Large connected components in sexual networks and their role in HIV transmission in Sub-Saharan Africa: A model-based analysis of HPTN 071(PopART) data.

The HIV epidemic in sub-Saharan Africa is historically characterised by high levels of prevalence and incidence. With the global effort to reach UNAIDS 95-95-95 targets, the scaling-up of HIV treatment, and focused preventive interventions, incidence has been declining over the past decade, albeit non-consistently across different sex and age groups. Two questions remain to be addressed to help tailor setting-specific interventions and allocate resources optimally. Firstly, are there unidentified demographic groups that are sources of transmission? Secondly, what are the patterns of decline in incidence across different groups? Model-based assessment is a valuable tool for the design of focused interventions and to answer these questions. PopART-IBM, an individual-based model calibrated to (anonymised) age-and-sex stratified data, was developed in the context of the HPTN-071 (PopART) trial, and it offers a unique opportunity to explore such questions in the context of high-burden HIV communities in Zambia and South Africa. The outputs of the model include the full HIV transmission and partnership networks. In this work, we explore these and show that the sexual partnership network exhibits a large connected component, usually comprising over 40 % of the population, in each of the studied communities. An analysis of the large connected component reveals that it is formed by young people (20-40 years old) and is centered around the most sexually active individuals of the community. At the same time, many individuals in the large connected component only have one partner, highlighting the complex dynamics of risk correlations in a population. Inspecting the transmission network reveals that, on average, more than 80% of transmissions occur among individuals belonging to the large connected component. These findings indicate that populations consisting of young and highly sexually active individuals should be given high priority when designing or deploying interventions.