AstraZeneca vaccine: more effective with half a dose?

The results offered by the Oxford-AstraZeneca coronavirus vaccine are encouraging, but they come with an ambiguity. The data seem to indicate contradictory efficacy rates depending on the doses injected.

Among the candidate vaccines in the fight against the coronavirus, that resulting from the collaboration between the University of Oxford and the AstraZeneca group stands out with encouraging results. Recently, a press release announced an efficiency rate of 70%. Nonetheless, it appears that the data is a bit more complex than this figure would suggest. It would all be a question of dosage , and what the researchers found is not intuitive …

Counter-intuitive results

The 70% efficacy would in fact be the average of two data: the efficacy rate for the injection of half a dose of the vaccine , and for the injection of the entire dose, for which the vaccine is normally designed. ci in order to guarantee optimal protection. But a surprise awaited the specialists: the vaccine was more effective when the injection remained incomplete! Indeed, with a half-dose, the observed effectiveness would be 90%, against only 62% of protection after the injection of the second half, one month later.

Human error or biological phenomenon? The jury is still out. ”  We are going to conduct a detailed investigation to understand exactly why we are getting better efficacy with half a dose, ” said Sarah Gilbert , professor of vaccinology and team leader at the University of Oxford. His intuition is that a half dose better “mimics” what happens when a person is infected. “  It may be that giving a small amount of the vaccine to start and follow with a larger amount is a better way to kick-start the immune system , and give us the strongest and most effective immune response. 

Next step: the ball in the United States

The results did not fail to intrigue scientists who had expected much better protection with two injections. It is unfortunately too late in the testing phases to change the protocol in the UK, but the trials in the US may well be adapted so that a single half dose is injected. The different methodology and means of measurement employed nevertheless make any comparison complex and place researchers in a dilemma. 

The Oxford-AstraZeneca vaccine promises to be cheaper and easier to store than solutions produced by Moderna or Pfizer , and the group has announced plans to produce nearly 3 billion doses next year. However, will it be necessary to judge the effectiveness of the vaccine against its competitors on the basis of a half-dose or a full dose? The debate remains open for the moment.

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