GMZ2 Vaccine-Induced Antibody Responses, Naturally Acquired Immunity and the Incidence of Malaria in Burkinabe Children

Abstract

GMZ2 is a malaria vaccine candidate evaluated in a phase 2b multi-centre trial. Here we assessed antibody responses and the association of naturally acquired immunity with incidence of malaria in one of the trial sites, Banfora in Burkina Faso. The analysis included 453 (GMZ2 = 230, rabies = 223) children aged 12-60 months old. Children were followed-up for clinical malaria episodes for 12 months after final vaccine administration. Antibody levels against GMZ2 and eleven non-GMZ2 antigens were measured on days 0 and 84 (one month after final vaccine dose). Vaccine efficacy (VE) differed by age group (interaction, (12-35 months compared to 36-60 months), p = 0.0615). During the twelve months of follow-up, VE was 1% (95% confidence interval [CI] -17%, 17%) and 23% ([CI] 3%, 40%) in the 12 - 35 and 36 - 60 months old children, respectively. In the GMZ2 group, day 84 anti-GMZ2 IgG levels were associated with reduced incidence of febrile malaria during the follow up periods of 1-6 months (hazard ratio (HR) = 0.87, 95%CI = (0.77, 0.98)) and 7-12 months (HR = 0.84, 95%CI = (0.71, 0.98)) in the 36-60 months old but not in 12-35 months old children. Multivariate analysis involving day 84 IgG levels to eleven non-vaccine antigens, identified MSP3-K1 and GLURP-R2 to be associated with reduced incidence of malaria during the 12 months of follow up. The inclusion of these antigens might improve GMZ2 vaccine efficacy.

Keywords: GLURP-R2; GMZ2; MSP3-K1; Plasmodium falciparum; malaria vaccine; naturally acquired immunity.

Figure 1

Clinical malaria incidence rate per randomization arm and parasite density by age group. (A) Incidence rate of febrile malaria by vaccine group per 1000-person year at risk (PYAR). (B) Geometric mean P. falciparum density by age group in both vaccine groups. P values were determined by t-test after log (base 10) transforming parasite density. Error bars represent geometric mean with 95% confidence intervals.

Figure 2

Vaccine efficacy stratified by age. Vaccine efficacy (VE) (using parasite density ≥ 5,000/uL and fever/history of fever) after 12 months of follow up stratified by age group. Error bars represent 95% confidence intervals. Cox regression model was used to calculate hazard ratios, 95% confidence intervals and p values for each age group. VE was defined as 100 × (1-HR), where HR is the hazard ratio from the Cox regression. The horizontal dashed line indicates (VE = 0). Asterisks represent P values (*P < 0.05).

Figure 3

Antibody responses after GMZ2 vaccination and association with parasite density. (A) Mean fluorescent intensity (MFI) representing GMZ2 specific IgG levels compared between the two vaccine groups at day 0 (D0) and day 84 (D84). (B) Mean fluorescent intensity (MFI) representing GMZ2 specific IgG levels compared between day 0 (D0) and day 84 (D84) for the two age groups. (C) Fold increase (day 84 GMZ2-IgG/day 0 GMZ2-IgG) in GMZ2-IgG levels between day 0 and day 84 compared between the vaccine groups for each age group. All P values in panels (A–C) respectively were determined by t-test after log (base 10) transforming data. Horizontal lines represent geometric means. (D) Association between GMZ2 specific IgG levels and parasite density during febrile malaria in the overall study population, the GMZ2 vaccine group alone and the rabies vaccine group alone. Beta (β) coefficients, confidence intervals and p values were calculated using separate multiple linear regressing models adjusting for age. Antibody and parasite density data were both log (base 10) transformed prior to use in the models. The vertical dotted line indicates no association with between GMZ2 IgG and parasite density (β = 0).

Figure 4

Association between GMZ2 antibody levels and clinical malaria stratified by age group. Cox regression model was used to calculate hazard ratios, 95% confidence intervals and p values for antibody levels in the rabies vaccine group (A) and GMZ2 vaccine group (B) for each age group. Error bars represent 95% confidence intervals. The horizontal dashed line indicates no association with protection (HR = 1). Asterisks represent P values (*P < 0.05). Malaria episodes were collected over 12 months of follow up.

Figure 5

GMZ2 bead OP index in relation to age and vaccine group. (A) GMZ2 coated bead OP compared between the vaccine groups at day 0 (D0) and day 84 (D84). (B). GMZ2 coated bead opsonic phagocytosis (OP) levels compared between day 0 (D0) and day 84 (D84) for the different age groups. P values were determined by t-test after log (base 10) transforming data. Horizontal lines represent geometric means.

Figure 6

Merozoite IFA levels in relation to age and febrile malaria. (A) Merozoite IFA levels compared between day 0 (D0) and day 84 (D84) for the different age and vaccine groups. (B) Merozoite IFA levels compared between the vaccine groups at day 0 (D0) and day 84 (D84). (C) Merozoite opsonic phagocytosis (OP) index compared between day 0 (D0) and day 84 (D84) for the different age groups. (D) Merozoite opsonic phagocytosis (OP) index compared between the vaccine groups at day 0 (D0) and day 84 (D84). P values were determined by t-test after log (base 10) transforming data. Error bars represent geometric mean and 95% confidence intervals. IFA, Immunofluorescence assay; AU, Antibody unit.