The first measles vaccine was inactivated, but after several years of common use, it turned out that it led to atypical measles, as well as to pneumonia and encephalopathy. The first attenuated vaccines were so potent that an immunoglobulin injection was also required with the vaccine administration. As a consequence, in 1965 and in 1967 the vaccine was attenuated again and again.Adverse Reactions Following Immunization with MMR Vaccine in Children at Selected Provinces of Iran. 2011, Esteghamati, Arch Iran Med
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.
A study of 48 children who suffered encephalopathy after MMR. The authors conclude that there is likely to be a causal link between MMR and encephalopathy.