A study of the side effects of MMR in Iran (43,000 vaccinated children). 1.8% of the vaccinated contracted mumps as a result of the vaccination. Two suffered encephalopathy and two more had anaphylactic shock. That is, the risk of encephalopathy and anaphylactic shock is about 1 in 20,000, rather than "one in a million", as usually claimed.
The risk of convulsions (not febrile) was 1 in 2,000 vaccinations in infants. The risk of febrile seizures was 1 in 1750.