Background In Samoa, lymphatic filariasis (LF) remains endemic. Targeted sampling strategies based on the locations of known infections could be more efficient, and therefore cost-effective, than random sampling for locating infected individuals and hotspots, providing valuable information to develop more efficient and effective interventions. The added efficiency of these strategies may depend on the prevalence of the chosen indicator in the area being surveyed. This study aims to assess the efficiency of targeted versus random sampling for identifying LF antigen (Ag)- and microfilaria (Mf)-positive individuals in Samoa across varying background Ag prevalence levels. Methodology In 2023, six primary sampling units (PSUs) were surveyed using random and targeted sampling strategies. PSUs were selected based on Ag prevalence in 2019, including two low (3-5%), medium (6-7%) and high Ag prevalence (13-17%). The randomly selected group included residents aged ³5 years in 15 houses per PSU. The targeted group included residents aged ³5 years in up to eight households within 200 metres of a household where Ag-positive resident(s) were identified in 2019.  Finger prick blood samples were tested for Ag and Ag-positive samples were examined for microfilaria (Mf). Principal Findings The targeted sampling strategy (n=400 people) identified more positives (57 Ag-positive, 23 Mf-positive) than the random sampling strategy (n=494, 39 Ag-positive, 16 Mf-positive), with an overall targeted:random sampled case ratio of 1.8 (95% CI 1.3-2.5) for Ag and 1.8 (95% CI 1.1-3.1) for Mf. Gain in efficiency was greatest in medium prevalence PSUs for both Ag-positives (ratio=2.4, 95% CI 1.3-5.2) and Mf-positives (ratio=2.6, 95% CI 0.9-12.8). Conclusions In Samoa, a targeted sampling strategy was a more efficient method for locating Ag-positive and Mf-positive individuals compared to random sampling, with the highest efficiency gain in medium Ag prevalence settings. The findings have design implications for LF surveillance in Samoa and other Pacific Island countries.

The authors have declared no competing interest.

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Ethics approvals were obtained from the Samoa Ministry of Health and The University of Queensland Human Research Ethics Committee (protocol 2021/HE000895). The study was conducted in close collaboration with the Samoa Ministry of Health, the World Health Organization (WHO) country office in Samoa, and the Samoa Red Cross. Prior to entering a village, permission was granted from village leaders to conduct the study. Verbal and written informed consent were obtained for all participants, or from the parents or guardians of participants who were less than 18 years old.

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Data used in this paper were collected during field surveys in Samoa. Communities in Samoa are small (some with less than 200 inhabitants) and sharing individual level data could enable identification of individual participants, and violating the conditions of the study’s ethics approval. For requests relating to data access, please contact the Human Ethics Department at the University of Queensland (humanethics@research.uq.edu.au) citing protocol 2021/HE000895”. if you would like access to the data. All relevant data at the primary sampling unit level has been included in the supplementary material.