The wakefield and o'leary studies do posit a mechanism.
It involves a PCR DNA test to find the M of the MMR in the intestines and lymph nodes.
There is quite a bit out there out that you are not likely aware of....
That is where the vested interest (bias)(motivation) issue comes into play, I believe. If you have that, your eyes are looking for the clues and your brain is putting the links in the chain together.