Why do people take part in vaccine trials? A mixed methods narrative synthesis

Objectives: To understand why individuals do or do not take part in vaccine trials, exploring the motivators and barriers to identify effective strategies to optimise recruitment in vaccine research.Methods: Qualitative studies and quantitative surveys capturing data on reasons for trial participation/decline were included. Six databases were searched from 1996 to October 2021. Two reviewers independently screened and assessed risk of bias. Results were reported narratively and analysed using thematic analysis.Results: We included 32 studies (17 qualitative; 12 quantitative; 3 mixed-methods) that covered a wide range of populations, geographical areas and disease types. Eight themes were identified 1) altruism; 2) potential for personal benefit; 3) perceived risks; 4) trust or distrust; 5) social networks; 6) stigma; 7) practical implications; 8) research vanguard.Conclusion: Our findings provide a detailed description of how potential participants weigh up their decisions to participate in vaccine trials, which could inform the planning and implementation of studies to enhance recruitment.Practice implications: Clinical trial researchers should consider a patient-centered approach to recruitment, tailoring promoting material and attempt to understand fears, stigma and perceived risks. In addition, recog-nising the importance of trust and the key role friends, communities, family, and those in supervisory positions play in decisions.