STI model overview

The EMOD STI model is an agent-based stochastic simulator of sexual and vertical disease transmission. The model utilizes microsolvers to combine detailed information on contact networks, pair formation, and human population dynamics. EMOD can be calibrated to particular geographic locations, and the microsolver framework enables the model’s functionality to be highly modifiable. To use the STI model, set the configuration parameter Simulation_Type to STI_SIM.

The STI transmission mode framework enables person-to-person transmission through a network of relationships or contact networks, each of which has a specific transmitter and recipient of every transmission event. To organize the networks, individuals form one or more relationships that are remembered over time.

Preference for partners is configurable through the model’s input files. Inside the model, the “supply and demand” for types of partners is balanced by a pair formation algorithm (PFA) that, if desired, can dynamically adjust the rates of relationship formation in each demographic category to produce a constant mixing pattern, even with demographic changes in the population. Alternatively, the dynamic adjustments can be turned off to allow mixing patterns to change in response to demographic changes. For more information on the PFA, see Relationships and contact networks.

The contact networks formed within the STI model are driven by the behavioral parameters found in the Society section of the demographics file. These parameters control pair formation, relationship types, and level of relationship concurrency. For more information about this section of the demographics file, see Society.

Because the STI model is built around person-to-person contact networks, human relationships impact nearly every aspect of model functionality. For example, migration is supported in the STI model, but is more complicated than in other disease types, as now it impacts the relationships experienced by the migrating individual (see Relationships and contact networks for more information).

The following pages describe the structure of the STI model and explore the model components. Additionally, the specifics of the HIV model, which is based on the STI model, are discussed in detail in the articles Bershteyn, Klein, Wenger, and Eckhoff,, and Klein, Bershteyn, and Eckhoff, AIDS 2014.