Acquisition and Longevity of Antibodies to Plasmodium vivax Preerythrocytic Antigens in Western Thailand

ABSTRACTPlasmodium vivaxis now the dominantPlasmodiumspecies causing malaria in Thailand, yet little is known about naturally acquired immune responses to this parasite in this low-transmission region. The preerythrocytic stage of theP. vivaxlife cycle is considered an excellent target for a malaria vaccine, and in this study, we assessed the stability of the seropositivity and the magnitude of IgG responses to three different preerythrocyticP. vivaxproteins in two groups of adults from a region of western Thailand where malaria is endemic. These individuals were enrolled in a yearlong cohort study, which comprised one group that remainedP. vivaxfree (by quantitative PCR [qPCR] detection,n= 31) and another that experienced two or more blood-stageP. vivaxinfections during the year of follow up (n= 31). Despite overall low levels of seropositivity, IgG positivity and magnitude were long-lived over the 1-year period in the absence of qPCR-detectable blood-stageP. vivaxinfections. In contrast, in the adults with two or moreP. vivaxinfections during the year, IgG positivity was maintained, but the magnitude of the response toP. vivaxcircumsporozoite protein 210 (CSP210) decreased over time. These findings demonstrate that long-term humoral immunity can develop in low-transmission regions.