Cohort Profile: The Melbourne Injecting Drug User Cohort Study (SuperMIX)

The Melbourne Injecting Drug User Cohort Study (SuperMIX) is a prospective cohort of people who inject drugs (PWID), established in 2008, when it was referred to as MIX. Most participants were recruited across 2008–10 for the original MIX cohort and 2017–present as SuperMIX. SuperMIX is Australia’s largest and only active longitudinal cohort of PWID. The cohort aims to describe the natural history and longitudinal trajectories of injecting drug use, including risk and protective factors of adverse health outcomes, cessation of and relapse into injecting drug use, and the impacts of health service access. Data collection involves baseline and annual follow-up interviews and venous blood samples with linked health service data. During interviews, a questionnaire that records demographics, drug purchase and use, health service utilisation, criminal behaviour and criminal justice system interactions is administered. As of July 2019, 1303 PWID had enrolled in SuperMIX, with 4835 interviews completed. Participants had an average of 6 years follow-up, with high retention (68% of participants had at least one follow-up interview in the 2 years preceding July 2019). MIX/SuperMIX has advanced the understanding of patterns of injection drug use, drug market dynamics, hepatitis C epidemiology, health service use, drug-related mortality and changes in well-being over time among PWID. Data are available for collaborative research through application to the cohort investigator team. The Melbourne Injecting Drug User Cohort Study (SuperMIX) is an ongoing longitudinal cohort on the natural history of drug use, service use and drug-related harms. SuperMIX is an evolution of the MIX cohort that was established in 2008 to identify trajectories of injection drug use among people who inject drugs (PWID) recruited from urban locations in Melbourne, Australia. Baseline data collection was initially funded by the Colonial Foundation Trust with funding for follow-up from the National Health and Medical Research Council (NHMRC). From 2013 onwards the cohort was referred to as SuperMIX. Upon receipt of a new round of NHMRC funding in 2017 (through to end 2021) the cohort was expanded and shifted to focus specifically on: (i) short- (1 year) and long-term (3-year) cessation of injection drug use and relapse in the cohort and key drivers of these outcomes; and (ii) evaluation of the impact of health care services oriented towards PWID, such as PWID-specific primary care, on the health and well-being of participants. We expect the cohort to continue through to 2026, depending on funding. The cohort was approved by the Victorian Department of Health Human Research Ethics Committee (approval number: 28.13.17) and the Australian Institute of Health and Welfare Ethics Committee (approval number: EO2013/3/48). Cohort participants were recruited through a combination of respondent-driven sampling, snowball sampling and street-based outreach methods. Recruitment took place in multiple Melbourne metropolitan locations, often in and around prominent drug markets (St Kilda, Footscray, Dandenong, Melbourne central business district, Collingwood/Fitzroy/Richmond and Frankston) and in the Greater Geelong region. Initial inclusion criteria included being aged between 18 and 30 years, having injected either heroin and/or amphetamines at least once a month for the 6 months preceding baseline interview, not enrolled in opioid agonist therapy (OAT) at the time of the baseline interview, willing to provide detailed contact information, holding a valid Australian universal health care insurance (Medicare) number and currently residing in Melbourne or the Greater Geelong region. Eligibility based on not receiving OAT was withdrawn 3 months into the cohort and the upper age limit removed. These criteria were relaxed due to difficulties recruiting sufficient participants without recent exposure to OAT under the original criteria. Written informed consent, including consent to access linked data, was obtained from all participants. Baseline data on the initial cohort of 688 participants recruited in 2008–10 were presented in a previous manuscript describing the MIX cohort.1 Since then, new recruitment has expanded the size of the cohort by 615 participants from three different sources. A total of 69 participants were from a cohort of PWID with similar recruitment criteria—the Networks 2 Cohort.2 This cohort ran from 2005 through 2009, and combined molecular and social network epidemiological methods to evaluate hepatitis C transmission dynamics among PWID recruited across metropolitan Melbourne. After the inclusion of these participants,the cohort was referred to as ‘SuperMIX’. Key baseline characteristics such as age, sex, age at first injection and injection frequency were similar to the original cohort.3 From July 2017, a new recruitment process was implemented through which an additional 507 participants were recruited to the cohort. Additionally, 