HIV transmission and source-sink dynamics in sub-Saharan Africa.

Multiple phylogenetic studies of HIV in sub-Saharan Africa have shown that mobility-driven transmission frequently occurs: many communities export and import strains. Mobility-driven transmission can result in source-sink dynamics: one community can sustain a micro-epidemic in another community in which transmission is too low to be self-sustaining. In epidemiology, the basic reproduction number (R) is used to specify the sustainability threshold. R represents the average number of secondary infections generated by one infected individual in a community in which everyone is susceptible. If R is greater than 1, transmission is high enough to sustain an epidemic; if R is less than 1, it is not. Here, we discuss the conditions that are needed (in terms of R) for source-sink transmission dynamics to occur in generalised HIV epidemics in sub-Saharan Africa, present an example of where these conditions could occur (ie, Namibia), and discuss the necessity of considering mobility-driven transmission when designing control strategies. Additionally, we discuss the need for a new generation of HIV transmission models that are more realistic than the current models. The new models should reflect not only geographical variation in epidemiology and demography, but also the spatial-temporal complexity of population-level movement patterns.