Using dual capture/recapture studies to estimate the population size of persons who inject drugs (PWID) in the city of Hai Phong, Vietnam

Background: Good estimates of key population sizes are critical for appropriating resources to prevent HIV infection. We conducted two capture/recapture studies to estimate the number of PWID currently in Hai Phong, Vietnam. Methods: A 2014 respondent-driven sampling (RDS) survey served as one capture, and distribution of cigarette lighters at drug use “hotspots” in 2016 served as another “capture.” A 2016 survey using RDS, conducted 1 week after lighter distribution, served as “recapture” for both captures. Recaptured participants in the two surveys were identified with a computerized fingerprint reader. Recaptured participants from the lighter distribution were asked to show their lighters. Results: 1385 participants were included in the “recapture” survey. They were 94% male and had a median age of 39. All (100%) injected heroin, and HIV prevalence was 30%. 144 of the 603 participants in the 2014 survey and 152 of the 600 PWID who had received lighters were “recaptured” in the 2016 survey. After adjusting for police suppression of drug use hotspots and conducting sensitivity analyses, our best estimate of the population size from the lighter recapture was 4617 (95% CI: 4090–5143), and our best estimate from the 2014 survey recapture was 5220 (95% CI: 4568–5872). A combined best estimate of the PWID population in Hai Phong is 5000, range 4000–6000. Conclusions: The capture/recapture studies produced consistent estimates. Adding a lighter/token distribution to planned RDS surveys may provide an inexpensive method for estimating PWID population size. Analyses of the estimates should include contextual information about the local drug scene.