An ODD Protocol for an Agent-Based Model of Hepatitis C Virus Transmission with Inter- and Intra-Group Structural and Behavioural Heterogeneous Syringe-Sharing Networks (5)

The model described in this ODD is an agent-based model of hepatitis C virus (HCV) transmission among people who inject drugs (PWID). The model incorporates structural heterogeneity through a three-group interaction framework and behavioural heterogeneity through group-specific syringe-sharing rates with additional intra-group variability. The syringe-sharing population in the model is divided into core, inner, and outer circle groups representing individuals with high, moderate, and low levels of syringe-sharing interaction intensity, respectively. In addition to differences in the number of daily interaction opportunities across groups, agents in each group are assigned syringe-sharing probabilities that vary at the individual level around their respective group means, reflecting heterogeneity in risk-taking behaviour within as well as between structural groups in the syringe-sharing social network. While epidemiological processes remain identical across agents, both the frequency of interaction events and the realised likelihood of sharing during those interactions vary across individuals. Interactions are generated dynamically using proximity-based sampling at each timestep (one day), allowing agents to form syringe-sharing interactions based on spatial closeness. Transmission occurs through direct syringe-sharing events, with infection probability determined by accumulated daily exposure to infectious partners. The model tracks transitions between susceptible, chronic infection, treatment, cure, and resistance states, and is calibrated to Irish HCV surveillance data. This model serves to examine how incorporating both structural heterogeneity and intra-group behavioural variability influences epidemic dynamics relative to models with only group-level heterogeneity and the homogeneous baseline model.