Serological evaluation of the effectiveness of reactive focal mass drug administration and reactive vector control to reduce malaria transmission in Zambezi Region, Namibia: Results from a secondary analysis of a cluster randomised trial

Abstract

Background: Due to challenges in measuring changes in malaria at low transmission, serology is increasingly being used to complement clinical and parasitological surveillance. Longitudinal studies have shown that serological markers, such as Etramp5.Ag1, can reflect spatio-temporal differences in malaria transmission. However, these markers have yet to be used as endpoints in intervention trials.

Methods: Based on data from a 2017 cluster randomised trial conducted in Zambezi Region, Namibia, evaluating the effectiveness of reactive focal mass drug administration (rfMDA) and reactive vector control (RAVC), this study conducted a secondary analysis comparing antibody responses between intervention arms as trial endpoints. Antibody responses were measured on a multiplex immunoassay, using a panel of eight serological markers of Plasmodium falciparum infection - Etramp5.Ag1, GEXP18, HSP40.Ag1, Rh2.2030, EBA175, PfMSP1₁₉, PfAMA1, and PfGLURP.R2.

Findings: Reductions in sero-prevalence to antigens Etramp.Ag1, PfMSP1₁₉, Rh2.2030, and PfAMA1 were observed in study arms combining rfMDA and RAVC, but only effects for Etramp5.Ag1 were statistically significant. Etramp5.Ag1 sero-prevalence was significantly lower in all intervention arms. Compared to the reference arms, adjusted prevalence ratio (aPR) for Etramp5.Ag1 was 0.78 (95%CI 0.65 - 0.91, p = 0.0007) in the rfMDA arms and 0.79 (95%CI 0.67 - 0.92, p = 0.001) in the RAVC arms. For the combined rfMDA plus RAVC intervention, aPR was 0.59 (95%CI 0.46 - 0.76, p < 0.0001). Significant reductions were also observed based on continuous antibody responses. Sero-prevalence as an endpoint was found to achieve higher study power (99.9% power to detect a 50% reduction in prevalence) compared to quantitative polymerase chain reaction (qPCR) prevalence (72.9% power to detect a 50% reduction in prevalence).

Interpretation: While the observed relative reduction in qPCR prevalence in the study was greater than serology, the use of serological endpoints to evaluate trial outcomes measured effect size with improved precision and study power. Serology has clear application in cluster randomised trials, particularly in settings where measuring clinical incidence or infection is less reliable due to seasonal fluctuations, limitations in health care seeking, or incomplete testing and reporting.

Funding: This study was supported by Novartis Foundation (A122666), the Bill & Melinda Gates Foundation (OPP1160129), and the Horchow Family Fund (5,300,375,400).

Keywords: Cluster randomised trials; Malaria; Serology.

Figure 1

Sero-prevalence ratio, qPCR prevalence ratio, and AUC ratio by antigen and intervention. (A) Adjusted prevalence ratios are shown for rfMDA vs RACD (black), RAVC vs. No RAVC (blue) and rfMDA plus RAVC vs. RACD only (magenta). (B) Adjusted ratio of log AUC values are shown for rfMDA vs RACD (black), RAVC vs. No RAVC (blue) and rfMDA plus RAVC vs. RACD only (magenta). All values are adjusted for EA incidence in 2016, proportion of EA index cases covered, proportion of target population covered, median time to intervention, and distance from villages receiving an MoHSS intervention.

Figure 2

Coefficient of variation, k, and number of clusters per arm, c, using serology compared to qPCR as a trial endpoint. Number of clusters per arm is estimated for serology (A) and qPCR (B) based on predicted decrease in prevalence of 75% in clusters receiving the combined intervention rfMDA plus RAVC arm (blue) and 50% in clusters receiving either rfMDA or RAVC alone (magenta). Mean cluster sample size, m (mean), is indicated by the dotted vertical black line, and the associated number of clusters required indicated by the horizontal dotted lines. Change in study power by relative reduction in prevalence (C) is shown for serology (black) and qPCR (red), with study power for predicted and observed relative reduction in prevalence indicated by filled and empty circles.

Figure 3

Number of clusters per arm, c, for a range of baseline prevalence and coefficient of variation values. Heatmaps show the number of clusters per arm required for a range of coefficient of variation values and sero-prevalence (A) or qPCR prevalence (B), assuming an average of 65 individuals per cluster and 50% reduction in sero- or qPCR- prevalence. Observed coefficients of variation and baseline sero- and qPCR-prevalence are indicated by asterisks.