In November 2009, a medical-educational institute (IME) for children and young adults with intellectual disabilities reported eight suspected cases of 2009 influenza A (H1N1) to the Departmental Directorate of Health and Social Affairs. At that stage of the pandemic, surveillance of clusters of cases was limited to populations at risk of complications. The investigation conducted by the Eastern Region Health Surveillance Unit at the IME was designed to characterize the outbreak, identify factors promoting viral transmission, and adapt control measures. A retrospective cohort study was conducted in the study population, defined as anyone working or residing at the institute between November 14 and December 14, 2009 (n=235). Questionnaires administered to each case documented sociodemographic and clinical characteristics, transmission factors, and the measures implemented. All samples collected were positive (n=3), thus confirming the outbreak. The outbreak lasted more than three weeks with 37 reported cases (attack rate (AR)=15.7%), the majority of whom were residents (n=25, AR=21.9%). The analysis revealed an increased risk for boarding students (relative risk (RR) = 2.2; p = 0.03) and, among children under 13 years of age, for those attending specialized classes (RR = 4.5; p = 0.002). Similarly, educators working at the boarding school were significantly more affected (p=0.02). Despite early and appropriate containment measures, the school had to resort to widespread vaccination and closure to control the outbreak. The scale of the outbreak and the identified factors contributing to viral spread appear to be more closely linked to the school’s organizational structure—which facilitates contact and socialization among children with developmental disabilities—rather than to any particular vulnerability of the affected subpopulations. The limited effectiveness of barrier measures in this context suggests that this type of facility should be considered a priority site for vaccination, not so much because of the children’s vulnerability to the disease, but due to the increased risk of an outbreak and the disruption of daily routines, which is detrimental to these children in need. (R.A.)