More than 20 years of research in implementing interventions for IDUs indicate that HIV transmission among these users can be prevented, slowed, and stopped with the appropriate intervention. Intervening early with multiple strategies can prevent epidemic spread and its consequences. Even after prevalence has increased substantially, prevention interventions can reduce the further spread of HIV in drug-using populations and transmission into other populations. Although research results have been promising, it is unknown if self-reported behavior change is correlated with reducing rates of HIV transmission. This study will determine the efficacy of a peer-educator, network-oriented intervention to prevent HIV transmission among substance users and their risk network members. IDUs will be recruited using street and community outreach methods in Chiang Mai and Chiang Rai, Thailand and in Philadelphia, Pennsylvania, USA.

Individuals at the study clinics will undergo a screening survey and HIV counseling and testing; they will return to the clinic to receive their HIV test results, post-test counseling, and further screening. Those who are HIV uninfected, meet preliminary eligibility requirements, and are willing to participate will be enrolled as index participants. Each index participant will be asked to identify and attempt to recruit at least two other people (potential network members) whom he/she has had sex with or taken drugs with in the 3 months prior to screening. Infected and uninfected network members will be eligible for enrollment. Index participants will be randomly assigned with their network members to either the intervention group or a control group. All participants will receive HIV counseling and regular HIV testing. Index participants assigned to the intervention group attend six, two-hour peer educator-training sessions over a period of 4 weeks. The training sessions focus on skills building and promotion of risk-reducing behaviors. A one-hour booster training session is held for index participants after their 6- and 12-month follow-up assessments.

The study will last approximately 48 months, with enrollment expected to take about 30 months. Participants will be followed for a minimum of 18 months and a maximum of 30 months. Both index participants and network members have follow-up visits every 6 months, at which they will complete surveys designed to assess HIV risk behavior, network characteristics, the intensity of the intervention delivery, and social harms that may be associated with study participation. Average network HIV incidence rates will be compared for the two study groups.

NOTE: as of January 31, 2006, the HPTN 037 trial was terminated. Participants enrolled in the study will continue to be followed at a reduced number of study visits.