Introduction: HIV spread rapidly among people who inject drugs in Bangkok in the late 1980s. In recent years, changes in drug use and HIV-associated risk behaviors have been reported. We examined data from the Bangkok Tenofovir Study, an HIV pre-exposure prophylaxis trial conducted among people who inject drugs, to assess participant risk behavior and drug use, and to identify risk factors for HIV infection. Methods: The Bangkok Tenofovir Study was a randomized, double-blind, placebo-controlled trial. HIV status was assessed monthly and risk behavior every 3 months. We used generalized estimating equations logistic regression to model trends of injecting, needle sharing, drugs injected, incarceration, and sexual activity reported at follow-up visits; and proportional hazards models to evaluate demographic characteristics, sexual activities, incarceration, drug injection practices, and drugs injected during follow-up as predictors of HIV infection. Results: The proportion of participants injecting drugs, sharing needles, and reporting sex with more than one partner declined during follow-up (p<0.001). Among participants who reported injecting at enrollment, 801 (53.2%) injected methamphetamine, 559 (37.1%) midazolam, and 527 (35.0%) heroin. In multivariable analysis, young age (i.e., 20-29 years) (p = 0.02), sharing needles (p<0.001), and incarceration in prison (p = 0.002) were associated with incident HIV infection. Participants reporting sex with an opposite sex partner, live-in partner, casual partner, or men reporting sex with male partners were not at a significantly higher risk of HIV infection compared to those who did not report these behaviors. Conclusion: Reports of HIV-associated risk behavior declined significantly during the trial. Young age, needle sharing, and incarceration were independently associated with HIV infection. Sexual activity was not associated with HIV infection, suggesting that the reduction in HIV incidence among participants taking daily oral tenofovir compared to those taking placebo was due to a decrease in parenteral HIV transmission.
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