Plasmodium falciparum circumsporozoite vaccine immunogenicity and efficacy trial with natural challenge quantitation in an area of endemic human malaria of Kenya

It has been hypothesized that antibody induced by Plasmodium falciparum circumsporozoite protein vaccine would be effective against endemic human malaria. In a malaria endemic region of Kenya, 76 volunteers, in 38 pairs sleeping adjacently, were immunized with subunit circumsporozoite protein Asn-Ala-Asn-Pro tetrapeptide repeat-pseudomonas toxin A, or hepatitis B vaccine. After quinine and doxcycycline, volunteers were followed for illness daily, parasitemia weekly antibody, T-lymphocyte responses, and treated if indicated Anopheles mosquitoes resting in houses were collected, and tested for P. falciparum antigen, or dissected for sporozoites and tested for blood meal ABO type and P. falciparum antigen. Vaccine was safe, with side-effects similar in both groups, and immunogenic, engendering IgG antibody as high as 600 μg ml-1, but did not increase the proportion of volunteers with T-lymphocyte responses. Estimation of P. falciparum challenge averaged 0.194 potentially infective Anopheles bites/volunteer/day. Mosquito blood meals showed no difference in biting intensity between vaccine and control groups. Both groups had similar malaria-free survival curves, cumulative positive blood slides, cumulative parasites mm-3, and numbers of parasites mm-3 on first positive blood slide, during three post-vaccination observation periods. Every volunteer had P. falciparum parastiemia at least once. Vaccinees had 82% and controls 89% incidences of symptomatic parasitemia (P=0.514, efficacy 9%, statistical power 95% probability of efficacy <50%). Vaccine-induced anti-sporozoite antibody was not protective in this study. Within designed statistical precisions the present study is in agreement with efficacy studies in Colombia, Venezuela and Tanzania.