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Abstract: Background: To investigate the safety and immunogenicity of a modified vaccinia virus Ankara vector expressing HIV-1 antigens from clade B (MVA-B), a phase-I, doubled-blind placebo-controlled trial was performed. Methods: 30 HIV-uninfected volunteers at low risk of HIV-1 infection were randomly allocated to receive 3 intramuscular injections (1×108 pfu/dose) of MVA-B (n =24) or placebo (n =6) at weeks 0, 4 and 16. All volunteers were followed 48 weeks. Primary end-points were adverse events and immunogenicity. Results: A total of 169 adverse events were reported, 164 of grade 1–2, and 5 of grade 3 (none related to vaccination). Overall 75% of the volunteers showed positive ELISPOT responses at any time point. The magnitude (median) of the total responses induced was 288SFC/106 PBMC at week 18. Antibody responses against Env were observed in 95% and 72% of vaccinees at week 18 and 48, respectively. HIV-1 neutralizing antibodies were detected in 33% of volunteers. Conclusions: MVA-B was safe, well tolerated and elicited strong and durable T-cell and antibody responses in 75% and 95% of volunteers, respectively. These data support further exploration of MVA-B as an HIV-1 vaccine candidate. Clinical Trials.gov identifier: NCT00679497. [Copyright &y& Elsevier]