Variation in relapse frequency and the transmission potential of Plasmodium vivax malaria

There is substantial variation in the relapse frequency of Plasmodium vivax malaria, with fast relapsing strains in tropical areas, and slow relapsing strains in temperate areas with seasonal transmission. We hypothesise that much of the phenotypic diversity in P. vivax relapses arises from selection of relapse frequency to optimise transmission potential in a given environment, in a process similar to the virulence trade-off hypothesis. We develop mathematical models of P. vivax transmission and calculate the basic reproduction number R0 to investigate how transmission potential varies with relapse frequency and seasonality. In tropical zones with year round transmission, transmission potential is optimised at intermediate relapse frequencies of 2-3 months: slower relapsing strains increase the opportunity for onward transmission to mosquitoes, but also increase the risk of being outcompeted by faster relapsing strains. Seasonality is an important driver of relapse frequency for temper...