PROJECT SUMMARY/ABSTRACT Antiretroviral (ARV) agents for treatment and prevention will be key to Ending the HIV Epidemic. Although oral pre-exposure prophylaxis (PrEP) is highly effective, new options less dependent on day-to-day adherence are needed to maximize impact. The era of long-acting PrEP has arrived, with rollout of two new products in 2022. The dapivirine vaginal ring (DVR), self-inserted monthly, is being implemented in sub-Saharan Africa, and injectable cabotegravir (CAB), given every 8 weeks, is now approved by the U.S. FDA. Despite the promise of these modalities to reduce HIV incidence, acute HIV infections (AHI) occur among PrEP users and were seen in DVR and CAB trials. Potential causes of AHI on PrEP modalities include inadequate ARV adherence/ exposure and resistance to the PrEP agent. As oral and long-acting PrEP are scaled up, 300,000 AHI could occur among PrEP users in the next 5 years. DVR showed modest efficacy in trials and high ring adherence will be critical. With high levels of circulating ARV resistance (from ARVs in the same class used for treatment), DVR could fail to block infection with resistant virus. While CAB efficacy was high in trials, AHI occurred despite on-time injections and resulted in resistance. Because the same drug classes are being used for HIV prevention and treatment, resistance from CAB could render U.S. and global first-line antiretroviral therapy (ART) regimens ineffective. Moreover, CAB levels persist for over 1 year in a pharmacokinetic (PK) tail that could further increase resistance risk. As DVR and CAB are scaled up, key knowledge gaps must be addressed: 1) What are causes of AHI on DVR and CAB in rollout settings? 2) What behavioral factors contribute to DVR/CAB non-adherence prior to AHI? 3) What is the risk of DVR/CAB resistance? 4) What are subsequent HIV treatment outcomes? This application proposes intensive studies of causes of AHI on DVR and CAB, leveraging three studies enrolling persons with AHI from large PrEP programs in western Kenya and the U.S. for sample collection, surveys, interviews, and ART outcomes assessment. Innovative methods will be deployed to detect drug resistance (next-generation sequencing), identify novel mutations (full-genome sequencing), retrospectively measure ARV exposure before diagnosis (PK analysis of hair samples cut into segments reflecting exposure over sequential 2-week intervals), and understand DVR/CAB adherence prior to AHI and ART adherence (longitudinal mixed methods). The proposed aims are: 1) To determine the causes of HIV infection among women using DVR in Kenya. 2) To identify the causes of HIV infection among persons using CAB for prevention in the U.S. 3) To evaluate subsequent outcomes on ART after HIV infection among persons using PrEP modalities. Collectively, these aims will provide the first and largest analysis of causes of and outcomes following AHI on DVR and CAB in rollout settings. This study will pave the way to address cru...