Previous studies have shown that vaccination and boosting of rhesus macaques with attenuated vesicular stomatitis virus (VSV) vectors encoding Env and Gag proteins of simian immunodeficiency virus-human immunodeficiency virus (SHIV) hybrid viruses protect rhesus macaques from AIDS after challenge with the highly pathogenic SHIV 89.6P (23). In the present study, we compared the effectiveness of a single prime-boost protocol consisting of VSV vectors expressing SHIV Env, Gag, and Pol proteins to that of a protocol consisting of a VSV vector prime followed with a single boost with modified vaccinia virus Ankara (MVA) expressing the same SHIV proteins. After challenge with SHIV 89.6P, MVA-boosted animals controlled peak challenge viral loads to less than 2 x 10(6) copies/ml (a level significantly lower than that seen with VSV-boosted animals and lower than those reported for other vaccine studies employing the same challenge). MVA-boosted animals have shown excellent preservation of CD4(+) T cells, while two of four VSV-boosted animals have shown significant loss of CD4(+) T cells. The improved protection in MVA-boosted animals correlates with trends toward stronger prechallenge CD8(+)-T-cell responses to SHIV antigens and stronger postchallenge SHIV-neutralizing antibody production.