Shot in the dark: Early Covid-19 vaccine efficacy explained

This week has seen a flurry of good news from Covid-19 vaccine developers, with Pfizer Inc and BioNTech SE trumpeting early data indicating their mRNA candidate is more than 90 percent effective.

A Russian project came out a day later, touting 92 percent efficacy for the Sputnik V candidate, named after the Soviet-era satellite, based on a smaller data set...