Information has been received from a published literature article, refers to 264 male of 9th-12th grade. It was stated in the article that 2009-2010, a large mumps outbreak occurred affecting two-dose vaccinated 9th-12th grade boys attending school. Our objective was to understand mumps transmission dynamics in this well-vaccinated population. We surveyed 9th-12th grade males with reported mumps case-students between 01-SEP-2009 and 30-MAR-2010. We assessed vaccination coverage, environmental factors (duration of school day, density, mixing, duration of contact), and whether environmental factors were associates with increased mumps attack rates. Ten schools compromising 1769 9th-12th grade students and 264 selfreported mumps cases were included. The average school attack rate was 14. So/o (median: 13.5%, range: 1-31%), despite two-dose measles-mumps-rubella vaccine coverage between 90-100%. School duration was 9-15.5 h/day; students averaged 7 h face-to-face/day with 1-4 study partners. Average daily mean density was 6.6 students per 100  square feet. The number of hours spent face-to-face with a study partner and the number of partners per day showed significant positive associations (p<0.05) with classroom mumps attack rates in univariate analysis, but these associations did not persist in multivariate analysis. This outbreak was characterized by environmental factors unique to the setting (e.g. densely populated environment, prolonged face-to-face contact, mixing among infected students). However, these features were present in all included schools, limiting our ability to discriminate differences. Nonetheless, mumps transmission requires close contact, and these environmental factors may have overwhelmed vaccine-mediated protection increasing the likelihood of vaccine failure among students. Inclusion. Of 93 schools with mumps cases reported to the Department of Health and Mental Hygiene (DOH), 57 did not meet the inclusion criteria (e.g., all female, not high school level, outside area) and 23 lacked contact information, leaving 13 (14%) school that met the inclusion criteria. Of the remaining 13, 11 (85%) completed the classroom survey where students were asked to selfreport that had mumps-like illness, whether they saw a healthcare provider for mumps symptoms, and whether they stayed home from school for mumps. One school was subsequently excluded because the cases were not in grades 9-12. Two schools did not respond to multiple attempted contacts. Thus, 10 schools comprising 1769 9th-12th grade students and 264 self-reported mumps cases met the inclusion criteria, completed the classroom survey, and were assessed. Although cases were selfreported, 241 (91%) reported visiting a healthcare provider for their symptoms and 100% were either sent home or stayed home from school. Vaccination coverage. In the 10 schools, 9th-12th graders compromised 39 grades and 63 classes (4 schools had multiple classes per grade). All 10 schools had overall one- and two-dose MMR coverage rates between 97-100% and 90-100%, respectively. Three 8%) grades had two-dose vaccination coverage rates< 90% (i.e., 82%, 87% and 89%), but all other grades had two-dose coverage rates equal or more than 90% (average:95%). The school setting was characterized by close contact: students averaged 7 h of face-face to exposure to an average of 2.5 meetings per day. Close contact is essential for mumps transmission; mumps spreads through respiratory droplets or secretions. To become infected, a person must be within three feet of an infected person or have direct contact with his or her secretions. Although in multivariate analysis we did not find a significant association between classroom mumps attack rates and face-to-face time or number of meetings/day (factors that may be critical determinants for spread of mumps among vaccinated persons), we postulate that these factors did not vary sufficiently across the included schools to allow significant difference to be discriminated. Nonetheless, the dynamics of prolonged face-to-face exposures with multiple meetings may have created a high enough force of infection to overwhelm vaccine-mediated protection and may have increased the likelihood of vaccine failure among school students. This paper describes class attack rates that were as high as 42%, despite high two-dose MMR vaccine coverage. Even with opportunities for mumps introductions in the surrounding community, the exposures were likely less frequent and less intense than in school, so the high vaccination coverage generally provided adequate population immunity for protection. These findings are consistent with a large mumps outbreak in 2009 where most cases also occurred among well-vaccinated adolescent males with minimal spread to the broader community, despite regular mixing with others. Mumps outbreaks have also occurred in other settings among highly vaccinated populations. Two MMR vaccine does provide 66-95% vaccine effectiveness against mumps. The two-dose policy has reduced mumps incidence by> 99% compared with the pre-vaccine era. Despite high two-dose MMR vaccine coverage, the average attack rate was 14.5% which was 10% higher than the 4.6% attack rate among two-dose vaccinated college students living in affected dormitories in the 2006 mumps outbreak, 20 and almost as high as attack rates during the one-dose era. Although it is possible that lesser-vaccinated grades seeded the outbreak ([8%] grades had two-dose MMR coverage< 90%), none of these had the highest grade-level attack rates, nor were they in school with the highest attack rates. Based on the age range of the students in the school included in this analysis (i.e., 13-18 y) and the ACIP guidelines for the standard age for receipt of the second MMR vaccine dose, it is likely that these students received their most recent dose between 7-14 y prior. Due to the limitations of an ecological design, we were unable to report individual-level data, such as identify who the classroom index case-students were, report the severity of their illness (as a proxy for infectiousness), document who were linked as friends, compare the attack rate among students who had zero of 1 dose of MMR vaccine, or identify who was uniquely exposed to whom. Two MMR doses are sufficient to prevent mumps in most settings, as evidenced by the broader community remaining generally protected from mumps during this outbreak. The unique school setting, with the density populated environment and prolonged face-to-face contact, may have led to viral inoculum that overcame immunity, resulting in vaccine failure. However, had vaccination coverage not been so high, there would likely have been many more cases, with disease that was more severe. Maintaining high MMR vaccination coverage remains the most effective way to prevent future outbreaks and limit their spread. MMR vaccine doses were considered valid if the first dose was given on or after the first birthday, the second dose at least 28 d later and if dates were recorded. Classroom survey. All schools that met the inclusion criteria were sent classroom surveys that asked administrative staff to report the number of students, classrooms and grades in the school. Students were asked to self-report whether they had mumps-like illness, which was defined as fever and swelling in the jaw or cheeks, from 01-SEP-2009 to 20-MAR-2010; they also reported whether they saw a healthcare provider for mumps symptoms and stayed home from school for mumps. Additional information has been requested.