For a child who will react to the MMR, for them that risk is 100%. Not 1 in a million.
And there are levels of knowing. No one would be certain that the MMR would have any bad affects on him after the DTP did, but it would be recommended that he didn't have it or any further vaccinations. His risk increases with the knowledge but that doesnt mean there are any certainty he will suffer from side effects of the MMR.
People can and do work out their own risk. Two of my children are unvaccinated against measles. One becaue he is too young and the other because she suffers from a number of autoimmune conditions where it has been recommended that she holds off for a while. My eldest with autism has had it, though it didn't cause his autism or as far as I can see make it any worse.
We've just been to a playground where a mother openly pushed around in a pram her child who had measles, declaring the fact
. Now that IS selfish. However I still won't be getting my children jabbed for the moment despite regular letters from the PCT and the GP.