MVA was chosen as the poxvirus vector to boost immunity induced by GeoVax DNA priming vaccination because of its safety features and because of the excellent protective responses that it stimulated in preclinical (monkey) models.
MVA was originally developed as a safe smallpox vaccine for use in immuno-compromised humans by further attenuating the standard smallpox vaccine. During this attenuation (loss of disease causing ability), MVA also lost essentially all of its ability to replicate in human cells. The attenuation was accomplished by making over 500 passages of the virus in chicken embryos or chick embryo fibroblasts (CEF). During passage the virus underwent 6 large genomic deletions. These deletions affected the ability of MVA to replicate and cause safety problems in humans, but did not compromise the ability of MVA to grow on the CEF cells that are required for manufacturing the virus.
The effectiveness of MVA as a vaccine vector is also accounted for by its loss of immune evasion genes during its passages in CEF cells. During the years of the dreaded human smallpox epidemics these immune evasion genes assisted the spread of smallpox infections, even in the presence of human immune responses.
MVA was safely administered to over 120,000 people in the 1970's to protect them against smallpox. With the advent of bioterrorism, our choice of the MVA vector becomes even more important, because of its potential for immunization for smallpox. GeoVax HIV vaccines may serve as both an HIV and a smallpox vaccine.