Figure 1. Serum IgG antibody levels to Spn proteins PhtD, LytB, PcpA, PhtE and Ply in healthy children increased with age. Natural acquisiton of serum IgG antibodies to Spn proteins PhtD, LytB, PcpA, PhtE and Ply. Plots of the geometric mean titers (end point titer) during 7 sampling visits at 6, 9, 12, 15, 18, 24 and 30 mo of age. The y axis is changed to log2 scale. The numbers of sera included at each time point were 107, 88, 65, 61, 55, 44, and 6. A linear regression fit from 9 to 30 mo is superimposed.