Over the next few months, the companies behind the leading [COVID] vaccine candidates will start releasing the first data from large clinical trials. Most likely, they will not be unalloyed good news or bad news. Keeping expectations measured will require understanding when a vaccine clears just one of many hurdles—it doesn’t have to be perfect, but it must be good enough.
If most of the infections are in the placebo group [in the first interim analysis]—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,” [chemist Derek] 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.