Participant recruitment during war: challenges and strategies from the longitudinal mental health assessment of the population study in Ukraine

Abstract Conducting longitudinal cohort studies in war conditions is accompanied by a range of challenges. This article describes the recruitment and implementation process of a large, population-based longitudinal mental health study conducted in Ukraine during the war. We focus on the evolution of the recruitment strategy, four categories of challenges encountered—technical, security, personal, and socio-cultural, along with the solutions, and key lessons learned, while reporting the number of individuals contacted and reached through different channels to achieve a substantial participation rate in a complex, war-affected setting. Recruitment for the baseline assessment began in April 2024, during an active phase of the war, and was intended to use semi-automated mass mailing via REDCap and Telegram with personal communication. However, even after extensive pilot testing initial outreach for the full study via REDCap proved largely ineffective, with most emails flagged as spam by Ukrainian email providers due to heightened national cybersecurity measures. Telegram outreach was similarly constrained by platform-level security limitations. To address these challenges, the team adopted platforms to those locally trusted, which significantly improved delivery rates. The involvement of local interviewers played a crucial role in enhancing participant trust and engagement. Integrating automated outreach with personalized phone calls by local interviewers significantly enhanced engagement. This adaptive recruitment strategy successfully reached 48.4% of invitees and achieved a 15.5% participation rate. Our experience demonstrates that even when working with randomly selected populations, participation rate can be significantly improved through flexible models that align with local social, cultural, and technological realities.