The first vaccines may provide only moderate protection, low enough to make it prudent to keep wearing a mask. By next spring or summer, there may be several of these so-so vaccines, without a clear sense of how to choose from among them. Because of this array of options, makers of a superior vaccine in early stages of development may struggle to finish clinical testing. And some vaccines may be abruptly withdrawn from the market because they turn out not to be safe.

“It has not yet dawned on hardly anybody the amount of complexity and chaos and confusion that will happen in a few short months,” said Dr. Gregory Poland, the director of the Vaccine Research Group at the Mayo Clinic…

The authorization of a vaccine will depend on how much protection the vaccine provides in the Phase 3 trial — what scientists refer to as its efficacy. In June, the F.D.A. set 50 percent efficacy as the target for a coronavirus vaccine.

But the efficacy in a trial may not necessarily match its effectiveness out in the real world. That’s because, like any statistical study, Phase 3 trials have margins of error. A vaccine that met the F.D.A.’s guidelines might actually be more than 50 percent effective, or might be less so. It might well turn out to be only 35 percent effective.