Typically half receive the experimental vaccine, while the remainder are given a placebo, though they do not know which one was administered to them.
The vaccine, called "Sputnik V" has been hailed as safe and effective by Russian authorities and scientists following two months of small-scale human trials.
Researchers say CanSino's vaccine which is co-developed with China's military research unit, appears to be safe and induced immune responses in most subjects.
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