Scale-free networks and sexually transmitted diseases: A description of observed patterns of sexual contacts in Britain and Zimbabwe

Background: Sexually transmitted infections spread through a network of contacts created by the formation of sexual partnerships. In physics, networks have been characterized as "scale-free" if they follow a power law with an exponent between 2 and 3. Objective: The objective of this study was to test statistically whether distributions of numbers of sexual partners reported from different populations are well described by power laws. Study Design: Power laws and an exponential null model are fitted by maximum likelihood techniques to reported distributions of numbers of partners. Data are taken from 4 population-based surveys, 3 from Britain and 1 from rural Zimbabwe. Results: The networks can be described by power laws over a number of orders of magnitude. In addition, the derived exponents differ significantly and meaningfully, with an "accelerating network" formed between men who have sex with men (MSM). Conclusions: A scale-free network approach provides a reasonable description of distributions of reported numbers of sexual partners.