Prevention of mother-to-child transmission of HIV (PMTCT) programs offer a range of services to women living with HIV (WLHIV) to maintain their health and minimize the risk of their HIV-exposed infants (HEI) acquiring HIV. Despite widespread implementation of PMTCT services in sub-Saharan Africa, only two-thirds of WLHIV are virally suppressed, and nearly 40% of HEI are lost to follow-up or die by 18 months of age. There is a critical need to develop person-centered strategies targeting the factors preventing WLHIV and their HEI from realizing the full benefits of PMTCT care. Mentor Mothers (MM) are peer advocates who support WLHIV and HEI to promote improved PMTCT outcomes. While MM show promise in this regard, it is clear a “one size fits all” approach lacks responsiveness to the fact that some WLHIV and HEI need minimal support while others have specific needs. Research is needed to understand what strategies MM can use to address factors known to influence PMTCT outcomes, and how best to implement these strategies with high-fidelity in real-world settings. This proposal aims to address knowledge gaps and promote optimal PMTCT outcomes through person-centered development of an enhanced MM strategy that focuses on both the specific problems faced by WLHIV and HEI, as well as the real-world constraints of PMTCT programs. The central hypothesis is that this enhanced MM strategy can be implemented with high acceptability and fidelity, resulting in improved PMTCT outcomes. We will test this hypothesis through research targeted to the following Specific Aims. AIM 1: Use a participatory research approach to develop an enhanced MM strategy. In Sub-Aim 1a we will perform qualitative interviews with MM and WLHIV (both those with and without unfavorable PMTCT outcomes [attrition, viremia, or vertical transmission]) to identify: (i) feasible enhancements to the current MM strategy, (ii) barriers to optimizing the MM strategy, and (iii) approaches to overcoming these barriers. In Sub-Aim 1b we will conduct participatory design workshops with key implementation stakeholders (MM, PMTCT program leadership, and clinic staff) to develop an enhanced MM strategy with optimal potential for sustainable implementation. AIM 2: Conduct a pilot hybrid type 3 implementation-effectiveness study of the enhanced MM strategy. The enhanced MM strategy will be implemented at one clinic. We will use a mixed-methods approach to thoroughly assess acceptability, feasibility, and fidelity to the enhanced MM strategy. A pre-post assessment of key PMTCT outcome measures will provide preliminary effectiveness data. We expect this K23 proposal to advance my expertise in HIV implementation science, qualitative and mixed-methods research, and person- centered participatory design while also generating the compelling preliminary data needed to support a future R01 application for a pragmatic cluster randomized clinical trial of the enhanced MM strategy. These K23 research and career deve...