But experts believe that the antibodies could prove far more effective than plasma. Last year, in the Democratic Republic of Congo, monoclonal antibody cocktails proved 90 percent effective at saving Ebola victims from death.

But this approach has limitations. It is believed to work only if administered soon after infection, and monoclonal antibodies are hard to produce and expensive, at least at the moment. If the treatment becomes popular, demand will quickly outstrip supply, forcing health officials to make hard choices.

Early testing in animals and humans suggests that a dose a fraction the size of the one Mr. Trump received can protect an uninfected person against the virus. If that finding holds up, the antibodies could be used like a fast-acting vaccine, lasting just a month or so but providing a crucial “bridge” to the arrival of the new vaccines.

Such a treatment could protect people at highest risk, such as health workers and nursing home residents. Or, in a “ring vaccination” strategy, antibodies could be given to the household contacts of known cases. Ring vaccination was how smallpox was defeated.