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Explained: What latest efficacy data of Oxford-AstraZeneca Covid-19 vaccine means

According to The University of Oxford and AstraZeneca, interim results from phase 3 clinical trial, the vaccine had an efficacy of 79 per cent against symptomatic Covid-19.

Written by Prabha Raghavan , Edited by Explained Desk | New Delhi |
Updated: March 24, 2021 9:06:37 am
A vial of the AstraZeneca Covid-19 vaccine in Luton, England. (AP Photo/Alberto Pezzali, File)

In a positive development for pharmaceutical company AstraZeneca, the company Monday shared encouraging results about the ability of its vaccine to bring down cases of symptomatic Covid-19 and prevent hospitalisations. A look at what the findings are and how they compare with the data that has been shared on this vaccine so far.

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What is AstraZeneca vaccine?

AstraZeneca, in collaboration with The University of Oxford, has developed AZD1222 vaccine. In India and other low- and middle-income countries, it is manufactured and supplied under the name “Covishield” by Serum Institute of India (SII) through a licence from the university and the Swedish-British drugmaker.

The vaccine works by using a weakened version of a common cold chimpanzee adenovirus to carry the code to make the protein that creates the spikes on the surface of the SARS-CoV-2 virus. Once injected into the body, the cells infected with the adenovirus start making the spike protein and the body is expected to develop an immune response to this.

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What are the latest findings and what do they mean?

According to The University of Oxford and AstraZeneca, interim results from phase 3 clinical trials conducted on 32,000 participants across the US, Chile and Peru show that the vaccine had an efficacy of 79 per cent against symptomatic Covid-19. More importantly, the efficacy in the cases of severe or critical symptomatic Covid-19 was 100 per cent.

This means that the possibility of getting symptoms of Covid-19 reduced by 79 per cent in those vaccinated in these trials compared with those who weren’t vaccinated. It also means that the vaccine was able to keep everyone who was inoculated with it from developing severe and critical symptoms that would require hospitalisation.

Why is this significant?

These interim findings show that the efficacy of the vaccine in these trials is much higher than its efficacy in trials conducted in countries like the UK and Brazil. The vaccine’s efficacy in the US, Peru and Chile trials was 79 per cent for symptomatic Covid-19 when the second dose was given four weeks after the first.

In the case of the trials conducted in the UK and Brazil, AstraZeneca in November 2020 had said that interim findings showed that two full doses of the vaccine given four weeks apart had an efficacy of 62 per cent. This number was even lower in an updated study that draws from phase 3 trials on 17,177 participants across the UK, Brazil and South Africa. According to this study, submitted as a preprint to The Lancet in February, the vaccine’s efficacy was around 54.9 per cent when the second dose was given less than six weeks after the first.

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What are the caveats in the latest study?

The results of the latest study are potentially a result of a difference in the criteria used to classify whether the participants have the disease between these trials.

“The absolute efficacy is higher in this new study than observed in the Oxford-led studies, as efficacy is affected by the protocol case definition (higher for more severe cases) and the population in which the study is conducted. Today’s findings are in line with findings from other major vaccine developers who studied efficacy in the US,” stated The University of Oxford in a release.

The population that was involved in the study also impacted the results. For instance, in the interim analysis conducted in the US trial, approximately 79 per cent were caucasian, 22 per cent were Hispanic, eight per cent were African American, four per cent Native American and four per cent were Asian.

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