But the unexpected demand for plasma has inadvertently undercut the research that could prove that it works. The only way to get convincing evidence is with a clinical trial that compares outcomes for patients who are randomly assigned to get the treatment with those who are given a placebo. Many patients and their doctors — knowing they could get the treatment under the government program — have been unwilling to join clinical trials that might provide them with a placebo instead of the plasma.

The trials have also been stymied by the waning of the virus outbreak in many cities, complicating researchers’ ability to recruit sick people. One of those clinical trials, at Columbia University, sputtered to a halt after the outbreak subsided in New York. One of its leaders, Dr. W. Ian Lipkin, looked for hospitals in other hot spots in the United States to continue the work. But he found few takers.

“Without a randomized control trial, it’s very difficult to be certain that what you have is meaningful,” he said.