Nurses are no longer taught to aspirate the plunger to check for blood when injecting vaccines because they are told they are completely inert. Some still do out of habbit, but its not official procedure. There is a nonzero chance the needle will pierce a blood vessel and the vax will be injected directly into the bloodstream. This explains why the lethality is rare. It is a sum of the probabilities of mechanics and vulnerability of the recipient to the inherently dangerous immune suppressants.