The reaction was detected with a secondary antibody HRP conjugated anti-human IgG (Chemicon, Australia) and enzyme substrate solution, TMB (3,3′,5,5′-tetramethylbenzidine, KPL, USA) followed by a 1 M H3PO4 stop solution. The absorbance (OD) was measured at 450 nm (reference filter 630 nm) on a Bio-Tek Elx808 (Bio-Tek Instruments, USA). OD was converted to antibody concentrations (μg/ml) using KCJunior software (Bio-Tek Instruments, USA). Sample dilutions were analyzed in duplicate and three controls (low, medium and high) were included on each plate to assess assay performance and inter-assay

variation. Results from HDAC inhibitor an inter-laboratory comparison between Wyeth Vaccines and the KTL Finland laboratory demonstrated a good correlation in measurement of serotype-specific antibody concentrations [28]. Laboratory staff members were

blinded to the group allocation of each serum sample. Cleaned data were exported to Stata version 9.0 (Stata Corporation, College Station, Texas) for analysis. Serotype-specific Protein Tyrosine Kinase inhibitor antibody concentrations by ELISA were log transformed (to base e) to calculate GMC. Comparisons of pre- and post-mPPS GMC and between group comparisons were performed using a paired t-test and two sample t-test, respectively. Simple and multi-variable regression analyses were undertaken to adjust for both the pre-mPPS log antibody concentration for all 23 serotypes, and the number of PCV doses check administered for all seven PCV serotypes. A p-value of <0.05 was considered statistically significant. The primary endpoint was serotype-specific

GMC response to mPPS at 18 months of age in children who had received the 12 months 23vPPS compared to children who had not received the 23vPPS. We defined hyporesponsiveness to a particular serotype as a significantly lower GMC observed post-mPPS, in the 12 month 23vPPS group compared to the no 12 month 23vPPS group, controlling for pre-mPPS antibody levels, using multivariable regression analysis. To prevent an inflated type 1 error due to multiple comparisons, and obtain a single p-value for the null hypothesis of mPPS having no impact on the antibody response to any of the 23 serotypes, a joint test of all the regression coefficients from the aforementioned multivariable regression analysis was performed [29]. The study was approved by the Fiji National Research Ethics Review Committee and the University of Melbourne Human Research Ethics Committee. There were 552 children enrolled in the study (Fig. 1) which represent a consent rate of 30.5%. There were 90 (16.3%) withdrawals and no child was withdrawn due to an adverse event resulting from administration of any of the vaccines. Characteristics and the number of children randomized to the eight groups are shown in Table 1. Following the 12 month 23vPPS, there were significantly higher GMC (each p < 0.001) for all PCV serotypes.