In late December 2003, a long-term care facility in the Pyrénées-Atlantiques region reported to the DDASS that approximately 30 cases of flu-like illness had occurred among residents, most of whom had been vaccinated against the flu. Given the persistence of the phenomenon, and in a European context where the Fujian viral strain—not included in flu vaccines—was circulating, a microbiological and epidemiological investigation was launched to characterize the outbreak and issue recommendations for its control. A suspected case was defined as a resident or staff member who, since December 15, 2003, had presented with a sudden deterioration in general health, possibly accompanied by respiratory and/or gastrointestinal symptoms, associated with a fever > 38 °C. The cohort study included 136 residents and 60 staff members, of whom 134 (99%) and 4 (7%), respectively, had received an influenza vaccination during the fall. The outbreak affected 57 residents (incidence rate = 42%), of whom 4 died (7%), as well as 19 staff members (incidence rate = 32%). The Fujian-type influenza A (H3N2) strain was identified. Vaccine efficacy was 59% [50%–66%] among residents. Attack rates showed significant differences by ward and staff category. Affected residents had a higher dependency score (GIR), were more likely to reside in a shared room, and more often had a sick neighbor. They were more frequently granted permission for unrestricted outings. This study confirmed the epidemic nature of the event and enabled the implementation of appropriate control measures, including antiviral treatment with oseltamivir. The main recommendation focused on encouraging vaccination among healthcare staff. (R.A.)