PROJECT SUMMARY Meeting targets set to end AIDS as a major public health threat by 2030 requires reaching all populations, particularly those with the highest burden, such as people who inject drugs (PWID). PWID continue to experience some of the fast-growing HIV epidemics globally. Injection drug use is increasingly accounting for new HIV infections in both low- and middle-income countries and countries that once saw notable declines in HIV incidence among PWID. Even in countries with notable declines in HIV incidence among PWID, such as the United States, the rise of prescription opioid use has resulted in increased heroin injection, increased overdose rates, and outbreaks of HIV. Despite decades of investigation into the behavioral drivers of HIV incidence, accurately predicting who is at the highest risk of becoming infected with HIV remains challenging. Yet, this information is critical for preventing outbreaks and tailoring interventions. These tools are even more critical at the final stages of disease control, elimination, or eradication programs which require disproportionately more effort and resources than in preceding stages to identify those at risk, especially in a setting of dwindling resources and emerging pandemics. As we rapidly approach the ambitious 2030 targets, the same will be true of HIV, even more so among hard-to-reach populations such as PWID, and we need early warning systems to guide a more targeted public health response We previously demonstrated that PWID accumulate blood-borne nonpathogenic viruses in the plasma before hepatitis C virus infection. We also have early data showing that sequences of these nonpathogenic viruses reveal epidemiologic links. This study builds on these findings to explore whether the plasma virome in PWID can be further used as a bioindicator of HIV risk and whether it can be leveraged to interrupt HIV outbreaks before they occur. To test this, we utilize a rare set of longitudinal social (injection partner) and spatial (injection venue) network data along with detailed individual-level risk factors and HIV sequences from a high-incidence cohort of over 2,500 PWID in New Delhi, India. To date, this cohort has observed over 159 HIV seroconversions. We plan to characterize the plasma virome using baseline specimens from participants who later acquired HIV to determine if virome richness independently predicts HIV incidence and to assess the added value of a bioindicator over established risk prediction tools. Additionally, we plan to sequence participants comprising complete contact networks and leverage next-generation sequencing approaches that capture the diversity of nonpathogenic viruses circulating within and between hosts to employ phylogenetic methods aimed at determining whether plasma virome sequences can accurately infer transmission networks. By using routinely collected samples, this work could lead to more robust molecular surveillance methods that can guide public health offi...