In the United States, a third measles-mumps-rubella (MMR) dose (MMR3) is suggested for people at higher risk for mumps during epidemics. MMR3 is also likely to be given to people who may have gotten two MMR doses but do not have documentation. Researchers described adverse occurrences in people who received MMR3 in a non-outbreak environment because MMR3 safety data is lacking. MMR3 was given to young adults who had already received 2 MMR doses. Participants reported daily on 11 requested, frequent symptoms potentially connected with MMR from 2 weeks prior to 4 weeks after receiving MMR3. Poisson regression was used to assess weekly rate differences in post-vaccination vs. pre-vaccination (baseline). To determine the number and percentage of individuals with an increased risk of symptoms after MMR3, baseline rates were subtracted from post-vaccination rates of significantly different symptoms. Three post-vaccination injection-site symptoms were subjected to descriptive analysis. The 662 participants’ age ranged from 18 to 28 (median=20), with women accounting for 56% of the total. Post Vaccination rates of headache, joint pain, diarrhea, and lymphadenopathy were considerably more significant than baseline. The researchers estimated that 119 participants (18%) reported higher symptoms after MMR3 vaccination than before the vaccine. Following MMR3, 13%, 10%, 8%, and 6% of people had increasing symptoms of headache, joint difficulties, diarrhea, and lymphadenopathy, respectively. The median onset was 3–6 days after vaccination, with a 1–2 day duration. One visit to the doctor for a possible vaccination-related illness (urticaria). 163 participants (25%) experienced injection-site symptoms, with a median duration of 1–2 days. Mild and temporary systemic and local events were reported. MMR3 is considered to be safe and tolerated in young adults.