Serological evaluation of a cluster randomised trial on the use of reactive focal mass drug administration and reactive vector control to reduce malaria transmission in Zambezi Region, Namibia
Authors: Wu Lindsey , Hsiang Michelle S. , Prach Lisa M. , Schrubbe Leah , Ntuku Henry , Dufour Mi-Suk Kang , Whittemore Brooke , Scott Valerie , Yala Joy , Roberts Kathryn W. , Patterson Catriona , Biggs Joseph , Hall Tom , Tetteh Kevin K.A. , Gueye Cara Smith , Greenhouse Bryan , Bennett Adam , Smith Jennifer L. , Katokele Stark , Uusiku Petrina , Mumbengegwi Davis , Gosling Roly , Drakeley Chris , Kleinschmidt Immo 
Published: April 2021

Due to challenges in measuring changes in malaria in low transmission settings, serology is increasingly being used to complement clinical and parasitological surveillance. Longitudinal cohort studies have shown serological markers, such as Etramp5.Ag1, to be particularly discriminatory of spatio-temporal differences in malaria transmission. However, these markers have yet to be used as endpoints in intervention trials. This study is an extended analysis of 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). A panel of eight serological markers of Plasmodium falciparum infection - Etramp5.Ag1, GEXP18, HSP40.Ag1, Rh2.2030, EBA175, PfMSP119, PfAMA1, and PfGLURP.R2 - was used on a multiplex immunoassay to measure population antibody responses as trial endpoints.

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 Etramp5.Ag1 prevalence ratio (aPR) was 0.77 (95%CI 0.65 – 0.90, p<0.001) for rfMDA and 0.79 (95%CI 0.67 – 0.92, p=0.001) for RACD. For combined rfMDA plus RAVC, aPR was 0.58 (95%CI 0.45 – 0.75, p<0.001). 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).

The use of serological endpoints to evaluate trial outcomes was comparable to qPCR and measured effect size with improved precision. 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.

Key questions

What is already known?

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Numerous serological studies across sub-Saharan Africa have found that malaria-specific antibody responses are highly correlated with malaria transmission.

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Serology is increasingly being used to complement traditional malaria surveillance data in settings where clinical or parasitological measures of incidence or infection may be less reliable due to fluctuations in parasite densities, limitations in health care seeking, or incomplete testing and reporting.

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The identification of new serological markers associated with recent malaria exposure hold promise as measures of malaria incidence. In previous longitudinal cohort studies, Etramp5.Ag1 has been shown to be a discriminatory serological marker capable of detecting spatio-temporal differences in malaria transmission. However, these markers have never been formally used as endpoints in a malaria cluster randomised trial.

What are the new findings?

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This study is the first application of serological endpoints in a malaria cluster randomised trial. Using a multiplexed immunoassay, a panel of sero-incidence markers of recent malaria exposure were used to evaluate the effectiveness of reactive focal mass drug administration (rfMDA) and reactive focal vector control (RAVC) compared to reactive case detection (standard of care) to reduce malaria transmission.

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Cluster-level antibody responses were significantly lower in all intervention arms compared to control, and effect sizes were measured with greater study power than other trial endpoints such as quantitative polymerase chain reaction (qPCR) parasite prevalence.

What do the new findings imply?

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The findings from this study, together with ongoing innovations in assay design and multi-disease platforms, illustrate the potential application of serological markers as endpoints in cluster randomised trials. The use of serological endpoints can help achieve trial efficiencies, such as reduced sample size, particularly in low transmission settings or multi-intervention trials where measuring differences between study arms may be challenging with clinical or parasitological endpoints alone.