Fig. 3.

Linear regression analysis of the relationship between Env divergence from vaccine strain and protection from detectable infection. The percentage of animals protected from detectable infection with viral strain in each trial group was plotted as a function of the level of amino acid divergence of the challenge strain envelope from the vaccine strain envelope. Linear regression (gray line) analysis (GraphPad Prism v. 4.0) revealed a direct, significant inverse linear relationship between the two variables, with an r² value of 0.99 (P = 0.02). Hence, “X” is predictive of “Y,” or divergence away from the Env of the vaccine strain is predictive of protection from detectable infection by challenge strain.