Estimating the Population Size of People Who Inject Drugs in 3 Cities in Zambia: Capture-Recapture, Successive...

Area of Science:

• Epidemiology
• Public Health
• Biostatistics

Background:

• Accurate population size estimates (PSE) are critical for HIV prevention and treatment programs, especially for key populations like people who inject drugs (PWID).
• Limited reliable data on PWID in Zambia hinders effective public health responses.
• Estimating the size of hidden populations presents significant methodological challenges.

Purpose of the Study:

• To estimate the population size of people who inject drugs (PWID) in three major Zambian cities.
• To compare the variability of PSEs derived from different estimation methods.
• To explore the strengths and limitations of each applied method for estimating PWID populations.

Main Methods:

• Application of 2-source capture-recapture (2S-CRC), 3-source capture-recapture (3S-CRC), and successive sampling population size estimation (SS-PSE) methods.
• 3S-CRC combined location-based 2S-CRC with respondent-driven sampling (RDS) surveys.
• Analysis utilized Bayesian nonparametric latent class models and a Bayesian consensus estimator for final city PSEs.

Main Results:

• Bayesian consensus PSEs for PWID ranged from 0.5% to 1.8% of the adult male population across Lusaka, Ndola, and Livingstone.
• Estimates were highest in Lusaka (3700), followed by Ndola (2200) and Livingstone (1200).
• Variability in estimates was observed across methods, with SS-PSE generally yielding lower figures. RDS participants captured via 2S-CRC showed different characteristics than those solely reached by RDS.

Conclusions:

• This study provides the first rigorous PSEs for PWID in major Zambian geographies.
• The integration of RDS within 3S-CRC improved reach to diverse PWID characteristics, enhancing the consensus estimate.
• Findings offer valuable guidance for public health program planning and future surveillance activities in Zambia.