SARS-CoV-2: effectiveness of the Oxford AstraZeneca vaccine

 

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Of the 58 vaccines under development to combat COVID-19, three vaccines so far have proceeded satisfactorily through the clinical trials process - one of these from the Oxford University - AstraZeneca partnership is scheduled for widespread inoculation of the Australian population during 2021 and onwards. This vaccine has been developed as a non-profit intervention aimed at "global supply, equity, and commitment to low-income and middle -income countries" (www.gavi.org/news) and has an agreement under the COVAX facility. Currently the vaccine is shown to have 70.4% efficacy.

Designated as ChAdOx1 nCoV-19, it utilises a chimpanzee adenovirus to enable a vectored vaccine. 

The four randomised controlled clinical trials of this vaccine took place in the UK, South Africa and Brazil with the UK and Brazil components being the first to report with interim efficacy results. Of note from these studies:

• 11, 636 participants were in the clinical trials
• 87.8% were aged 18-55 years
• 82.7% were white
• 60.5% were female

The phase 1 stage of the trial had supported a two dose regimen administered 28 days apart. There were no hospital admissions for those who received the vaccine whereas ten admissions occured in the control groups. An interesting development for the Oxford-AstraZeneca trial was the accidental provision of a low dose of the vaccine for some participants in the UK which was then boosted by a standard dose 28 days later that appears to have not reduced the level of effectiveness.

The clinical trials for this vaccine had substantial strengths including -

• a large sample size
• randomisation to vaccine groups
• inclusion of diverse sites with different races and ethnicities
• standardisation of key elements between trials
• balance of participant characteristics between the vaccine groups 
• similar results in Brazil and the UK for those receiving the standard dose regime providing credibility.

There have been limitations for the clinical trial studies as well such as -

• less than 4% of participants were older than 70 years of age
• no participants older than 55 years received the mixed dose regime
• unavailable results as yet on those participants with comorbidities and any impact.

The two dose regime poses its own challenges for health systems to coordinate and manage. A substantial practical advantage of the the Oxford AstraZeneca vaccine is that it can be transported and stored through the routine refrigerated cold chain whereas in contrast the Pfizer vaccine requires ultra-low temperature freezers.

[Information for this post has been drawn from The Lancet December 8, 2020]

A link is provided below.

Oxford- AstraZeneca vaccine - The Lancet