https://unherd.com/2021/02/how-worrying-is-the-south-africa-variant/
First: this is an
extremely uncertain finding. As the virologist Müge Çevik
points out, the study was small: it had 1,700 participants, which sounds reasonable enough, but your study’s statistical power comes from the number of cases, not the number of people in the study. And there were only 42 cases of coronavirus in the control and the treatment arm put together.
Of those 42, 19 had been given the real vaccine, and 23 had the placebo. The study itself isn’t available, but
screenshots taken from an online briefing say that it is believed to be 22% effective. But because of the small numbers, the uncertainty around them is huge: the 95% uncertainty interval stretches from 58% to -49% efficacy.