39 participants from the Prison and Transition Health Cohort Study (PATH)4 and the Treatment and Prevention study (TaP)5 who met the criteria were also enrolled into SuperMIX. The study design, methods and baseline characteristics of the initial cohort were reported in 2013.1 By July 2019, a total of 1303 PWID had been enrolled in SuperMIX and key sociodemographic characteristics of this expanded cohort are shown in Table 1. At baseline, the median age was 30 years [interquartile range (IQR) = 2, 35]; 67% were male, 12% identified as Aboriginal and Torres Strait Islander and 83% were born in Australia. The main source of income for most participants was government benefit or pension (86%), 87% were unemployed, 34% did not complete year 10 of schooling (final year of compulsory education in Australia), 36% resided in unstable accommodation, 27% lived alone and 42% had children. Two-thirds (66%) had been incarcerated at least once, 29% had been incarcerated in the 12 months preceding baseline interview, the median time since first injection was 12 years (IQR = 8, 18) and for the majority (73%) heroin was the most injected drug in the previous month. Sociodemographics, drug behaviours and health-related characteristics among 1303 participants in the SuperMIX cohort and differences between initial cohort participants and new cohort participants, baseline interviews, April 2008–June 2019 Totals might not add up due to missing values. IQR, interquartile range; OAT, opioid antagonist therapy; HCV, hepatitis C virus. SuperMIX: a combined cohort of 1303 people who inject drugs. MIX: initial cohort participants recruited in the first 5 years of the cohort (2008–13), as reported in Horyniak et al. 2013.1 Participants recruited between 2013 and June 2019. Pearson chi square or Fisher’s exact statistics for categorical variables, Kruskal–Wallis tests for continuous variables. Table 1 also shows the comparison across the various sociodemographic and health-related variables reported in the original MIX baseline paper1 with the participants recruited after 2013. Compared with the initial cohort, newer recruits were older, in circumstances that made them more likely to be socially vulnerable (e.g. history of incarceration, homelessness) at the time of their baseline interview and to report higher frequency of risk behaviours such as frequent injecting at baseline. See Supplementary material, available as Supplementary data at IJE online, for more details on methods for the self-report data. SuperMIX is an open-ended longitudinal natural history cohort. In face-to-face and telephone (mostly during COVID-19 restrictions) interviews participants are administered a structured, quantitative annual questionnaire that records information on demographic characteristics, current and past drug purchases and use, personal well-being, health service utilisation, criminal behaviour and interactions with criminal justice systems. Interviews take approximately 1 h to administer. From 2009 onwards, venous blood samples have been collected for serological testing for hepatitis C and hepatitis B virus, and HIV. On 1 July 2019, the proportion of participants lost to follow up was 32% (n = 417) (i.e. who had no follow-up interview in the 2-year period preceding 1 July 2019). This attrition rate is comparable to what has previously been found for the initial 2008–13 cohort (29%).1 Across the cohort, the total follow-up time was 4354 person-years. Among participants with at least two follow-up interviews (n = 734; 56% of participants), the average follow-up time was 5.9 years [standard deviation (SD) = 3.4], with an average of 5.8 interviews in total (SD = 3.1). Compared with those who were retained in the cohort, participants lost to follow-up were more likely to be younger (32 vs 28 years), and less likely to identify as Aboriginal and Torres Strait Islander (15% vs 6%), be born in Australia (86% vs 76%) or reside in unstable housing (42% vs 21%) (Table 2). Participants retained in SuperMIX versus those lost to follow-up, April 2008–June 2019 Totals might not add up due to missing values. SuperMIX: combined cohort of 1303 people who inject drugs. Lost to follow-up is defined as not having had a follow-up interview in the 2 years preceding 1 July 2019. Pearson chi square or Fisher’s exact statistics for categorical variables, Kruskal–Wallis tests for continuous variables. A detailed description of the measures collected during each baseline interview was published in the 2013 baseline paper.1 The cohort questionnaire has undergone varied developments and adaptations since recruitment. Key changes include the addition of the Kessler-10 (K10), a 10-item measure of psychological distress, in 2010. This scale was replaced in 2015 by the Patient Health Questionnaire (PHQ-9), a nine-item psychological screening tool assessing levels of distress and functional impairment, and the General Anxiety Disorder-7 (GAD7), a seven-item anxiety scale.6 Further, in 2015 the Oral Health Impact Profile (OHIP) instrument was introduced to capture oral health-related quality of life among participants.7 Participants are also recruited to do qualitative in-depth interviews to glean insights into topics such as experiences of opioid agonist therapy (OAT), a treatment for dependence on opioids.8 From 2009 onwards, participants are asked to provide venous blood samples for serological testing. In bloods analysed up to February 2015, we found 19 incident hepatitis C virus (HCV) infections, with an incidence of 7.6 per 100 person years.9 Participant self-report data are enhanced by linkage to health records from the Medicare Benefits Schedule (MBS) for government-subsidized medical services, the Pharmaceutical Benefits Scheme (PBS) for government-subsidized prescription drugs (excluding OAT which is provided under a special access arrangement), Ambulance Victoria (VACIS) for ambulance attendances, the Victorian Admitted Episode Dataset (VAED) for inpatient hospital separations, the Victorian Emergency Minimum Dataset (VEMD) for emergency department (ED) presentations, the Alcohol and Drug Information System (ADIS) for specialist drug treatment use (excluding OAT), and the National Death Index (NDI) for mortality. SuperMIX studies that have linked self-report data to these databases have reported on hospital separations,10 emergency department presentations,11–13 and mortality.14Table 3 shows the total number of records available from linkage with the above health service datasets, except for NDI. Data linkage will be updated periodically through to the end of the cohort. Below we describe data available through to 1 July 2018. Records and participants from six health services datasets that were linked to the SuperMIXa self-report data, 2008–18 SuperMIX: combined cohort of 1303 people who inject drugs. Figure 1A shows the total number of general practitioner (GP) visits over 2008–18 from MBS data. There was a 3% annual decrease in GP visit incidence over the study period [incidence rate ratio (IRR) = 0.97; 95% CI 0.97 to 0.97, P <0.001). Figure 1B shows the number of prescriptions that are mental health-related (antidepressants and antipsychotics), sleeping-, stress- and anxiety-related (benzodiazepines), pain management-related (opioids and pregabalin) and others over 2008–18 using PBS data. There was a 4% annual increase in mental health-related prescription incidence (IRR = 1.04; 95% CI 1.03 to 1.04, P <0.001), a 3% decrease in benzodiazepine (IRR = 0.97; 95% CI 0.97 to 0.97, P <0.001) and a 4% per year increase in other prescription incidence (IRR = 1.04; 95% CI 1.03 to 1.04, P <0.001) over the study period. There was no evidence of a change in opioid prescription incidence over the study period (IRR = 0.99; 95% CI 0.99 to 1.000, P = 0.075). Figure 1C shows the total number of ambulance attendances, ED presentations and hospital separations over 2008–18. There was a 4% annual increase in ambulance attendances (IRR = 1.04; 95% CI 1.03 to 1.05, P <0.001) and a 3% annual increase in ED presentation incidence over the study period (IRR = 1.03; 95% CI 1.03 to 1.04, P <0.001). There was no evidence of a change in hospital separations over the study period (IRR = 1.01; 95% CI 0.99 to 1.01, P = 0.181). Figure 1D shows the number of completed alcohol and other drugs treatment episodes over 2008–18. There was a 7% annual decrease in treatment episode incidence over the study period (IRR = 0.93; 95% CI 0.93 to 0.94, P <0.001). See Supplementary material for more details on the record linkage process. Total number of GP visits (A), prescriptions (B), ambulance attendances, emergency department presentations and hospital admissions (C), and completed specialist drug treatment episodes (D) per year (2008–18) Since the first publication in 2013, 37 manuscripts have been published using data from the cohort. A complete list of MIX/SuperMIX publications can be found at [https://www.burnet.edu.au/projects/89_supermix_the_melbourne_injecting_drug_user_cohort_study]. Recent publications are summarized below, focused on published findings with use of participants recruited from 2017 onwards, with analyses under way around the main aims of injection drug use cessation and relapse. Among SuperMIX participants, drug use was found to be a persistent behaviour, with gradual and minor declines observed in the frequency of the cohort’s overall drug use. However, around a third reported that they had engaged in co-injecting substances (use of concurrent substances), associated with a range of sociodemographic and drug use-related factors.15 In-depth interviews revealed that the motivations for PWID to co-use methamphetamines and opioids were shaped by achieving and maintaining intoxication, but also by external factors such as the availability of drugs at the time.16 Recently, data from our cohort have contributed to the review of Melbourne’s first Medically Supervised Injecting Room (MSIR), undertaken by the Victorian Government. During its first 18 months of operation, the MSIR attracted socially