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Nonsynonymous (A) and synonymous (B) divergence, or mean pairwise distance between visits, of envelope gene sequences (753 nt; 10 sequences per study subject per visit, for a total of 20 sequences per subject) was calculated using the method of Nei and Gojobori with the Jukes-Cantor correction. Error bars indicate standard deviations. P values were calculated using a nonparametric comparison of medians.
