The epidemiology of multiple Plasmodium falciparum infections - 12. Reduction in the mean number of Plasmodium falciparum genotypes in Gambian children immunized with the malaria vaccine SPf66

SPf66, a synthetic peptide Plasmodium falciparum vaccine, did not protect y oung Gambian children against clinical attacks of malaria. Nevertheless, Ga mbian children who had been vaccinated with SPf66 and who were parasitaemic at the end of the first malaria transmission season after vaccination had significantly fewer detectable P. falciparum genotypes than control childre n, as determined by polymerase chain reaction analysis of 3 polymorphic loc i-the msp1 block 2 repeat region, the msp2 repeat region, and the R11 regio n of the glutamate-rich protein gene (glurp). Geometric mean numbers of gen otypes were 1.66 vs. 1.87, 1.95 vs. 2.43, and 1.21 vs. 1.50 for msp1, msp2 and glurp, respectively (P=0.31, P=0.04 and P<0.01). Differences between gr oups became;I little more marked for msp1 and msp2 when children with sympt omatic malaria were excluded. Mo significant difference was found between p arasites obtained from SPf66-vaccinated or control children in the prevalen ces of amino acid alleles at positions 44 and 47 in the II amino acid seque nce of the merozoite surface protein 1 molecule, which is present in SPf66. The reduction in the number of genotypes observed could not be explained b y a difference in parasite densities between SPf66-vaccinated and control c hildren, as geometric mean parasite densities were almost identical in the 2 groups. These observations suggest that SPf66 vaccine may have induced an immune response which reduced the incidence of new infections in immunized children or accelerated the rate of clearance of parasites of individual g enotypes. However, no reduction in the prevalence or density of parasitaemi a was recorded in SPf66-vaccinated children, suggesting the existence of so me kind of density-dependent mechanism for controlling low levels of malari a parasitaemia.