In 2001, an outbreak of typhoid fever occurred among the members of the French Armed Forces. All had received a typhoid vaccination as per the immunization schedule practiced in the Armed Forces (every 5 years). A retrospective cohort study was conducted in 94 personnel. The objectives were to confirm the diagnosis, determine the source of contamination and identify the factors associated with defective vaccinal efficacy. Twenty-four cases were clinically identified. A cucumber salad was identified as the contaminating dish (Risk Ratio=3.6; 95%CI 1.5–8.9). Only one factor was related to defective vaccinal efficacy; the risk of typhoid fever was twofold higher in people vaccinated more than 3 years previously (Risk Ratio = 2.2; 95%CI, 1.1–4.2). Compliance with food hygiene rules could have prevented 24 cases of typhoid fever. Nevertheless, repeat vaccination against typhoid fever is now conducted every 3 years in the French Forces, in compliance with the manufacturers’ recommendations.