Applying open population capture–recapture models to estimate the abundance of injection drug users in Victoria,Canada

Background: Injection drug users (IDUs) are considered a hidden, hard to reach population that is difficult to measure. Multi-list recapture methods are commonly used to estimate IDU population sizes but do not allow inference on population dynamics. In Victoria, Canada, closed population capture–recapture methods have been used to estimate the abundance of IDUs. In this study, we make use of a newer sample of a survey of IDUs to relax the closure assumption.

Methods: The I-Track survey of IDUs was carried out in Victoria on three occasions (2003, 2005 and 2009). Data from the three samples were linked using unique subject identifiers. A Jolly-Seber model was used to estimate the number of IDUs.

Results: The results were very similar to a two-sample closed population analysis. However, when using open-population models, it is possible to get estimates for each time period of abundance and survival. The estimate of the number of the IDUs in Greater Victoria was relatively stable with fewer than 3000 individuals over the six-year study.

Discussion: Improved estimates of population size and dynamics will assist in improving access to harm reduction services, which may reduce higher risk drug use practices.