In September 2009, a mumps outbreak began in a village that by December 31, 2009 had resulted in 392 cases. During this time, the incidence of mumps continued to steadily rise. Nearly three quarters (72%) of those cases were in children aged 11 - 17 years who had previously been vaccinated with two doses of MMR vaccine. The high rate of 45 transmission in the village's three main schools coupled with the 46 community's large household sizes (on average, 5.7 individuals per household) made case isolation, social distancing, and school closure impractical control measures. Therefore, a third dose of MMR vaccine was offered to children who had not developed mumps during the outbreak and had previously received 2 doses of the vaccine. The present study is the first known study to measure adverse events (AE) occurring after administration of a third dose of MMR vaccine in an outbreak setting. A M-M-R II was offered in offered in vaccination clinics in the three schools in a village from 19-JAN-2010 to 02-FEB-2010. Students in grades 6-12 were eligible for the intervention if they had no history of mumps since 01-JUL-2009, and obtained written consent from their parent or guardian. Clinic staff collected baseline surveys and monitored all children for immediate AE for 15 min after receiving the vaccine. From 05-APR-2010 to 18-APR-2010, follow-up surveys were distributed to all 6-12th grade students in the eligible schools regardless of their participation in the intervention. Information captured on the survey included demographic characteristics, occurrence of mumps or mumps-like symptoms since the intervention, and whether any adverse event was experienced within a 14-day window starting on day of vaccination. The AE were categorized as local (pain/redness/swelling at the injection site) or system (fever, difficulty breathing, rash, dizziness, fainting, and joint or muscle aches). Fever was defined as a temperature 100.4 F. Parents were asked to report if their child experienced any of the above local reactions or systemic symptoms on the follow-up survey. Parents were asked to report if their child experienced any of the local reactions or systemic symptoms on the follow-up survey. Parents were also asked to report if they sought medical attention for any reason during the 14 days post-vaccination, the expected interval of occurrence for vaccine-related reactions; outpatient records were collected and reviewed for individuals who sought medical attention. In order to achieve a comprehensive assessment of serious AE that might have occurred following the intervention, a list of all physician practices was compile to search for occurrences of the following serious AE that may have resulted in death, life-threatening illness, hospitalization, prolongation of hospitalization, or permanent disability: anaphylactic shock; encephalopathy/encephalitis; flaccid hemiplegia; meningitis; Guillain-Barre syndrome; convulsions or seizures; shock thrombocytopenia. Results: Parent-reported AE: Of the 1597 third dose recipients who returned the follow-up survey, 87 (5.4%) reported only one event , 24 (1.5%) reported two, and 4 (0.3%) reported three. No respondents reported more than three AEs. The most frequently reported adverse event was "pain, redness, or swelling at the injection site" (3.6%), which was followed by "joint or muscle aches" (1.8%), "dizziness or lightheadedness" (1.7%), and "fever (100.4 F or higher)" (1.3%). Three individuals reported fainting at any time during the 2-week period following vaccination (0.2%), but only one 12-year-old girl fainted during the 15-min monitoring period following vaccination. Seventeen individuals reported seeking medical attention for an adverse event on the follow-up survey. Outpatient records from 01-JAN-2010 to 30-APR-2010 were available for 9 (52.9%). Of those, 7 were seen for unrelated medical conditions, including respiratory symptoms, gastrointestinal symptoms, urinary problems, and pharyngitis. Physicians diagnosed the other 2 children, whose symptoms included parotitis, with mumps, the first 1 day after receiving the M-M-R II intervention dose and the second 8 days after receiving it. Seven children (41.2%) whose parents reported having sought medical attention on the follow-up survey had no documentation of a clinical visit, and one did not indicate a physician practice at all. Serious AE in the 2 months/allowing vaccination: After staff and physicians at each practice reviewed billing and clinical records, one individual was found to have been diagnosed with a potential serious adverse event during the follow-up period. A 12-year-old female was diagnosed with "thrombocytopenia" when she presented to her physician with upper respiratory symptoms 28 days after receiving a third dose of M-M-R II. Laboratory investigations indicated Group Abeta-hemolytic Streptococci (throat culture) and a complete blood profile (CBP) revealed a platelet count of 95 (normal: 150-400 x 10^9 per liter). A follow-up CBP 2 days later revealed a platelet count of 112 x 10^9 per liter. There were no reported clinical manifestations of thrombocytopenia such as petechiae, purpura, bleeding gums, or nosebleed. Four years prior in 2006, two blood tests were conducted 2 months apart with the first showing platelet counts of 140 x 10^9 and 147 x 10^9 per liter. A discharge summary in 2001 did not indicate low platelet counts. Medical records before 2001 were not available. VAERS: In 2010, VAERS received 2311 reports of AE following any dose of M-M-R II vaccination, however, they did not occur during the period of interest (01-JAN-2010 to 30-APR-2010) and were not associated with a third dose of M-M-R II. Literature review: The proportions of AE found in the present study were lower than or within the range of those reported in prior studies of first- and second-dose MMR vaccine studies. The patient in this report is a female patient who experienced fainting at any time during the 2-week period following vaccination. Discussion: The results of this study suggest that a third dose of M-M-R II administered in an outbreak setting was safe, with injection site reactions reported more frequently than systemic reactions. However, to assess risk for rare or serious AE after a third dose of M-M-R II, longer term studies would be required. This is one of several reports received from the same source. Additional information had been requested.