Background
Disrupted social connections may negatively impact youth mental health. In contrast, sustained quality social connections (QSC) can improve mental health outcomes. However, few studies have examined how these quality connections impact depression and anxiety outcomes within digital interventions, and conceptualisation is limited.

Objective
The study aim was to conceptualise, appraise and synthesise evidence on quality social connection within digital interventions (D-QSC) and the impact on depression and anxiety outcomes for young people (14-24).

Methods
A systematic scoping review and meta-analysis was conducted using the Johanna Briggs Institute methodological frameworks and guided by experts with lived experience. Reporting was guided by the Preferred Reporting Items for Systematic reviews and Meta-Analyses extension for Scoping Reviews (PRISMA-ScR). Medline, Embase, PsycInfo and Cumulative Index to Nursing and Allied Health Literature (CINAHL) databases were searched against a comprehensive combination of key concepts on 24th June 2020. Search concepts included young people, digital intervention, depression/anxiety, and social connection. Google was also searched. One reviewer independently screened abstracts/titles and full-text and 10% were screened by a second reviewer. A narrative synthesis was used to structure findings on indicators of D-QSC and mechanisms that facilitate the connection. Indicators of D-QSC from included studies were synthesised to produce a conceptual framework.

Results
5715 publications were identified and 42 were included. Of these, there were 23,319 participants. Indicators that D-QSC was present varied and included relatedness, having a sense of belonging and connecting to similar people. However, despite the variation, most of the indicators were associated with improved outcomes for depression and anxiety. Negative interactions, loneliness and feeling ignored indicated D-QSC was not present. In ten applicable studies, a meta-analysis showed a significant decrease in depression (-25.6%, 95% CI [-0.352, -0.160], p<0.0005) and anxiety (-15.1%, 95% CI [-0.251, -0.051], p<0.0005) following a D-QSC. Digital mechanisms that helped create a quality connection included anonymity, confidentiality and peer support. In contrast, mechanisms that hindered the connection included disconnection from the real world and inability to see body language. Data synthesis also identified a five-component conceptual framework of D-QSC that included Rapport, Identity and commonality, Valued interpersonal dynamic, Engagement, and Responded to and accepted, referred to as RIVER.

Conclusions
D-QSC is an important and under-considered component for youth depression and anxiety outcomes. Researchers and developers should consider targeting improved QSC between clinicians and young people within digital interventions for depression. Future research should build on our framework to further examine relationships between individual attributes of QSC, various digital interventions and different populations.

Funding
This work was funded by a Wellcome Trust Mental Health Priority Area 'Active Ingredients' commission awarded to Dr Lindsay Dewa at Imperial College London.