: A lay health worker delivered digital intervention for depression in Zimbabwe – optimisation and effectiveness trial with embedded testing of mediators.

Digital interventions for depression are hardly available in Sub-Saharan Africa despite high mobile penetration. The Inuka app is a chat-based digital platform based on the successful Friendship Bench model developed in Zimbabwe. Its advantages for scalability over face-to-face therapy include better access for clients and lay counsellors, better reach for a wider sector of the population, and in-built quality control for fidelity. Pilot studies in African countries suggest Inuka is feasible for implementation and improves depression. However, high data costs and a lack of robust evidence on effectiveness impede broader adoption.We will optimise the Inuka app for scale in Zimbabwe and other low-resource settings by expanding existing partnerships with lived experience experts and organisations which can take Inuka to scale and co-producing a data-lite version of Inuka which is acceptable and feasible for end-users and lay counsellors. We will then conduct a randomised controlled trial in Zimbabwe to test if Inuka is at least as effective as enhanced usual care for depression, to test its mechanisms of action, and to describe its costs. This research will leverage the global Friendship Bench network to expand Inuka's reach across Sub-Saharan Africa and beyond, enhancing mental health support for low-resourced users.