It doesn't have anything to do with commonest choices, or with any particular region of the world. The obvious answer is for.
You could try to argue that against could make some sense in the context, but there's no reason to think it's a better answer than for, given the limited context we have.
It's not that for and against are two different ways of saying the same thing—it's that they are two ways of saying different things. The sense of for here, as it so often does, relates to special purpose. There are vaccines designed for those diseases, but there isn't one for this one.
Probus—I think you're confusing this context with others that you may have in mind, where against is indeed the correct preposition. Look:
They've been vaccinated against polio.
Here, for is not appropriate, as there is no sense of special purpose. Remember that different prepositions have different meanings.