marginalized PWID most at risk of harms related to injecting drug use and, therefore, most in need of the service.17 The Government’s response to the review resulted in decisions to extend the MSIR operations for another 3 years and to open a second site. SuperMIX data have also been included in a key review related to injection drug use, viral infections and homelessness.18 SuperMIX has several strengths. It is the largest ever cohort of PWID conducted in Australia and one of few cohorts internationally that is still active, with over 4000 years of person follow-up time and an attrition rate of 32% (i.e. participants who had at least one interview in the 2-year period preceding 1 July 2019). Recruitment of PWID early in their injecting careers, coupled with long-term follow-up and detailed characterization of participants, makes SuperMIX unique and ideally positioned to examine cessation of and relapse to injection drug use, viral infection risk and overdose deaths, to optimize existing interventions for PWID. The cohort has collected a broad range of data on reported behaviours, housing, well-being, employment, drug purchasing and needle sharing, and there is the ability to add in questions as services change, such as the opening of a safe-injecting facility, or to measure the impact of COVID-19 restrictions.19 SuperMIX data have been linked to a range of health services datasets, which has never been previously undertaken in studies of PWID in Australia. Most of the participants of SuperMIX consented to linkage, and linkage continues for those lost to follow-up for interviews. Several weaknesses of the cohort should also be mentioned. The frequency of interviewing is once a year. Higher interview frequency would provide more power to estimate effects of self-reported exposures across all cohort outcomes. Furthermore, the cohort questionnaire has undergone varied developments and adaptations since baseline recruitment, which have generally improved the questionnaire but impaired comparability in some domains. For linking SuperMIX data to Victorian health-linked data sets, deterministic linkage methods are used. This provides a potentially conservative (under)estimate of the use of emergency department and hospital separations. Also, no reliable linked data are available on OAT, because the Victorian government permit database still does not routinely update inactive permits, meaning that it is not possible to determine whether a participant’s permit is active or whether they have since ceased therapy. Data access requires permission from the Chief Investigators for the cohort, for collaborative work. Researchers interested in data access should contact Professor Paul Dietze directly at the Burnet Institute [[email protected]] to discuss potential collaborations and to obtain a copy of the application form for data access developed for the project. Supplementary data are available at IJE online. P.D., M.S. and L.M. are funded by National Health and Medical Research Council (NHMRC) Senior Research Fellowships. P.L.H. is funded by an NHMRC Postgraduate Scholarship. Baseline data collection for SuperMIX was funded by the Colonial Foundation Trust and the NHMRC (#545891, #1126090), with ongoing data collection funded by the NHMRC alone. The authors gratefully acknowledge the support of the Victorian Operational Infrastructure Fund. The funders had no input into the work. M.H. acknowledges funding from NIHR Health Protection Research Unit in Behavioural Science and Evaluation. We would like to acknowledge the contribution of the SuperMIX participants, the Burnet Institute fieldwork team and supporting community services and organizations. The authors would like to acknowledge the Victorian Department of Health as the source of VAED, VEMD and ADIS data for this study, and the Centre for Victorian Data Linkage (Victorian Department of Health) for the provision of data linkage for these datasets; Ambulance Victoria for linkage and provision of the VACIS dataset, Services Australia for the provision of MBS and PBS datasets and the Australian Institute of Health and Welfare for the linkage of the MBS and PBS datasets and overall management of data linkage and management processes. W.V. and P.D. initiated the manuscript. W.V. and M.Q. acquired the data, conducted the main data analyses and liaised with data linkage providers. P.A. and P.D. contributed to the analyses and interpretation. All authors contributed to the writing of the manuscript and approved the final version. P.D., P.Hig., N.S. and M.S. have received investigator-driven research funding from Gilead Sciences for work on hepatitis C unrelated to this work. P.D. has served as an unpaid member of an Advisory Board for an intranasal naloxone product. P.H. and M.S. have received funding from Abbvie for work on hepatitis C unrelated to this work. M.H. has received unrestricted and unrelated speaker fees and travel expenses in the past 3 years from Gilead and MSD.

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