Implementing a complex psychosocial intervention for unstably housed Veterans: A realist-informed evaluation case study.

Background: Only 7% of individuals with co-occurring mental health and substance use disorder (COD) receive services for both conditions. We implemented and evaluated maintaining independence and sobriety through systems integration, outreach and networking-Veteran's edition (MISSION-Vet), an evidence-based manualized psychosocial intervention for Veterans with CODs. This paper identifies the generative mechanisms that explain "how, for whom, and under what conditions" MISSION-Vet adoption, implementation, and fidelity work when applied in a complex adaptive system with facilitation support. Methods: Within two VA healthcare systems (Sites A and B), a hybrid III trial tested facilitation to implement MISSION-Vet. We conducted a two-site case study based on 42 semi-structured consolidated framework for implementation research (CFIR) guided interviews with site leadership, implementers (social workers, peer specialists), and team members (facilitators, site leads). Interviews were coded and CFIR constructs used to generate "Context  +  Mechanism  =  Outcome" configurations to understand the conditions of MISSION-Vet adoption, implementation, and fidelity. Results: Site A was low, and Site B was high in adoption, implementation, and fidelity. Adoption hesitancy/eagerness (outcome) resulted from the interaction of "external policy" (context) dampening/encouraging a "tension for change" (mechanism). Implementation intensity (outcome) was based on how "peer pressure" or practice culture (context) activated staff "self-efficacy" (mechanism) to engage with MISSION-Vet and appraise its "relative advantage" over current practices (mechanism). Fidelity relied on how "staffing structure and availability" (context) activated/muted "facilitation" (mechanism) to result in strategy and intervention adaptation (outcome). Conclusions: We delineated how specific contexts activated certain mechanisms to drive the different stages of implementation of a multi-faceted COD treatment intervention. Trial registration: ClinicalTrials.gov, NCT02942979. Plain language abstract: Implementation is inherently dynamic and influenced by interdependent factors operating at the individual, organizational, and system levels. This is especially true for complex interventions addressing co-occurring mental health and substance use disorders because such interventions involve multiple treatment modalities delivered simultaneously, in busy practice settings, with challenging populations. This paper pairs consolidated framework for implementation research (CFIR) constructs with a realist evaluation approached to generate configurations important to the adoption, implementation, and adaptation stages of a highly complex intervention addressing the behavioural health and housing needs of a vulnerable population. Each configuration describes how contextual factors trigger mechanisms to generate implementation outcomes and answers "what works for whom, in what circumstances and in what respects, and how?" These findings further our understanding of possible mechanisms of change and push us to be more precise about identifying causal relationships among constructs that contribute to the success of implementing complex interventions. This work also moves us to think theoretically and methodologically in a more dynamic fashion, thereby leading to more responsive implementation practice.