Impact of unstable housing on all-cause mortality among persons who inject drugs

The Vancouver Injection Drug Users Study (VIDUS) and AIDS Care Cohort to Evaluate Access to Survival Services (ACCESS) are open prospective cohorts of illicit drug users in Vancouver. The recruitment and follow up procedures for the two studies are largely identical to allow for analyses of merged data, with the only key differences being that HIV positive individuals are followed in ACCESS whereas HIV-negative individuals are followed in VIDUS. In both studies the primary modes of enrollment were self-referral, word of mouth, and street outreach. Detailed sampling and recruitment procedures for these two cohorts have been described elsewhere [18,19].

To be eligible, participants were 18 years of age or older, had ever injected illicit drugs and resided in the greater Vancouver region. Participants with no completed follow up surveys were excluded. All participants provided written informed consent. Participants were given a $20 stipend at each study visit for their time and transportation. The study was approved by the University of British Columbia/Providence Healthcare Research Ethics Board.

At baseline and semianually thereafter, participants completed an interviewer-administered questionnaire that elicited a range of data, including demographic characteristics, housing status, injection and non-injection drug use, and sexual risk behaviors. In addition, venous blood samples were drawn at each visit and tested for HIV and hepatitis C virus (HCV) antibodies among those previously testing negative for these diseases. Venous blood samples taken from HIV positive individuals were assessed for HIV disease progression [18]. All participants had private interviews and were offered both pre- and post-test counseling with trained nurses. Referral for free healthcare was provided to those who tested HIV positive and these individuals were subsequently followed in ACCESS.

The present study included PWID who were recruited and completed at least one follow up visit between May 1996 and December 2012. To avoid potential bias relating to long durations between the last study visit where behavioural information was assessed and the date of death, individuals who were identified as deceased more than 24 months after their last follow up visit were censored on the date of the last follow up.

The primary endpoint in this analysis was all-cause mortality. Here, we ascertained all-cause mortality rates among participants through a confidential record linkage using personal health numbers with the British Columbia Vital Statistics Agency and through ongoing follow up with contacts provided by participants. The primary explanatory variable of interest was unstable housing in the previous six months. As previously, unstable housing was defined as living in a single room occupancy hotel, shelter or other transitional housing, or living on the street [20,21].

Potential confounders that were considered included gender (male vs. female); age (per year older); ancestry (Caucasian vs. non-Caucasian); HIV serostatus (positive vs. negative); and sex work involvement (yes vs. no). A number of substance use behaviors (in the previous six months) were also considered, including ≥ daily heroin injection (yes vs. no), ≥ daily cocaine injection (yes vs. no), ≥ daily crack cocaine smoking (yes vs. no), and current enrolment in a methadone program (yes vs. no). Other covariates that were considered included incarceration (yes vs. no) and HCV serostatus (positive vs. negative). With the exception of age, gender and ancestry, all variables were in reference to the prior six months and measured at each semiannual follow up visit and were treated as time-updated.

As a first step, we used Chi-square test and Wilcoxon rank sum test to compare the baseline characteristics of the participants who did and did not report unstable housing at baseline. Those who did not report unstable housing at baseline were maintained as the reference group. All-cause mortality rate and 95% confidence interval [CI] were calculated using the Poisson distribution. Survival probabilities from all-cause mortality were estimated using the Kaplan-Meier product limit method, and compared using the two-sample log-rank test.

Next, we used bivariate Cox proportional hazards regression to examine the associations between each explanatory variable and time to all-cause mortality. To fit the multivariate model, we employed a conservative stepwise backward selection approach which considered the magnitude of change in the coefficient of unstable housing [22]. Specifically, we included all variables found to be associated with time to all-cause mortality in bivariate analyses at p < 0.10 in a multivariate model and used a stepwise approach to fit a series of reduced models. After comparing the value of the coefficient associated with unstable housing in the full model to the value of the coefficient in each of the reduced models, we dropped the secondary variable associated with the smallest relative change. We continued this iterative process until the minimum change exceeded 5%. Remaining variables were considered as potential confounders in a final multivariate model. All statistical analyses were performed using SAS software version 9.3 (SAS, Cary, NC). All p-values were two-sided.

