PfSPZ-CVac efficacy against malaria increases from 0% to 75% when administered in the absence of erythrocyte stage parasitemia: A randomized, placebo-controlled trial with controlled human malaria infection

Abstract

PfSPZ-CVac combines 'PfSPZ Challenge', which consists of infectious Plasmodium falciparum sporozoites (PfSPZ), with concurrent antimalarial chemoprophylaxis. In a previously-published PfSPZ-CVac study, three doses of 5.12x104 PfSPZ-CVac given 28 days apart had 100% vaccine efficacy (VE) against controlled human malaria infection (CHMI) 10 weeks after the last immunization, while the same dose given as three injections five days apart had 63% VE. Here, we conducted a dose escalation trial of similarly condensed schedules. Of the groups proceeding to CHMI, the first study group received three direct venous inoculations (DVIs) of a dose of 5.12x104 PfSPZ-CVac seven days apart and the next full dose group received three DVIs of a higher dose of 1.024x105 PfSPZ-CVac five days apart. CHMI (3.2x103 PfSPZ Challenge) was performed by DVI 10 weeks after the last vaccination. In both CHMI groups, transient parasitemia occurred starting seven days after each vaccination. For the seven-day interval group, the second and third vaccinations were therefore administered coincident with parasitemia from the prior vaccination. Parasitemia was associated with systemic symptoms which were severe in 25% of subjects. VE in the seven-day group was 0% (7/7 infected) and in the higher-dose, five-day group was 75% (2/8 infected). Thus, the same dose of PfSPZ-CVac previously associated with 63% VE when given on a five-day schedule in the prior study had zero VE here when given on a seven-day schedule, while a double dose given on a five-day schedule here achieved 75% VE. The relative contributions of the five-day schedule and/or the higher dose to improved VE warrant further investigation. It is notable that administration of PfSPZ-CVac on a schedule where vaccine administration coincided with blood-stage parasitemia was associated with an absence of sterile protective immunity. Clinical trials registration: NCT02773979.

Fig 1. Consort flowchart.

Fig 2. Vaccination phase AEs in vaccinated and placebo control participants.

(A) Specific solicited systemic AEs shown as the proportion of subjects who reported a maximum grade of 3, 2, or 1 in the 7–10 days after each vaccination by group and vaccination status. (B) The proportion of participants in each group and amongst placebo controls who reported at least one Grade 3, Grade 2 (Grade 3 excluded), or Grade 1 (Grades 2–3 excluded) event by dose. Grade 1 events are those that required minimal or no treatment and did not interfere with daily activities. Grade 2 events are those that resulted in a low level of inconvenience or required therapeutic measures and may have interfered with functioning and daily activities. Grade 3 events are those that interrupted the subject’s usual daily activities. Temperature values are noted only if ≥38.0°C (lower limit of graded fever) and are reported as Grade 1 (38.0°C—38.4°C), Grade 2 (38.5°C—38.9°C), or Grade 3 (>38.9°C). Vomiting was a solicited systemic adverse but was not reported by any subject in the 7- to 10- days after a vaccination and so is not represented in the graphs. Blue fill (top), Grade 1; orange fill (middle), Grade 2; red fill (bottom), Grade 3.

Fig 3. Parasite density estimated by qRT-PCR in vaccinated subjects after each vaccination by group.

(A) Triangle symbols indicate the first, second, and third days of vaccine administration as shown. The numbers above each peak of parasitemia display the number of persons positive divided by the total number of vaccine recipients for that dose of vaccine. Days are listed relative to the first dose of vaccine. The number of participants completing each vaccination are listed above each peak of parasitemia. Group 1 received a PfSPZ Challenge dose of 5.12x10⁴ PfSPZ for each of three doses given by DVI seven days apart. Group 2 received a PfSPZ Challenge dose of 1.024x10⁵ PfSPZ for each of three doses given by DVI seven days apart. Group 3 received a PfSPZ Challenge dose of 1.024x10⁵ PfSPZ for each of three doses given by DVI five days apart. (B) Inset table indicates the geometric mean peak parasite density and the minimum/maximum ranges for each post-vaccination interval in estimated parasites/mL. *The value for the single subject with parasitemia.

Fig 4. Kaplan-Meier curves for time to first P. falciparum qRT-PCR positive after CHMI in vaccinated subjects versus placebo or control subjects in Groups 1 and 3.

(A) Group 1 (seven-day interval vaccine cohort, n = 9) and control (n = 3) data. (B) Group 3 (five-day interval vaccine cohort, n = 9) and control (n = 3) data. qRT-PCR limit of detection, 20 estimated parasites/mL. p values calculated using log-rank test.

Fig 5. Pre-CHMI antibody comparisons between vaccination and placebo groups.

(A) Antibodies to PfCSP (net OD 1.0) prior to CHMI. Filled circles are uninfected (protected) subjects and open circles are infected subjects. (B) Antibodies to PfSPZ (net aIFA^200K) measure prior to CHMI. The y-axis refers to aIFA fluorescence intensity of 2x10⁵. Filled circles are uninfected (protected) subjects and open circles are infected subjects. (C) Net reciprocal serum dilution for 80% inhibition of PfSPZ invasion of hepatocytes (HC-04 cells) prior to CHMI. Filled circles are uninfected (protected) subjects and open circles are infected subjects. Horizontal lines represent medians and bars are interquartile ranges for all panels. p values for Wilcoxon-Mann-Whitney tests are shown in A-B.

Fig 6. Cellular immune parameters in vaccinated and control participants.

(A) Background-adjusted percent of CD4 T-cells expressing specific cytokines upon stimulation with malaria antigens by time point among vaccinated subjects in Groups 1 and 3, placebo recipients, and infectivity controls. (B) Background-adjusted percent of CD8 T-cells expressing specific cytokines upon stimulation with malaria antigens by time point and treatment group. (C) Fold Change in γδ T-cells and in the Vγ9+/Vδ2+ subset from pre-vaccination to 14 days post-vaccination and to pre-CHMI by treatment group. Bars and text show medians in all panels.