title: A complex systems approach to evaluate HIV prevention in metropolitan areas: preliminary implications for combination intervention strategies.
abstract: BACKGROUND
  HIV transmission among injecting and non-injecting drug users (IDU, NIDU) is a significant public health problem. Continuing propagation in endemic settings and emerging regional outbreaks have indicated the need for comprehensive and coordinated HIV prevention. We describe the development of a conceptual framework and calibration of an agent-based model (ABM) to examine how combinations of interventions may reduce and potentially eliminate HIV transmission among drug-using populations.
  
  
  RESULTS
  A multidisciplinary team of researchers from epidemiology, sociology, geography, and mathematics developed a conceptual framework based on prior ethnographic and epidemiologic research. An ABM was constructed and calibrated through an iterative design and verification process. In the model, "agents" represent IDU, NIDU, and non-drug users who interact with each other and within risk networks, engaging in sexual and, for IDUs, injection-related risk behavior over time. Agents also interact with simulated HIV prevention interventions (e.g., syringe exchange programs, substance abuse treatment, HIV testing) and initiate antiretroviral treatment (ART) in a stochastic manner. The model was constructed to represent the New York metropolitan statistical area (MSA) population, and calibrated by comparing output trajectories for various outcomes (e.g., IDU/NIDU prevalence, HIV prevalence and incidence) against previously validated MSA-level data. The model closely approximated HIV trajectories in IDU and NIDU observed in New York City between 1992 and 2002, including a linear decrease in HIV prevalence among IDUs. Exploratory results are consistent with empirical studies demonstrating that the effectiveness of a combination of interventions, including syringe exchange expansion and ART provision, dramatically reduced HIV prevalence among IDUs during this time period.
  
  
  CONCLUSIONS
  Complex systems models of adaptive HIV transmission dynamics can be used to identify potential collective benefits of hypothetical combination prevention interventions. Future work will seek to inform novel strategies that may lead to more effective and equitable HIV prevention strategies for drug-using populations.
authors.affiliation: Department of Epidemiology, Columbia University Mailman School of Public Health, New York, New York, United States of America. brandon_marshall@brown.edu
authors.forename: Brandon D L
authors.initials: BD
authors.lastname: Marshall
authors.forename: Magdalena M
authors.initials: MM
authors.lastname: Paczkowski
authors.forename: Lars
authors.initials: L
authors.lastname: Seemann
authors.forename: Barbara
authors.initials: B
authors.lastname: Tempalski
authors.forename: Enrique R
authors.initials: ER
authors.lastname: Pouget
authors.forename: Sandro
authors.initials: S
authors.lastname: Galea
authors.forename: Samuel R
authors.initials: SR
authors.lastname: Friedman
mesh_terms: D002947:Cities / Q000706:statistics & numerical data*; D003140:Communicable Disease Control / Q000379:methods* / Q000706:statistics & numerical data*; D055030:Drug Users / Q000706:statistics & numerical data; D005260:Female; D015658:HIV Infections / Q000453:epidemiology / Q000517:prevention & control* / Q000635:transmission; D006716:Homosexuality / Q000706:statistics & numerical data; D006801:Humans; D007267:Injections; D008297:Male; D015233:Models, Statistical; D012306:Risk; D012309:Risk-Taking