The studies outlined above strongly suggest that only intranasal immunization can successfully afford protection against Ebola virus in the absence and presence of pre-existing immunity. It was also clear that additional strategies were needed to improve vaccination by either the oral or intramuscular route in the presence of preexisting immunity. Covalent attachment of activated monomethoxypolyethylene glycol to the protein capsid of the Ad5-ZGP vector was evaluated as a potential way to improve the immune response after oral vaccination. Based upon previous observations, we hypothesized that this modification would protect the virus from neutralization by immune sera and improve survival in gastrointestinal tract [26, 27, 30, 44] . The frequency of Ebola glycoprotein peptide-specific activation of IFN-c positive CD8+ T cells was not significant in naïve animals given the PEGylated virus (0.360.3%) with respect to those given the unmodified virus (2.060.5%, p = 0.09, Figure 6A ). Similar results were also seen in the presence of pre-existing immunity in either treatment group (0.460.2% PEGylated vaccine, 1.760.5%, unmodified vaccine).