On February 15, 2012, a high number of influenza cases were reported to the Provence-Alpes-Côte d'Azur (PACA) Regional Health Agency (ARS) among residents and staff at the Marseille Departmental Gerontology Center (CGD). Given the scale of the outbreak, an epidemiological investigation was launched to characterize the outbreak, guide control measures, and assess the effectiveness of the influenza vaccine. A total of 104 probable cases of influenza were identified between January 10 and February 29, 2012: 75 residents and 29 staff members (attack rates of 21% and 11%, respectively). Seventeen cases were biologically confirmed as influenza type A, with the H3N2 subtype identified in several cases. The epidemic peak was reached in week 6. Several wings were affected. Vaccination coverage (VC) was measured at 88% among residents and 20% among staff. Vaccine efficacy (VE) was estimated at 39% among residents and 70% among staff. Droplet-based measures and antiviral treatment for residents were implemented. The financial cost to the facility was estimated at 26,000 EUR. This influenza outbreak was large-scale due to its duration and the number of recorded cases, disrupting the facility’s operations for several weeks. The occurrence of this outbreak may be linked to the circulation of the H3N2 virus following two seasons in which the pandemic H1N1 virus had been predominant. The low vaccination coverage among residents, combined with very low vaccination coverage among staff, may have facilitated the spread of the outbreak within the facility. This outbreak underscores the importance of achieving high vaccination coverage, particularly among healthcare staff, and of identifying the cause of the illness as early as possible when clusters of cases occur in order to rapidly implement containment measures. (R.A.)