Short Report: Study variability in recent human challenge experiments with Plasmodium falciparum sporozoites (PfSPZ Challenge)

There has been renewed interest in the use of sporozoite-based approaches for malaria vaccination and controlled human infections, and several sets of human challenge studies have recently completed. A study undertaken in Tanzania and published in 2014 found dose-dependence between 10,000 and 25,000 sporozoite doses, as well as divergent times-to-parasitemia relative to earlier studies in European volunteers. However, this analysis shows that these conclusions are based upon suboptimal analytical methods; with more optimal analysis, there is no evidence for dose-dependence within this dose range; and more importantly, no evidence for differences in event times between Dutch and Tanzanian study sites. While these finding do not impact the reported safety and tolerability of PfSPZ, they highlight critical issues that should be comprehensively considered in future challenge studies.