Statistical Methods for Standard Membrane-Feeding Assays to Measure Transmission Blocking or Reducing Activity in Malaria

Transmission blocking vaccines for malaria are not designed to directly protect vaccinated people from malaria disease, but to reduce the probability of infecting other people by interfering with the growth of the malaria parasite in mosquitoes. Standard membrane-feeding assays compare the growth of parasites in mosquitoes from a test sample (using antibodies from a vaccinated person) compared to a control sample. There is debate about whether to estimate the transmission reducing activity (TRA) which compares the mean number of parasites between test and control samples, or transmission blocking activity (TBA) which compares the proportion of infected mosquitoes. TBA appears biologically more important since each mosquito with any parasites is potentially infective; however, TBA is less reproducible and may be an overly strict criterion for screening vaccine candidates. Through a statistical model, we show that the TBA estimand depends on μ_<i>c</i>, the mean number of parasites in the control mosquitoes, a parameter not easily experimentally controlled. We develop a standardized TBA estimator based on the model and a given target value for μ_<i>c</i> which has better mean squared error than alternative methods. We discuss types of statistical inference needed for using these assays for vaccine development. Supplementary materials for this article are available online.

疟疾传播阻断疫苗（Transmission Blocking Vaccines）并非旨在直接保护接种者免受疟疾感染，而是通过干扰疟原虫在蚊虫体内的增殖，降低其传染其他个体的概率。标准膜饲实验（Standard membrane-feeding assays）会对比两组样本中疟原虫在蚊虫体内的增殖情况：一组为使用疫苗接种者抗体的测试样本，另一组为对照样本。目前学界对于应采用传播降低活性（TRA，transmission reducing activity）还是传播阻断活性（TBA，transmission blocking activity）存在争议：前者对比测试组与对照组蚊虫体内的平均寄生虫数量，后者则对比感染蚊虫的比例。从生物学层面而言，TBA的意义更为关键，因为任何携带疟原虫的蚊虫均具备潜在传染性；但TBA的可重复性较差，且作为疫苗候选株的筛选标准可能过于严苛。通过构建统计模型，我们发现TBA的估计量依赖于对照组蚊虫体内的平均寄生虫数量μ_c，该参数难以通过实验手段实现精准控制。我们基于该模型与μ_c的给定目标值，开发了一种标准化的TBA估计方法，其均方误差（mean squared error）优于其他替代方法。此外，我们还讨论了将这类实验应用于疫苗开发所需的各类统计推断方法。本文的补充材料可在线获取。