Modest results from a federal trial of an experimental drug helped send the stock market soaring on Wednesday, another sign of the desperation for a viable treatment against the coronavirus.

Just before markets opened, Gilead, maker of the antiviral drug remdesivir, announced that it was “aware of positive data” about the drug’s performance in a federal trial, sending futures upward. Trading in the company’s shares was briefly halted.

Later, in a briefing at the White House, Dr. Anthony S. Fauci, director of the National Institute of Allergy and Infectious Diseases, said the trial had shown that treatment with the drug could modestly speed recovery in patients infected with the coronavirus.

The improvement in recovery times “doesn’t seem like a knockout 100%,” Fauci conceded, but “it is a very important proof of concept, because what it has proven is that a drug can block this virus.”

Sitting at Fauci’s side, President Donald Trump said, “Certainly it’s positive, it’s a very positive event.” In past weeks, he has repeatedly hailed remdesivir as a potential “game changer,” despite spotty evidence.

Business leaders, scientists and politicians alike are scrambling to find ways to fight an insidious epidemic and to reopen a devastated economy. The virus has claimed at least 60,000 lives in the United States and more than 200,000 worldwide. There have been few reasons for optimism, and the markets seized on the news.

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The trial sponsored by the National Institute of Allergy and Infectious Diseases enrolled 1,063 patients who were given remdesivir or a placebo. The time to recovery averaged 11 days among those who got the drug, compared with 15 for those on the placebo.

There were fewer deaths in the remdesivir group, but the result did not reach statistical significance, Fauci said. Deaths were not a primary measure in the trial.

Fauci cautioned that the results of the study still needed to be properly peer-reviewed, but he was optimistic that remdesivir would become “the standard of care” for patients with COVID-19.

Some scientists were unsettled by the way in which the findings were reported. The disclosure of trial results in a political setting, before peer review or publication, is very unusual, said Dr. Steven Nissen, a cardiologist at the Cleveland Clinic who has conducted dozens of clinical trials.

“Where are the data?” he asked. Scientists will need to see figures on harms associated with the drug in order to assess its benefits, he added.

“This is too important to be handled in such a sloppy fashion,” Nissen said.

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Dr. Michele Barry, a global health expert at Stanford University, said she had faith in Fauci’s assessment. Still, she added, “It is unusual to call a drug the ‘standard of care’ until peer review of data and publication, and before studies have shown benefit in mortality.”

The Food and Drug Administration is likely at some point to announce an emergency approval for remdesivir, a senior administration official told The New York Times. Another drug touted by the president, hydrochloroquine, also was granted such an approval, with disappointing results.

In one study of veterans with COVID-19, those receiving hydrochloroquine and an antibiotic died at higher rates than those given ordinary supportive care.

Trump also hopes to put in place a crash program to develop a vaccine, an undertaking being seen with skepticism even inside the administration. The accelerated process, known internally as Operation Warp Speed, would aim to produce hundreds of millions of doses by the end of this year.

Medical experts, including Fauci, have warned that developing a vaccine will require a year to 18 months at the earliest and that rushing the process could endanger public health.

For now, drug treatment seems a more attainable goal.

“Remdesivir is not a magic bullet, but it’s as good as we get right now,” said Dr. Peter Chin-Hong, an infectious disease specialist at the University of California, San Francisco, and one of the trial’s investigators.

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“Patients come to the hospital thinking we have a treatment, and by treatment, they mean a drug,” he added. “We have been impotent in not having any options.”

Remdesivir has never been approved as a treatment for any disease. It was developed to fight Ebola, but results from a clinical trial in Africa were disappointing.

But as the coronavirus pandemic took hold, the drug emerged as one of the more promising potential treatments. It interrupts the production of the virus in lab studies and seems safe in animals.

Until now, high expectations for remdesivir have been fueled largely by anecdotal reports of COVID-19 patients who took the drug and recovered.

Two such reports were published in the New England Journal of Medicine, lending credibility to what researchers said were actually uncertain results.

Without trials comparing the drug to a placebo, it has been impossible to know whether the drug made a difference or patients got better on their own with normal supportive care.

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A separate study of remdesivir, published on Wednesday in The Lancet, found no benefit to the drug, compared with a placebo.

“Unfortunately, our trial found that while safe and adequately tolerated, remdesivir did not provide significant benefits over placebo,” said the lead investigator of the new study, Dr. Bin Cao of the China-Japan Friendship Hospital and Capital Medical University in Beijing.

“This is not the outcome we hoped for,” he added.

The results are hard to interpret, because the study was far smaller than planned — enrolling 236 patients instead of the 453 that had been expected, because there were too few severely ill patients now in China.

Dr. Eric Peterson, a clinical trials expert at Duke University, said that with too few patients, “all you can say is it doesn’t seem to work in this population.” If there had been a big effect of the drug, he added, that would have been seen.

He added that the trial should not be repeated with this population but instead in those who are less severely ill.

“This is a flawed study,” Barry said, but results might improve if the drug were given at a higher dose or earlier in the course of the disease.

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Acceding to demands, Gilead has distributed the drug to hundreds of patients under so-called compassionate use, a regulatory exemption by which patients may receive a drug apart from a clinical trial.

Gilead itself published reports of uncontrolled studies. On Wednesday, in another news release, the company announced that a study comparing a five- to 10-day course of treatment with the drug showed that those getting the shorter course of treatment did just as well.

That study had no control group and was “noninformative,” Peterson said.