AstraZeneca's under-fire COVID-19 vaccine has been dealt another blow, with confirmation it simply doesn't work at preventing mild-to-moderate infections caused by the South African variant of the virus.
South Africa halted its planned rollout of the AstraZeneca vaccine in early February, when evidence emerged it might not be effective against a local mutated version of the SARS-CoV-2 virus, called 501.V2 or B.1.351.
The variant has a number of mutations on its spike protein - the pointy bit the virus uses to enter human cells - which make it more infectious.
The decision to halt the rollout of the vaccine - also known as ChAdOx1 nCoV19 - was made based on early data from a study by scientists at the University of the Witwatersrand in Johannesburg. The full details of that research have now been peer-reviewed, and were published Wednesday in The New England Journal of Medicine.
In the study, 750 people got two shots of the AstraZeneca vaccine and 717 a placebo. Of those who got the vaccine, 19 went on to develop "mild-to-moderate" COVID-19, compared with 23 in the placebo group. Of the 42 COVID-19 cases, 39 were the B.1.351 variant.
All-up, this gave it an efficacy of just 10.4 percent.
"A two-dose regimen of the ChAdOx1 nCoV19 vaccine did not show protection against mild-to-moderate COVID-19 due to the B.1.351 variant," the study concluded.
In contrast, rival vaccines such as those made by Pfizer and Moderna have shown more than 90 percent efficacy against the original strain of the virus, though there are ongoing concerns they too might not work as well against B.1.351.
While studies have found the AstraZeneca vaccine - like the others - is effective at preventing severe illness and death, a mysterious condition known as 'long COVID' is possible even after mild infections. People showing symptoms are also believed to be more infectious than those who catch the virus but don't get sick, potentially putting more people at risk.
The AstraZeneca vaccine, developed in conjunction with the University of Oxford, has had a troubled development. Mistakes were made in its trial phase, which led to some volunteers getting injected with the wrong amount; its effectiveness during its trials wasn't quite as good as that achieved by vaccines made by some of its rivals; and there have been recent reports it could be causing blood clots, prompting several countries to stop using it.
The World Health Organization (WHO) has urged countries to keep using it, saying there's no evidence linking use of the vaccine to blood clots, and even if there was, current data suggests the risk is still extremely low compared to that posed by COVID-19.
ChAdOx1 nCoV19 is cheaper to produce and easier to transport and store than some of the other vaccines developed so far, making it a key weapon in the WHO's arsenal against COVID-19, particularly in poorer countries.
Most vaccine manufacturers expect they'll need to make booster shots as new variants of the virus emerge, but they won't be available for months, at least. Some medical experts have predicted COVID-19 will become endemic like influenza - another disease that is here to stay, constantly mutating to evade existing vaccines.
Its deadlier predecessor SARS was wiped out after killing less than 1000 people because it wasn't as infectious as SARS-CoV-2.