## 2 Materials and Methods
### Study Design  
In this qualitative study, we adopted a realist evaluation method. Realist evaluation studies have the purpose of identifying 'what works in which circumstances and for whom?' [28]. Therefore, realist evaluation aims to identify the hidden causal forces behind empirically observable patterns or changes in those patterns [29]. This is done through'retroduction': going back from observed patterns and looking below the surface for what might have produced them. As such, realist studies focus on context and necessary conditions for generating social mechanisms, making it a practical approach for studying complex social issues such as psychosocial health and building trust [30]. The central research question of this project is realist in nature: it aims to identify underlying mechanisms and the circumstances in which these mechanisms emerge. The theory developed as an answer to this research question is subject to further refinement and enrichment in consequent research loops, which is characteristic of realist evaluation. In this study, the realist evaluation formed the logic of inquiry and theory was developed following the principles of grounded theory [28; 31]. We chose this approach because of the appreciation of realism as a 'logic of inquiry that generates distinctive research strategies and designs, and then utilizes available research methods and techniques within these' [28]. Realist evaluation starts from an Initial Programme Theory (IPT) formulated in a context-mechanism outcome-structure (CMO). In this study, we started from an IPT, tracing processes backward to study the question 'what is it about CHWs that allows them to build trust with vulnerable communities, why is that, and under which circumstances?'. The IPT in this study was drawn from a literature review and from input we collected from the CHWs and the project coordinators in the course of the intervention. In addition, the Gilson framework was used as a structure to build the IPT and to guide the analytical steps. The IPT was formulated as follows: 'CHWs can build trust with vulnerable citizens during the lockdown and physical distancing measures in a primary care setting through the following mechanisms: by offering a sympathetic ear and standing close to the community, CHWs can profile as accessible persons, allowing vulnerable clients to feel connected with others and to feel less lonely.'. The IPT was then tested in focus group discussions with CHWs and in in-depth interviews with CHWs and clients. Testing the hypotheses in the IPT, sourcing from the already existing Gilson framework, led to the development of a middle-range theory (MRT), consisting of CMO hypotheses on how, why and when CHWs can build trust with vulnerable communities. A MRT is a program theory, developed at a mid-level range of abstraction, i.e., a theory concrete enough to test yet generalizable to different contexts. This program theory clarifies why, how, and in which circumstances CHWs can build trustful relationships with vulnerable clients (referring to mechanisms and influencing context factors of improved psychosocial health outcomes) [32]. This MRT can be tested and refined through further studies.