We don’t know yet whether it provides any protection against severe disease (SA strain) but it seems very unlikely given it’s efficacy of 10-20% against mild - moderate, don’t you think?
But there is lots of uncertainty around those figures:
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.
So, while it is very likely that the vaccine is less effective against the South African strain than against the more old-school varieties, it is still likely to give some protection against mild to moderate disease, and it is very possible that it gives much better protection than this study suggests.
The study only looked at younger people — the median age was 31 — so there were no severe cases at all, in either the treatment or the control groups.
Intuitively, you might think that if a vaccine doesn’t protect against mild disease, it won’t have a chance against severe disease. But that’s not how it works. “Most severe disease is a consequence of someone’s immune system not dealing with the level of viral load,” says Stephen Evans, a professor of pharmacoepidemiology at the London School of Hygiene and Tropical Medicine. “If you drive the viral load down in someone they’re less likely to get severe disease.” It may not be enough to stop them from developing a PCR-detectable, or even a symptomatic, case of the disease, but it might stop them from ending up in hospital.
Or at least that’s the theory, according to our understanding of virology. But it’s not actually been easy to show in the case of Covid because severe disease is, mercifully, rare. Even in the main Lancet study into the safety and efficacy of the Ox/AZ vaccine, there were only 11 severe cases, including one death, among the 11,600 participants in the study. Ten of them, including the death, were in the control group, suggesting a greater-than-90% efficacy against severe disease; but the uncertainty interval (determined using an odds-ratio calculator) is huge, from 99% to 22%. Pfizer’s results are similar; Moderna’s are a bit more robust, but still hard to draw firm conclusions from.
unherd.com/2021/02/how-worrying-is-the-south-africa-variant/