As you said, the exposure amount is quite low, so barring something really toxic, I wouldn't expect a danger from chemical reactions.

However, the body is a complex system; if something in the vaccine triggers a change in function, that could lead to side effects that are only detected years later; most likely through epidemiologic studies comparing those who got the different types of vaccines with those who didn't.

Of course, the disease itself has potential long term effects as well; and has already been studied to have significant medium term effects.