It was recently reported - at the National Foundation for Infectious Diseases 15th Annual Conference on Vaccine Research - that the rate of adverse effects from a third dose of the measles, mumps and rubella (MMR) vaccine is the same as those of the second dose. This was conducted as part of a Centres for Disease Control study and led by Glen Abedi, an epidemiologist at the CDC and Masters student. I'm basing this on a media report of the conference as the paper has not yet been published. You can find more results here.
This is the first study to look at the safety of receiving a third dose of the vaccine in school children during an outbreak. The study has some obvious caveats but what it brings up is the question of whether we should extend 3-dose coverage to the population as a whole?