Is a reproduction number of one a threshold for Plasmodium falciparum malaria elimination?

BACKGROUND: The basic reproduction number (R(0)) is an important summary of the dynamics of an infectious disease. It is a threshold parameter: an infection can only invade a population if R(0) is greater than 1. However, a number of studies using simple models have suggested that for malaria, it is in theory possible for infection to persist indefinitely even if an intervention has reduced R(0) below 1. Such behaviour is known as a bistable equilibrium. Using two published mathematical models which have both been fitted to detailed, age-stratified data on multiple outcomes, the article investigates whether these more complex models behave in such a way, and hence whether a bistable equilibrium might be a real feature of Plasmodium falciparum malaria in Africa. RESULTS: With the best-fitting parameter values, neither model has a bistable state, because immunity reduces onwards infectiousness. The results imply that there is a threshold such that if interventions can reduce transmission so that R(0) is below 1 for long enough, then malaria will be locally eliminated. CONCLUSIONS: This means that calculations of the reduction in R(0) that interventions can achieve (the effect size) have a useful and straightforward interpretation, whereas if the theoretical possibility of a bistable equilibrium were the real behaviour, then such effect size calculations would not have a clear interpretation. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1186/s12936-016-1437-9) contains supplementary material, which is available to authorized users.