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> (AstraZeneca using meningitis vaccine as control instead of true placebo).

AFAICR, this was done on purpose to prevent people from figuring out they got a vaccine shot, because an inert placebo wouldn't cause any side effects.

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A shot with saline would help demonstrate some of the side effects (localized aches or damage to the muscle), but would not elicit the same response as different vaccine with roughly known side effects (feedback on vaccine administration reactions is far from ideal). The other possibility is minimizing the appearance of side effects by comparing against the control group.

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