Figure 6.

Possible epistatic effects for vaccine efficacy and R₀. Panel A describes vaccine efficacy. Efficacy is bounded at 1.0 and we assume that no antigen would be considered with a base efficacy less than 0.5. Here, the vector has an efficacy of 0.0, a vaccine with one antigen has an efficacy of 0.8, and a vaccine with two antigens can have an efficacy between 0.8 and 1.0 depending on the degree of negative epistasis. Panel B reflects the reduction in transmission resulting from expressing a second antigen. Here, the vector has an $R_0^0$ of 4, vaccine strains with one antigen have an $R_0^1$ of 3, and the pure vaccine with two antigens can have an $R_0^V$ between 1 and 3. Although $R_0^V$ is shown here, epistasis is actually only affecting one of its components, the transmission rate.