How common are adverse reactions after the second dose of MMR?
We know that on average 95% (range, 90%-98%) of vaccinees respond to the first dose of MMR vaccine. Therefore, only 5% (range, 2%-10%) would be susceptible to adverse events associated with the 2nd MMR. We also know 5%-15% of those vaccinated experience one of the more common adverse events associated with MMR (e.g., fever or rash). Therefore, we would expect that only about 5% of the 5% who are still susceptible at the 2nd MMR vaccination would experience an adverse reaction.
