Heterologous protection against malaria by a simple chemoattenuated PfSPZ vaccine regimen in a randomized trial

Abstract

Immunization with Plasmodium falciparum (Pf) sporozoites under chemoprophylaxis (PfSPZ-CVac) is the most efficacious approach to malaria vaccination. Implementation is hampered by a complex chemoprophylaxis regimen and missing evidence for efficacy against heterologous infection. We report the results of a double-blinded, randomized, placebo-controlled trial of a simplified, condensed immunization regimen in malaria-naive volunteers (EudraCT-Nr: 2018-004523-36). Participants are immunized by direct venous inoculation of 1.1 × 105 aseptic, purified, cryopreserved PfSPZ (PfSPZ Challenge) of the PfNF54 strain or normal saline (placebo) on days 1, 6 and 29, with simultaneous oral administration of 10 mg/kg chloroquine base. Primary endpoints are vaccine efficacy tested by controlled human malaria infection (CHMI) using the highly divergent, heterologous strain Pf7G8 and safety. Twelve weeks following immunization, 10/13 participants in the vaccine group are sterilely protected against heterologous CHMI, while (5/5) participants receiving placebo develop parasitemia (risk difference: 77%, p = 0.004, Boschloo's test). Immunization is well tolerated with self-limiting grade 1-2 headaches, pyrexia and fatigue that diminish with each vaccination. Immunization induces 18-fold higher anti-Pf circumsporozoite protein (PfCSP) antibody levels in protected than in unprotected vaccinees (p = 0.028). In addition anti-PfMSP2 antibodies are strongly protection-associated by protein microarray assessment. This PfSPZ-CVac regimen is highly efficacious, simple, safe, well tolerated and highly immunogenic.

Conflict of interest statement

B.M. and P.G.K. received funding from the DZIF. The remaining authors declare no competing interests. All authors associated with Sanaria Inc. (N.K.C., A.M., B.K.L.S., T.L.R., and S.L.H.) have a potential conflict of interest.

Figures

Fig. 1. Diagram illustrating the differences between the established and current vaccination regimen for PfSPZ-CVac (CQ).

a The previously used vaccination regimen included 13 clinic visits during 9 weeks, 10 for administration of CQ, and 3 for immunizations. b The immunization regimen assessed in this study included three clinic visits during 4 weeks, three for administration of CQ, and the same three for immunizations. Red arrows: vaccination time points; blue arrows: chloroquine treatment time points.

Fig. 2. Study flow chart.

CONSORT flow diagram showing study participant flow through for each individual stage of the randomized controlled vaccination trial (enrollment, allocation, follow-up, and analysis).

Fig. 3. Parasitemia during immunization phase.

a Parasitemia (parasites/ml) was estimated by quantitative PCR during the immunization phase after immunizations 1, 2, and 3. Individual parasitemia levels are shown. Red circles: vaccinated group (protected); turquoise triangles: vaccinated group (unprotected). During the first vaccination period, one unprotected vaccinee missed visits of Study days 9 and 10 after immunization, two volunteers missed the visit on Study day 10 after immunization. At second immunization, one protected volunteer missed the visit on Study day 15. For the third immunization, one unprotected and one protected vaccinee missed the visit on Study day 38 after immunization. b Number of subjects positive per number injected, median peak parasite density among positives, and study day of median peak parasite density among positives after each dose of PfSPZ-CVac. Values are given for all vaccinated volunteers as well as separated by outcome of CHMI.

Fig. 4. Kaplan–Meier plot of time from infection to treatment initiation.

The overall time to treatment (in days) after injection of 3.2 × 103 PfSPZ Challenge (onset of CHMI) is presented for both the placebo (n = 5) and the vaccinated (n = 13) groups. The cross represents the censoring event at the end of the follow-up period of the specific CHMI (day 28 after CHMI). Two volunteers receiving placebo withdrew consent before CHMI; these individuals were not included in the survival analysis. Time to treatment: treatment was initiated at time of parasitemia. Parasitemia as the CHMI endpoint was defined as at least one qPCR result above 100 parasites/ml among three positive results at least 12 h apart or as a positive thick blood smear.

Fig. 5. Adverse events (AEs) over time during immunization period.

AEs during the immunization period were recorded for each individual. The total number of mild and moderate AEs per individual (rows) and per day (columns) are plotted as a heatmap. The study population was divided into the subgroups of treatment allocation and CHMI outcome. Numbers of AEs are represented as given by the color scheme in the legend. Arrows highlight the time points of injection of the investigational product. Bars in burgundy indicate the period of transient parasitemia in vaccinees. AEs on Day 1 occurred after administration of investigational product. There were no AEs on days 2–5 after dose I or on days 30–35 after dose III.

Fig. 6. The number of adverse events (AEs) during the immunization period in the placebo and vaccine groups.

Each bar represents one volunteer sorted on the number of AEs from the time of first injection with normal saline or PfSPZ Challenge until the end of the vaccination period. Mild (grade 1) AEs are depicted in red, moderate (grade 2) in turquoise. Non-protected volunteers are marked with an “M” on the x-axis.

Fig. 7. IgG antibody reactivity against Plasmodium falciparum antigens.

a IgG antibody levels specific for PfCSP were measured by ELISA using sera from volunteers before the first immunization (D-1), 15 days after the last immunization (D 44), as well as 1 day before CHMI (C-1). PfCSP-specific antibody levels of baseline-corrected post-immunization time points (net OD 1.0) are shown. Green triangles: vaccinated group (protected); red squares: vaccinated group (unprotected); blue circles: placebo controls. Reported data were derived from a single assessment with three technical replicates. N = 18 biologically independent study participants. Lines shown are median with 95% CI. P values were estimated by two-sided Wilcoxon–Mann–Whitney test. b–d Sera from all volunteers collected before immunization (baseline, D-1) and one day before CHMI (C-1) were assessed on protein microarrays containing 262 P. falciparum proteins representing 228 unique antigens. Analysis was performed on C-1 data after subtraction of the individual baseline reactivity. b To estimate PfSPZ-CVac immunogenicity, antigen reactivity in vaccinated donors (to the right) was compared to the placebo controls (to the left). Differentially recognized antigens (p value <0.05 and fold change >2) are depicted in red. P values were estimated using the two-sided Welch-corrected Student’s t-test. c The fraction of seropositive vaccinated and placebo samples (seropositivity defined as at least fourfold overall baseline reactivity) of the 25 most immunogenic antigens were assessed. d Individual antibody reactivities to the five most immunogenic proteins representing sporozoite, liver, and early blood stage are presented stratified by the group allocation (placebo, unprotected, and protected vaccinees). The boxplot gives median signal intensities, interquartile ranges (IQR), and whiskers of the length of 1.5 × IQR. N = 18 biologically independent study participants.