Methods: A global systematic review collated data on the percentage of young PWID, IDU duration, average age of PWID, average age at IDU initiation, and percentage reporting sexual/injecting-risk behaviours. World Bank data were used for development indicators. Hypotheses were tested through generalized linear models.
Findings: Globally, 25.3% (95% Uncertainty Interval [95%UI]: 19.6-31.8) of PWID were aged ≤25 years. Eastern Europe had the highest percentage of young PWID (43.4%, 95%UI: 39.447.4%), Middle East and North Africa had the lowest (6.9%, 95%UI: 5.1%-8.8%). At the country-level higher GDP was associated with longer median injecting duration (0.11 years per $1000 GDP increase, 95% Confidence Interval [95%CI]: 0.04-0.18), and older median age of PWID (0.13 years per $1000 increase, 95%CI: 0.06-0.20). Urbanisation growth was associated with higher age at IDU initiation (1.40 years per annual percentage change, 95%CI 0.41-2.40). No associations are observed between indicators of youth IDU and country-level youth unemployment, GINI, or OST provision. There was no evidence for associations between injecting/sexual-risk behaviours and age of PWID.
Interpretation: Variation in the age profile of PWID is associated with GDP and urbanisation. Notably, regions with the highest prevalence of PWID aged ≤25 years have low coverage of interventions to prevent blood-borne virus spread. Data quality demonstrates required improvements in monitoring.
Funding: Australian National Drug and Alcohol Research Centre, Australian National Health and Medical Research Council, Open Society Foundation, WHO, the Global Fund, UNAIDS, NIHR HPRU EI, The Wellcome Trust.