1. Darke S, Degenhardt L, Mattick R. Mortality Amongst Illicit Drug Users. Cambridge: Cambridge University Press; 2007.Google Scholar
2. Degenhardt L, Hall W. Extent of illicit drug use and dependence, and their contribution to the global burden of disease. Lancet. 2012;379(9810):55–70.View ArticlePubMedGoogle Scholar
3. Oppenheimer E, Tobutt C, Taylor C, Andrew T. Death and survival in a cohort of heroin addicts from London clinics: a 22-year follow-up study. Addiction. 1994;89(10):1299–308.View ArticlePubMedGoogle Scholar
4. Clausen T, Waal H, Thoresen M, Gossop M. Mortality among opiate users: opioid maintenance therapy, age and causes of death. Addiction. 2009;104(8):1356–62.View ArticlePubMedGoogle Scholar
5. Degenhardt L, Singleton J, Calabria B, Kerr T, Mehta S, Kirk G, et al. Mortality among concaine users: a systematic review of cohort studies. Drug Alcohol Depend. 2011;113(2–3):88–95.View ArticlePubMedGoogle Scholar
6. Mathers BM, Degenhardt L, Bucello C, Lemon J, Wiessing L, Hickman M. Mortality among people who inject drugs: a systematic review and meta-analysis. Bull World Health Organ. 2013;91(2):102–23.View ArticlePubMedPubMed CentralGoogle Scholar
7. Gossop M, Stewart D, Treacy S, Marsden J. A prospective study of mortality among drug misusers during a 4-year period after seeking treatment. Addiction. 2002;97(1):39–47.View ArticlePubMedGoogle Scholar
8. Aidala A, Cross JE, Stall R, Harre D, Sumartojo E. Housing status and HIV risk behaviors: implications for prevention and policy. AIDS Behav. 2005;9(3):251–6.View ArticlePubMedGoogle Scholar
9. Tompsett CJ, Domoff SE, Toro PA. Peer substance use and homelessness predicting substance abuse from adolescence through early adulthood. Am J Community Psychol. 2013;51(3–4):520–9.View ArticlePubMedPubMed CentralGoogle Scholar
10. Thompson Jr RG, Wall MM, Greenstein E, Grant BF, Hasin DS. Substance-use disorders and poverty as prospective predictors of first-time homelessness in the United States. Am J Public Health. 2013;103 Suppl 2:S282–8.View ArticlePubMedPubMed CentralGoogle Scholar
11. Topp L, Iversen J, Baldry E, Maher L, Collaboration of Australian NSP. Housing instability among people who inject drugs: results from the Australian needle and syringe program survey. J Urban Health. 2013;90(4):699–716.View ArticlePubMedGoogle Scholar
12. Linton SL, Celentano DD, Kirk GD, Mehta SH. The longitudinal association between homelessness, injection drug use, and injection-related risk behavior among persons with a history of injection drug use in Baltimore, MD. Drug Alcohol Depend. 2013;132(3):457–65.View ArticlePubMedPubMed CentralGoogle Scholar
13. Hibbs J, Benner L, Klugman L, Spencer R, Macchia I, Mellinger A, et al. Mortality in a cohort of homeless adults in Philadelphia. N Engl J Med. 1994;331(5):304–9.View ArticlePubMedGoogle Scholar
14. Baggett T, Hwang S, O'Connell J, Porneala B, Stringfellow E, Orav J, et al. Mortality among homeless adults in Boston: Shifts in causes of death over a 15-year period. JAMA Intern Med. 2013;173(3):189–95.View ArticlePubMedPubMed CentralGoogle Scholar
15. Cheung A, Hwang S. Risk of death among homeless women: a cohort study and review of the literature. Can Med Assoc J. 2004;170(8):1243–7.View ArticleGoogle Scholar
16. Morrison DS. Homelessness as an independent risk factor for mortality: results from a retrospective cohort study. Int J Epidemiol. 2009;38(3):877–83.View ArticlePubMedGoogle Scholar
17. Tyndall M, Craib K, Currie S, Li K, O'Shaughnessy M, Schechter M. Impact of HIV infection on mortality in a cohort of injection drug users. J Acquir Immunodeficiency Syndr. 2001;28(4):351–7.View ArticleGoogle Scholar
18. Strathdee SA, Palepu A, Cornelisse PG, Yip B, O'Shaughnessy MV, Montaner JS, et al. Barriers to use of free antiretroviral therapy in injection drug users. JAMA. 1998;280(6):547–9.View ArticlePubMedGoogle Scholar
19. Tyndall MW, Currie S, Spittal P, Li K, Wood E, O'Shaughnessy MV, et al. Intensive injection cocaine use as the primary risk factor in the Vancouver HIV-1 epidemic. AIDS. 2003;17(6):887–93.View ArticlePubMedGoogle Scholar
20. Craib KJ, Spittal PM, Wood E, Laliberte N, Hogg RS, Li K, et al. Riskfactors for elevated HIV incidence among Aboriginal injection drug users in Vancouver. Can Med Assoc J. 2003;168(1):19–24.Google Scholar
21. Spittal P, Craib K, Wood E, Laliberté N, Li K, Tyndall M, et al. Riskfactors for elevated HIV incidence rates among female injection drug users in Vancouver. Can Med Assoc J. 2002;166(7):894–9.Google Scholar
22. Maldonado G, Greenland S. Simulation study of confounder-selection strategies. Am J Epidemiol. 1993;138(11):923–36.PubMedGoogle Scholar
23. Galea S, Vlahov D. Social determinants and the health of drug users: socioeconomic status, homeless and incarceration. Public Health Rep. 2002;117(1):S135–45.PubMedPubMed CentralGoogle Scholar
24. The Universal Declaration of Human Rights [http://www.un.org/en/documents/udhr/index.shtml]
25. Palepu A, Patterson M, Moniruzzaman A, Frankish J, Somers J. Housing first improves residential stability in homeless adults with concurrent substance dependence and mental disorders. Am J Public Health. 2013;103(2):e30–6.View ArticlePubMedPubMed CentralGoogle Scholar
26. Darke S. Self-report among injecting drug users: a review. Drug Alcohol Depend. 1998;51(3):253–63.View ArticlePubMedGoogle Scholar
27. Rachlis B, Wood E, Li K, Hogg R, Kerr T. Drug and HIV-related risk behaviors after geographic migration among a cohort of injection drug users. AIDS Behav. 2010;14(4):854–61.View ArticlePubMedGoogle Scholar