LETTERS HOW “HIDDEN” ARE UNOBSERVED NETWORKS AMONG PEOPLE WHO INJECT DRUGS? We read with interest the article by Hunter et al., in which they argue that “hidden” social network members are crucial in understanding behavior change and suggest that public health interventions can harness social networks, including those that are “hidden.”1 While it is generally true that “hidden” social networks “have typically been overlooked, unobserved, and subsequently underused” in public health, there are examples in harm reduction---focused work with people who inject drugs (PWID) where individuals and their social networks have been trained to facilitate behavior change that aims to limit the transmission of blood-borne viruses.2 We have been exploring this concept of social network analysis through a cohort study of PWID and HCV transmission in Melbourne, Australia, over the past 10 years,3 and also believe there are lessons to be learned in developing behavior change interventions. In our longitudinal study, we asked our 258 participants (recruited from street-based drug markets) about their injecting relationships and collected behavioral and serological data4; this information has revealed the changing nature

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of both injecting and social networks and their impact on HCV transmission.5 Individuals (nodes) with at least one other HCV-positive person in their network have increased likelihood of infection, and participants reporting injecting together (same time and place) were more likely to have genetically related HCV.5,6 Modeling of the cohort has been used to account for the effects of unobserved or missing network members on HCV transmission.7 Careful characterization and follow-up of the cohort revealed that HCV infection was highly dynamic, with multiple reinfections and clearances.3 Importantly, our network models demonstrated that a “treat your friends” approach reduced HCV incidence and prevalence more than treating PWID randomly.4 Our models have informed the HCV Treatment and Prevention Study, a multimillion dollar investigation of the impact of new-generation treatments on HCV incidence and prevalence among PWID using a network-based approach.8 Studying the social and injecting networks of PWID provides essential epidemiological information about HCV transmission and the potential for behavioral change interventions. However, our field-based observations also made us acutely aware of “hidden” networks, including those that may go unreported to us in structured surveys. Such work provides the ideal foundation for developing and evaluating the effectiveness of health and human services, and the delivery of behavioral interventions that improve the health of PWID. j

Suit 6, 19-35 Gertrude St., Fitzroy 3065 Australia (e-mail: [email protected]). Reprints can be ordered at http://www.ajph.org by clicking the “Reprints” link. This letter was accepted February 28, 2015. doi:10.2105/AJPH.2015.302667

Contributors All of the authors contributed equally to this letter.

References 1. Hunter RF, McAneney H, Davis M, Tully MA, Valente TW, Kee F. “Hidden” social networks in behavior change interventions. Am J Public Health. 2015;105 (3):513---516. 2. Mateu-Gelabert P, Gwadz MV, Guarino H, et al. The staying safe intervention: training people who inject drugs in strategies to avoid injection-related HCV and HIV infection. AIDS Educ Prev. 2014;26(2):144---157. 3. Sacks-Davis R, Aitken CK, Higgs P, et al. High rates of hepatitis C virus reinfection and spontaneous clearance of reinfection in people who inject drugs: a prospective cohort study. PLoS ONE. 2013;8(11):e80216. 4. Hellard M, Rolls DA, Sacks-Davis R, et al. The impact of injecting networks on hepatitis C transmission and treatment in people who inject drugs. Hepatology. 2014;60(6):1861---1870. 5. Sacks-Davis R, Daraganova G, Aitken C, et al. Hepatitis C virus phylogenetic clustering is associated with the social-injecting network in a cohort of people who inject drugs. PLoS ONE. 2012;7(10):e47335. 6. Aitken C, Lewis J, Hocking J, Bowden D, Hellard M. Does information about IDU’s injecting networks predict exposure to the hepatitis C virus? Hepat Mon. 2009;9 (1):17---23. 7. Rolls DA, Wang P, Jenkinson R, et al. Modelling a disease-relevant contact network of people who inject drugs. Soc Networks. 2013;35(4):699---710. 8. Hellard ME, Jenkinson R, Higgs P, et al. Modelling antiviral treatment to prevent hepatitis C infection among people who inject drugs in Victoria, Australia. Med J Aust. 2012;196(10):638---641.

Peter Higgs, PhD, MA, BSW Rachel Sacks-Davis, PhD, BA, BSc (Hons) Campbell Aitken, PhD, BSc (Hons) Margaret Hellard, PhD, FRACP, FAFPHM

About the Authors Peter Higgs is with the Melbourne Office of the National Drug Research Institute, Melbourne, Australia. Rachel Sacks-Davis is with the Doherty Institute, Melbourne. Campbell Aitken and Margaret Hellard are with the Centre for Population Health, the Burnet Institute, Melbourne. Correspondence should be sent to Peter Higgs Curtin University Research Fellow, Faculty of Health Sciences, NDRI,

June 2015, Vol 105, No. 6 | American Journal of Public Health

Letters | e3

How "hidden" are unobserved networks among people who inject drugs?

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