The Oxford COVID-19 Vaccine: Here’s What We Know So Far

AstraZeneca’s COVID-19 vaccine candidate, which late-stage trials showed could be up to 90 percent effective, uses a modified version of a chimpanzee common cold virus to deliver instructions to cells to fight the target virus, a traditional approach to vaccine development. 

The effectiveness of AstraZeneca’s vaccine depended on the dosing, and fell to 62 percent when given as two full doses rather than a half-dose first.

Scientists have cautioned against seeing this as evidence it would be less useful than rivals. Pfizer and Moderna vaccines each prevented about 95 percent of cases according to interim data from their late-stage trials. 

Andrew Pollard, director of the Oxford Vaccine group, which invented the vaccine, said that work is ongoing to determine exactly why the half-dose/full-dose worked better. 

He added, “We think that by giving a smaller first dose we are priming the immune system differently, we are setting it up to respond.”


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