PSQ assay based on a SNP within the csp gene detects the strain-specific protective effect.

Toque monkeys were strain-specifically immunized with live sporozoites (by infective mosquito bites) of Pc746 and PcCeylon strains under chloroquine and challenged with a mixture of bites of PcCeylon and Pc746 live sporozoite-infected mosquitoes. Open portions of columns represent Pc746 proportions and shaded portions of columns represent PcCeylon proportions in challenge infections of (A) non-immunized, n = 4 (B) Pc746 immunized, n = 5 and (C) PcCeylon immunized monkeys, n = 4. The absolute asexual parasitaemia, as a percent of red blood cells infected, are indicated by the lines connecting filled triangles. Error bars represent the standard deviation between the 3 to 6 repeated PSQ measurements of each monkey represented in each experimental condition. The proportion of parasites of each strain was recorded for 4 to 8 consecutive days starting from and including the day on which a monkey first reached or passed 0.03% asexual parasitaemia.The day of the first such record in each monkey is designated “Day 1” in the text. The Mann-Whitney U test for two independent variables (at 95% confidence level) was used to compare the proportions of Pc746 in Pc746 immunized and PcCeylon immunized groups on each day from “Day 1” to “Day 4”. P = 0.027 for “Day 1”, “Day 3”, and “Day 4”; P = 0.014 for “Day 2”.