Participant recruitment and retention in a longitudinal study: experience from SARS-CoV-2 cohort in Ethiopia

Longitudinal cohort studies are essential for understanding changes in health over time, yet participant recruitment and retention remain major challenges, especially in low-resource settings. This study describes the strategies, outcomes, and lessons learned from a two-year longitudinal severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) cohort study conducted in Ethiopia, focusing on engagement among healthcare workers (HCWs) and community members. This prospective observational cohort study was nested within a larger longitudinal SARS-CoV-2 project conducted from November 2022 to December 2024. The study enrolled 500 participants, including 350 HCWs and 150 community members, at Jimma Medical Center (JMC). Participants were followed every three months over two years to assess SARS-CoV-2-specific antibody responses. Data collection included venous blood samples, nasopharyngeal swabs (NPS) during symptomatic episodes, and clinical and demographic information. Recruitment used multiple approaches such as phone calls, in-person outreach, and community visits. Retention strategies included flexible scheduling, regular communication, voluntary home visits, and engagement by culturally aware, participant-focused staff. The study reached 100% of its target enrollment within 48 days. Over the two-year follow-up, the overall retention rate was 88%, with 82% of participants completing all follow-up visits. High retention was supported by structured contact tracking, regular reminders, culturally sensitive communication, and fair participant compensation. A total of 60 participants were lost to follow-up. Among healthcare workers, the main reasons were relocation (36, 65.5%), health-related issues (6, 10.9%), and unstated reasons (7, 12.7%), with smaller numbers lost due to religious considerations, family influence, or death. In the community group, attrition was mainly due to health-related issues (2, 40%), relocation (1, 20%), religious perspectives (1, 20%), and family influence (1, 20%). High retention is achievable in longitudinal studies in resource-limited settings when supported by well-planned, context-specific recruitment and retention strategies. This study highlights the value of transparency, participant-centered practices, and operational flexibility, offering practical guidance for future public health research in low- and middle-income countries.