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Background: Development of multiple antigens in combined vaccines offers the advantages of reducing costs, increasing compliance and provides dual protection. Hepatitis A is an endemic disease in Mexico and hepatitis B, notwithstanding low prevalence, confers risk of progression to cirrhosis, hepatocellular carcinoma, and high medical costs in consequence., Objective: Determine immunogenicity and reactogenicity of a combined vaccine when compared with use of conventional vaccines simultaneously., Methods: The present study was a prospective, open, and randomized trial; 73 healthy children and adolescents were included, all with negative serologic markers. They were assigned to one of the following groups: Group 1, combined vaccine (n = 49) Twinrix (HAV 720 UE/HBV 20 micrograms), and group 2, separate vaccines (n = 24) Engerix B 20 micrograms/Havrix 720 UE. Both groups were given two-dose series at months 0 and 6. Geometric titles of antibody production (GMT) anti-HAV and anti-HBV were determined in months 1, 2, 6 and 7. Adverse reactions were registered during the study., Results: No difference was observed between the two groups in age or gender. Immunogenicity anti-HAV: 100% of vaccines in both groups reached seroprotective levels (> or = 33 mUI/mL). Antibody titles in group 1 were three times higher than those in group 2 (9,696 mIU/mL vs. 3,940 mIU/mL [p = 0.003]) at the end of the study. Immunogenicity anti-HBV: All subjects in both groups reached seroprotective levels (> or = 10 mIU/mL) with similar antibody titles at the end of the study (group 1: 5,603 mIU/mL vs. group 2: 5,201 mIU/mL [p = 0.55 NS]). Reactogenicity: No serious adverse reactions were observed; main were local, and frequency and characteristics were similar in both groups., Conclusions: Seroprotective levels and reactogenicity obtained from use of a combined vaccine against hepatitis A/B are acceptable when compared with use of conventional vaccines administered separately.