Low-level malaria infections detected by a sensitive polymerase chain reaction assay and use of this technique in the evaluation of malaria vaccines in an endemic area

Abstract

The feasibility of using a sensitive polymerase chain reaction (PCR) to evaluate malaria vaccines in small group sizes was tested in 102 adult Gambian volunteers who received either the malaria vaccine regimen FP9 ME-TRAP/MVA ME-TRAP or rabies vaccine. All volunteers received the antimalarial drugs primaquine and Lapdap plus artesunate to eliminate malaria parasites. Volunteers in a further group received an additional single treatment with sulfadoxine-pyrimethamine (SP) to prevent new infections. There was substantially lower T-cell immunogenicity than in previous trials with this vaccine regimen and no protection against infection in the malaria vaccine group. Using the primary endpoint of 20 parasites per mL, no difference was found in the prevalence of low-level infections in volunteers who received SP compared with those who did not, indicating that SP did not reduce the incidence of very low-density infection. However, SP markedly reduced the incidence of higher density infections. These findings support the feasibility and potential of this approach to screen pre-erythrocytic vaccines for efficacy against infection in small numbers of vaccinees in endemic areas.

Figures

FIGURE 1

Trial profile.

FIGURE 2

Ex vivo IFN-γ ELISPOT responses to P. falciparum TRAP vaccination, 7 days after final vaccination in the malaria vaccine (FFM) and the rabies-vaccinated volunteers. PBMC were stimulated with 6 pools of TRAP peptides containing 7–10 20-mer peptides overlapping by 10 amino acids. Median values (gray boxes) and individual values (black spots) are shown.

FIGURE 3

Ex vivo IFN-γ ELISPOT responses to P. falciparum TRAP and multi-epitope (ME) peptide pools. Responses are from volunteers in the malaria vaccine (FFM) group 7 days after the final vaccination. Median values (gray boxes) and individual values (black spots) are shown. Significant differences were observed between traps 1 and 2 (P = 0.0058), traps 1 and 5 (P = 0.0058), traps 2 and 4 (P = 0.003), and traps 3 and 4 (P = 0.001). Tests for significance were carried out using the Wilcoxon signed rank test.

FIGURE 4

Kaplan-Meier survival estimates showing the probability of remaining free of parasitemia during the 28-day follow-up period: estimates were done using thresholds of (a) 20, (b) 100, and (c) 1000 parasites/mL followed by a second positive PCR result. The population used for efficacy analyses included volunteers who received a full course of malaria or rabies vaccines, excluding volunteers who were PCR-positive on the first day of their surveillance (Group 1 = FFM; Group 2 = rabies + SP; Group 3 = rabies alone). This figure appears in color at www.ajtmh.org.