Patients given a 10-day course of the antiviral drug fared no better than those who didn't receive it, and were just as likely to die, the data showed.
Those that were administered a five-day course of remdesivir saw an improvement, "but the difference was of uncertain clinical importance," according to the researchers.
"Among patients with moderate COVID-19, those randomized to a 10-day course of remdesivir did not have a statistically significant difference in clinical status compared with standard care at 11 days after initiation of treatment," the researchers, from remdesivir maker Gilead Sciences, wrote.
Originally developed to treat Ebola virus, remdesivir works by slowing the production of enzymes that play a key role in the replication of viruses, including coronaviruses, according to Gilead Sciences.
Earlier research suggested that the drug might be effective in people with severe COVID-19.
A study published by the New England Journal of Medicine found that seriously ill patients infected with the new coronavirus had an average recovery time of 11 days after receiving the drug, compared to 15 days for those given a placebo.
On the strength of this data, remdesivir was granted an emergency authorization for use in hospitalized patients with COVID-19 by the U.S. Food and Drug Administration on May 1.
For the new study, the researchers enrolled 584 people with confirmed cases of COVID-19, all of whom were 12 years old or older and hospitalized with moderate pneumonia caused by the virus.
Roughly one-third of the study participants were given 10-day course of intravenous remdesivir, while one-third received a five-day course, the researchers said.
The rest did not receive the drug but were administered "standard of care" for COVID-19, which includes corticosteroids to reduce inflammation caused by the virus, they said.
All patients who received remdesivir were given 200 milligrams of the drug on the first day of treatment, followed by 100 mg. per day thereafter.
On day 11 of the study, 65% of the patients in the 10-day remdesivir group had been discharged from the hospital, compared to 60% of the patients not given the drug, the data showed.
However, 23% of the patients in both groups remained hospitalized and 6% in each group required supplemental oxygen to maintain their breathing, according to the authors.
In both groups, 2% of the patients died, the data showed.
In a related commentary also published Friday in JAMA Network Open, Erin K. McCreary and Dr. Derek C. Angus from the University of Pittsburgh School of Medicine wrote that, based on these findings, "whether remdesivir offers incremental benefit over corticosteroids, which are widely available and inexpensive, is unknown."