Abstract
The prevalence of injection drug use has been of especial interest for assessment of the impact of blood-borne viruses. However, the incidence of injection drug use has been underresearched. Our 2-fold aim in this study was to estimate 1) how many other persons, per annum, an injection drug user (IDU) has the equivalent of full responsibility (EFR) for initiating into injection drug use and 2) the consequences for IDUs' replacement rate. EFR initiation rates are strongly associated with incarceration history, so that our analysis of IDUs' replacement rate must incorporate when, in their injecting career, IDUs were first incarcerated. To do so, we have first to estimate piecewise constant incarceration rates in conjunction with EFR initiation rates, which are then combined with rates of cessation from injecting to model IDUs' replacement rate over their injecting career, analogous to the reproduction number of an epidemic model. We apply our approach to Scotland's IDUs, using over 2,500 anonymous injector participants who were interviewed in Scotland's Needle Exchange Surveillance Initiative during 2008–2009. Our approach was made possible by the inclusion of key questions about initiations. Finally, we extend our model to include an immediate quit rate, as a reasoned compensation for higher-than-expected replacement rates, and we estimate how high initiates' quit rate should be for IDUs' replacement rate to be 1.
Highlights
OF SCOTLAND’S injection drug user (IDU) EPIDEMICIn the early 1980s, Scotland experienced an epidemic of injection drug use [7] and of injection-related blood-borne viruses, notably human immunodeficiency virus [8], hepatitis B virus [9], and hepatitis C virus [10] infections
Explanations of our high equivalent of full responsibility (EFR) rate include the following: 1) IDU instructor thinks he/she is initiating but, the novice has had previous initiations; 2) systematic bias is underestimated; 3) recall bias is exaggerated; 4) mismatch between Needle Exchange Surveillance Initiative (NESI) respondents and Scotland’s current IDUs; 5) changes over time, which gave rise to Scotland’s number of current IDUs; and 6) many 21st century novices do not persist with injecting beyond the initiation event
We have presented a novel approach for estimating IDU incidence in terms of EFR initiation rates
Summary
In the early 1980s, Scotland experienced an epidemic of injection drug use [7] and of injection-related blood-borne viruses, notably human immunodeficiency virus [8], hepatitis B virus [9], and hepatitis C virus [10] infections. Day et al [26] surveyed 399 heroin users in Sydney, Australia (mean age at interview = 31 (standard deviation, 8.2) years; median injecting-career length, 9.5 years), of whom almost all had injected heroin. One in 6 (17%) reported having taught someone to inject in the past year, the median being 2 novices (range, 1–50), which implied an initiation rate of 0.34 per IDU per annum. Kermode et al [28] recruited 200 early career IDUs (age at interview: mean = 24.5 (standard deviation, 2.2) years) in northeast India, 138 of whom (69%) had initiated 690 others over injecting careers of a mean length = 3.4 years, which implied an initiation rate of 1.01 per IDU per annum. There were between 3 and 4 people present (including the person being initiated), so the 690 initiations may have related to many fewer initiates because of multiple counting by those present at the same time
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