Pfizer is still likely to be the first out of the gate with interim results on efficacy. “I very much hope that Pfizer does their first interim, and it kicks ass. But who knows?” Lowe says. “They also then have a better chance of doing an interim readout and having it not quite come out as strong as they want it, because they’re doing it earlier.” With such small numbers—the first interim analysis will occur after only 32 infections—it may still be unclear how well the vaccine actually protects against disease.

If most of the infections are in the placebo group—say 26 out of 32—that would suggest the vaccine is at least 76 percent effective. That’d be pretty good. But scientists have cautioned that a COVID-19 vaccine might be less effective than we’d like, based on how vaccines against respiratory viruses tend to work. The FDA has set a bar of at least 50 percent efficacy for a COVID-19 vaccine. It’ll take longer and more cases for trials to reach a conclusion if vaccine efficacy is on the lower side. So if the first interim results are a little disappointing, that “doesn’t mean this is a failed vaccine,” Lowe says. “We’re just going to keep on rolling.” We’ll have a better idea of efficacy once we’ve seen how the vaccine performs in more people.

Conversely, it shouldn’t come as a shock if some of these vaccine candidates do turn out to be ineffective. The development process from Phase 1 to 2 to 3 has gone very smoothly so far. But, in general, more than 90 percent of drugs and treatments fail, and close to 50 percent of them fail in Phase 3. Lowe says he expects COVID-19 vaccine candidates to do much better because scientists are building on research into MERS and SARS, two related coronaviruses. But the whole point of conducting clinical trials is to find out if a vaccine works, so we shouldn’t expect them all to succeed. With 46 vaccine candidates already in clinical trials around the world, scientists are optimistic that at least some will be effective.