Background. The development of an effective human immunodeficiency virus (HIV) vaccine is a high global priority. Here, we report the safety, tolerability, and immunogenicity of a replication‐defective recombinant adenovirus serotype 5 (rAd5) vector HIV‐1 candidate vaccine.

Methods. The vaccine is a mixture of 4 rAd5 vectors that express HIV‐1 subtype B Gag‐Pol fusion protein and envelope (Env) from subtypes A, B, and C. Healthy, uninfected adults were randomized to receive 1 intramuscular injection of placebo () or vaccine at dose levels of 10⁹ (), 10¹⁰ (), or 10¹¹ () particle units and were followed for 24 weeks to assess immunogenicity and safety.

Results. The vaccine was well tolerated but was associated with more reactogenicity at the highest dose. At week 4, vaccine antigen–specific T cell responses were detected in 28 (93.3%) and 18 (60%) of 30 vaccine recipients for CD4⁺ and CD8⁺ T cells, respectively, by intracellular cytokine staining assay and in 22 (73%) of 30 vaccine recipients by enzyme‐linked immunospot assay. Env‐specific antibody responses were detected in 15 (50%) of 30 vaccine recipients by enzyme‐linked immunosorbant assay and in 28 (93.3%) of 30 vaccine recipients by immunoprecipitation followed by Western blotting. No neutralizing antibody was detected.

Conclusions. A single injection induced HIV‐1 antigen–specific CD4⁺ T cell, CD8⁺ T cell, and antibody responses in the majority of vaccine recipients. This multiclade rAd5 HIV‐1 vaccine is now being evaluated in combination with a multiclade HIV‐1 DNA plasmid vaccine.