I am not all qualified to answer this but I will guess. I would assume "separation of duties". A Syringe's job is to be available to administer any vaccine at t

I am not all qualified to answer this but I will guess. I would assume "separation of duties". A Syringe's job is to be available to administer any vaccine at the time of delivery. If you prefill a syringe, you now have to ensure it is only used for that vaccine purpose and then you also have to label it properly. More work. Vaccine package anyway needs to be properly labeled but now you are introducing another overhead to track/manage/label the syringes. Logically, it makes sense to keep the 2 separate. This way, you can use any good syringe when needed while ensuring there is no confusion on which vaccine was administered. Also, this ensures that you don't need a 1-1 relationship between syringe and vaccines. Lets say you produced a million vaccines. There is no need to produce a million syringes at the same time. You can just ship the vaccine to whichever clinic/hospital/authority and they can decide how to distribute and get syringes at the time. How is this for a guess ?