Abstract: Efficacy of vaccination with Brucella abortus S19 vaccine as control measure against bovine brucellosis has been controversial; therefore, it is necessary to know the efficacy of this vaccine under different field conditions. In this study, a clinical assay was performed to establish the efficacy of this vaccine on double purpose cattle. Two groups of one hundred animals each were formed. Infected cattle were not eliminated or segregated. One herd was identified as infected, with four animal reactors to Card Test (CT) and confirmed by Rivanol Test (RT) with a serum reaction rate of 1.2%. Confirmation of infected herd was carried out by isolation and identification of Brucella abortus colonies and PCR on milk samples from RT reactor animals. In 18 months, the number of infected animals increased to eight females, seven within the non-vaccinated group and one in the vaccinated group for a serum reaction rate in the non-vaccinated group of 5.8% and the vaccinated one of 0.8%. Thus, in this period the accumulated serum reaction rate for both groups was increased from 1.2 to 3%. Vaccination efficacy of strain S19 was 86% and the risk of getting the disease in these animals was very low (RR = 0.112; I.C.95% 0.014-0.887). It is concluded that strain S19 vaccine is efficacious in the control of brucellosis in herds with a 3% prevalence of the disease; yet, before its use, interference of diagnosis problems that are produced should be assessed to properly evaluate economics and vaccination efficiency.