Estimating sizes of hidden or hard-to-reach populations is an important problem in public health. For example, estimates of the sizes of populations at highest risk for HIV and AIDS are needed for designing, evaluating and allocating funding for treatment and prevention programmes. A promising approach to size estimation, relatively new to public health, is the network scale-up method (NSUM), involving two steps: estimating the personal network size of the members of a random sample of a total population and, with this information, estimating the number of members of a hidden subpopulation of the total population. We describe the method, including two approaches to estimating personal network sizes (summation and known population). We discuss the strengths and weaknesses of each approach and provide examples of international applications of the NSUM in public health. We conclude with recommendations for future research and evaluation.

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BackgroundWe evaluated the effectiveness and cost-effectiveness of interventions targeting hepatitis C virus (HCV) and HIV infections among people who inject drugs (PWID) in Eastern Europe/Central Asia. We specifically considered the needle-syringe program (NSP), opioid substitution therapy (OST), HCV and HIV diagnosis, antiretroviral therapy (ART), and/or new HCV treatment (direct acting antiviral [DAA]) in Belarus, Georgia, Kazakhstan, Republic of Moldova, and Tajikistan.MethodsWe developed a deterministic dynamic compartmental model and evaluated the number of infections averted, costs, and incremental cost-effectiveness ratios (ICERs) of interventions. OST decreased frequencies of injecting by 85% and NSP needle sharing rates by 57%; ART was introduced at CD4 <350 and DAA at fibrosis stage ≥F2 at a $2370 to $23 280 cost.ResultsIncreasing NSP+OST had a high impact on transmissions (infections averted in PWID: 42% in Tajikistan to 55% in Republic of Moldova for HCV; 30% in Belarus to 61% in Kazakhstan for HIV over 20 years). Increasing NSP+OST+ART was very cost-effective in Georgia (ICER = $910/year of life saved [YLS]), and was cost-saving in Kazakhstan and Republic of Moldova. NSP+OST+ART and HIV diagnosis was very cost-effective in Tajikistan (ICER = $210/YLS). Increasing the coverage of all interventions was always the most effective strategy and was cost-effective in Belarus and Kazakhstan (ICER = $12 960 and $21 850/YLS); it became cost-effective/cost-saving in all countries when we decreased DAA costs.ConclusionIncreasing NSP+OST coverage, in addition to ART and HIV diagnosis, had a high impact on both epidemics and was very cost-effective and even cost-saving. When HCV diagnosis was improved, increased DAA averted a high number of new infections if associated with NSP+OST.

Background: This study examines socio-demographic profiles, injecting risk, and use of health services among young injectors (15)(16)(17)(18)(19)(20)(21)(22)(23)(24) in Albania, Moldova, Romania, and Serbia. The objective was to provide age-disaggregated data to identify differences between adolescents (<18) and youth (18)(19)(20)(21)(22)(23)(24), and help fill the gap in knowledge on the youngest injectors in this region. Methods: Cross sectional surveys were conducted in each country using chain-referral sampling to reach diverse networks of people who use drugs (PWID). In Albania and Romania, surveys were conducted in the capitals, respectively Bucharest and Tirana. Respondents were recruited from 3 cities in Moldova (Chisinau, Balti and Tiraspol) and Serbia (Belgrade, Novi Sad and Nis). Data were collected on risk behaviours, service use, and contact with police and other authorities. Analysis focused on associations between unsafe injecting behaviour and key determinants including demographic background, source of needles/syringes, use of harm reduction services, and interactions with law enforcement. Results: Although drug use and health-seeking varied across settings, sources of injecting equipment were significantly associated with sharing needles and syringes in Moldova, Romania and Serbia. Obtaining equipment from formal sources (pharmacies, needle-exchange programmes) reduced likelihood of sharing significantly, while being stopped by the police or incarcerated increased it. Adolescents relied on pharmacies more than public sector services to obtain equipment. Conclusion: Adolescents comprise a small proportion of PWID in this region, but have poorer access to harm reduction services than older peers. Engaging young PWID through private and public sector outlets might reduce unsafe practices, while use of the justice system to address drug use complicates efforts to reach this population.

Monitoring injecting drug users' (IDUs) health is challenging because IDUs form a difficult to reach population. We examined the impact of recruitment setting on hepatitis C prevalence. Individual datasets from 12 studies were merged. Predictors of HCV positivity were sought through a multilevel analysis using a mixed-effects logistic model, with study identifier as random intercept. HCV prevalence ranged from 21% to 86% across the studies. Overall, HCV prevalence was higher in IDUs recruited in drug treatment centres compared to those recruited in low-threshold settings (74% and 42%, respectively, P < 0·001). Recruitment setting remained significantly associated with HCV prevalence after adjustment for duration of injecting and recent injection (adjusted odds ratio 0·7, 95% confidence interval 0·6-0·8, P = 0·05). Recruitment setting may have an impact on HCV prevalence estimates of IDUs in Europe. Assessing the impact of mixed recruitment strategies, including respondent-driven sampling, on HCV prevalence estimates, would be valuable.