Presumably they're using different syringes for different people in any one sitting, right? If there were a plan to inject some people with something different,

Presumably they're using different syringes for different people in any one sitting, right? If there were a plan to inject some people with something different, the people administering the vaccine could hypothetically decide which syringe to use based on some tiny, subtle marking or alteration that the public wouldn't notice. If you could see that every syringe used was filled from the same "output port" (don't know the actual term) on the same storage device, then it'd be a lot harder to accuse them of anything.