CD4 and infection: After 10, 13 and 20 months post-infection the frequency of non-S-specific cells significantly decreased to 0.019% (p = 0.0007); 0.026% (p = 0.02) and 0.025% (p = 0.01), respectively for CD4 + memory T cells, and 0.040% (p = 0.03); 0.030% (p = 0.004) and 0.023% (p = 0.001), respectively, for CD8 + memory T cells (Fig. 1C).