This study clearly shows that there are interference IgGs in the 2-3 year old children who are the main targets of a blood-stage vaccine. From a vaccine development point of view, one of the other concerns is whether the AMA1 vaccination by itself induces such interfering IgGs. In our previous study, we did not detect any interference effect of non-AMA1 IgG from U.S. vaccinees [27] . That study suggests that the AMA1 vaccination per se is unlikely to induce the interference IgG at least in a malaria naïve population. In this study, the volume of plasma from each Malian child was too small to make sufficient amounts of AMA1-specific and/or non-AMA1 IgGs for experiments. Even if we collected a larger amount of plasma from each individual child, it is practically impossible to differentiate AMA1-specific and/or non-AMA1 IgG induced by the vaccination from those induced by a natural infection in children living in a malaria endemic area. For that reason, it is difficult to exclude the possibility that AMA1 vaccination induces interfering IgGs in the target population. However, as shown in Figure 4 , non-AMA1 IgGs from children immunized with the AMA1 vaccine showed the same (AMA1-3) or less (AMA1-1 and AMA1-2) interference than those from children without AMA1 vaccination with the same AMA1 levels in the original total IgG pools. Therefore, there is no evidence to suggest that AMA1 vaccination induced more interfering IgGs in this study, rather the data from AMA1-1 and AMA1-2 indicates that the interference antibody may be induced by natural infection, not by the AMA1 vaccination. AMA1 is a highly polymorphic protein [44, 45] and it is obvious that polymorphism is one of the major obstacles to make a broadly effective vaccine [11] . Indeed, when AMA1 vaccines are administered in malaria naïve individuals, the anti-AMA1 antibodies induced by vaccination show growth-inhibitory activity against the homologous strain of parasites, but weaker or no activity against heterologous strains of parasites, regardless of adjuvant used [16, 19, 22] . In this study, we investigated both 3D7 and FVO strains of parasites and the results demonstrate that the interference effect occurs in both allelic forms of AMA1. However, we did not test the heterologous combination, e.g., a mixture of non-AMA1 (3D7) IgGs and US-total IgG tested against FVO strain of parasites. Because the non-AMA1(3D7) IgGs still had anti-AMA1(FVO)-allele-specific antibody in the preparation, which was confirmed by ELISA and GIA (data not shown), it is very difficult to interpret the result of mixture experiments with FVO parasites. We tested a tandem purification method (i.e., total IgGs were applied onto an AMA1(3D7) purification column and an AMA1(FVO) column sequentially) using rabbit anti-AMA1 antibodies, but there were technical problems with completely depleting anti-AMA1(3D7) and anti-AMA1(FVO)-allele-specific antibodies from the total IgGs (data not shown). Further studies could be conducted to test the cross-reactivity of the interfering antibodies if enough volume of starting blood materials is available.