  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.
