ABSTRACT The human papillomavirus (HPV) is the most common sexually transmitted infection in the U.S. and is responsible for causing 70 to 90 percent of all genital, anal, and oral cancers. Racial and ethnic minorities are disproportionately affected by these outcomes. Vaccination during adolescence is recommended before sexual activity to prevent HPV and associated cancers. However, national adolescent vaccine rates remain low with completion rates lower than initiation rates, especially among minority youth. Recently, the number of doses required for vaccine completion among some age groups has been reduced. For example, only two doses are now required for vaccine completion for adolescents ages 11 to 14, while adolescents ages 15 to 17 remain on the three-dose series. Despite these changes, the HPV vaccine series remains on a 6- month administration schedule for completion. The gap between vaccine initiation and completion rates and disparities in HPV vaccination may be influenced by social determinants. To date, the majority of studies have explored individual-level determinants to refusing or starting the HPV vaccine series. Much less is known regarding determinants of vaccine completion, especially relationship and community level factors. The purpose of this study is to identify individual-, relationship-, and community-level determinants of HPV vaccine completion among adolescents ages 11 to 14 and barriers and facilitators to completion among parents and adolescents. More specifically we aim to: 1) Determine the effects of social determinants of health for adolescent HPV vaccine completion (age, sex, race/ethnicity, insurance type, area deprivation index for residence, and distance to clinic) within a 14-month period using EHR data from retrospective chart reviews; and 2) Explore barriers and facilitators to vaccine completion by describing parent and adolescent experiences with the HPV vaccine, including individual, relationship, and community level determinants influencing vaccine completion, and integrating interview data with social determinants explored in Aim 1. A parallel mixed-methods design will be used to integrate information from electronic health records (EHR) with qualitative data from individual interviews with parents and adolescents to determine social determinants influencing vaccine completion. A modified version of the Social Ecological Model will be used as a guide to understand various determinants affecting HPV vaccine completion across three levels: Individual, relationship, and community. Thirty-two adolescents and parents will be recruited from a local pediatric clinic. Quantitative data will be analyzed using logistic regressions and directed content analysis will be used to analyze interview data. Results from both analyses will be presented using data displays where an integrative summary will provide further description of how determinants serve as barriers and facilitators to vaccine completion. Findings from this study will guide future research on multi-level intervention studies that prioritize social determinants of HPV vaccination completion to reduce barriers experienced among parents and adolescents.