposed to covid-19.

“As we say in Tennessee, ‘That dog won’t hunt’ — it didn’t work,” said William Schaffner, professor of preventive medicine and infectious diseases at Vanderbilt University Medical Center. Schaffner, who was not involved in the trial, praised it as “rigorously done.”…

Researchers at the University of Minnesota Medical School launched the trial in mid-March. They enrolled more than 800 adults in the United States and Canada who were exposed to someone with covid-19 because of their jobs as health care workers or first responders, or because they lived with someone with the disease. The study was a randomized placebo-controlled trial, and was double-blinded, meaning neither the participants nor the researchers knew what the participants received. Such a study is considered the gold standard for clinical trials.