Kat, I apologise for fobbing you off with hand waving, but that is because I don't either know the taste you describe, nor do I know what preservatives, adjuvents, solvents and the like they put into the injection formulation. That is why I did not answer at first, because I thought that someone better in formed would have chimed in. But since no one has...

So the best I can do is offer a generalisation and some obviosities in case you had not come to similar conclusions yourself.

Firstly, you know of course, that there are a number of components in such a brew, for all sorts of purposes, ranging from preservation and prevention of clotting to promoting absorption, combating inflammation,  and so on. 

Well, various substances migrate to various parts of the body, depending on their chemistry and interaction with your physiology. They might get there in their original form, or modified by your enzymes, or they might cause the formation of different effective compounds from some of your bodily components.

One example that springs to mind is that a patient who receives a thiamine injection can vividly detect thiamine's yeasty smell on his own skin, even before the injection is complete. Some substances stimulate the sense of smell directly from inside, and others the sense of taste. Sometimes it is because the substance is secreted into one's saliva, or your oral or pharyngeal secretions. It also might be, not because of a taste as such, but because the active ingredient affected the way in which your taste buds interpreted some of the substances normally present in your mouth.

As you can see, the range of reasonable possibilities is far too large for any definitive answer without knowing the ingredients of the vaccine or being in a position to try some experiments to determine what is happening and what was responsible for the effect.

Sorry about that.

Jon