SUMMARY Worldwide, over 3 billion people are at risk of infection and disease caused by dengue virus 1-4 (DENV1-4) and Zika virus (ZIKV), both potentially severe flaviviral diseases transmitted by Aedes mosquitoes. The devastating effects of endemic dengue across the tropics and subtropics are well documented. The recent Zika pandemic galvanized research as Zika swept across Latin America. Three years after the peak of the Zika pandemic, major dengue epidemics have started to re-occur; however, the future of flaviviral disease across areas with widespread ZIKV immunity is unknown. In this R01, we propose to develop new tools and address key knowledge gaps in flaviviral transmission and immunological interactions between DENV and ZIKV to understand how widespread ZIKV immunity impacts subsequent dengue disease and to inform evaluation of dengue and Zika clinical vaccine trials and post-licensure studies. Based on our serological, epidemiological, and clinical data to date, our overall hypothesis is that DENV1-4 and ZIKV are antigenically closely related and that immune interactions mutually affect transmission and disease severity. We will address this hypothesis with the ongoing Pediatric Dengue Cohort Study (PDCS, 2004-present), a community-based prospective cohort study in Managua, Nicaragua, following ~4,000 children, now in its 17th year. Samples from the PDCS, as well as companion studies in Managua, provide documented infection and disease data, as well as banked serum samples for over a decade before the arrival of ZIKV. The proposed study extends the cohort, ensuring that we are able to fully document the interactions of these viruses from the pre- to post-Zika eras. In Aim 1, we will develop innovative serologic tools based on glycan-fusion-loop-masked envelope proteins and new algorithms to distinguish DENV and ZIKV infection histories, critical for vaccination and epidemiological studies of dengue and Zika. We will then test our hypothesis that pre-existing ZIKV immunity can enhance disease severity caused by DENV3 but protect against DENV1. In Aim 2, we will measure changes in anti-DENV and anti-ZIKV antibody-mediated immunity over time, estimate annual changes in protective and enhancing population immunity to each virus, collect entomological data, and use modeling approaches to evaluate popula- tion susceptibility to DENV and ZIKV infection and the potential for future epidemics by incorporating immunolo- gical and entomological data. In Aim 3, we will identify determinants of protective and disease-enhancing anti- body-mediated immunity of prior DENV infection on Zika and prior ZIKV infection on dengue disease and severity. With support of expert collaborators, we will use state-of-the-art tools (e.g., new monoclonal antibodies, innovative flavivirus antigens, and antibody Fc profiling) to analyze specific infection histories and uncover potential immune correlates. Overall, this program will define new vaccine companion diagnosti...