Abstract Introduction: School-entry vaccination mandates have been widely used as a mechanism to ensure high immunization coverage rates. In August 2018, Puerto Rico (PR) mandated a Human papillomavirus (HPV) vaccine school-entry policy for student’s ages 11 to 12 years. This new requirement in PR presents an opportunity to study the implementation process across a 5 years period (2018-2023). In this ongoing study, we conducted Key Informant (KI) interviews to document factors that facilitate or impede a successful HPV vaccine school policy implementation in PR. Methods: We conducted 29 KI semi-structured interviews with stakeholders in the Department of Health (DOH), school system, healthcare organizations, community-based organizations and coalitions in PR (in favor and against the school-entry policy) from July 2018 to June 2019. The interview guide included relevant domains based on the Consolidated Framework for Implementation Research, such as Intervention characteristics, Inner setting, Outer setting and Individual characteristics. We transcribed interviews, coded transcripts and analyzed data to identify emergent themes. Results: Potential facilitators of HPV policy implementation included: clear enforcement messages and use of personal anecdotes by nurses from health and school fields, the coverage of the vaccine by medical insurers, and the power of the Secretary of Health to include vaccines required for school-entry. Perceived barriers to the implementation process were: lack of communication between the DOH and the Department of Education, lack of knowledge about the HPV vaccine, uncertainty about the consistency of implementation in schools, lack of vaccine availability and disproportionate burden for regional school nurses. Other barrier to implementation mentioned included: school directors and teachers were detached from the implementation process, and lack of compliance from private health providers and public schools towards the immunization registry. Coalitions against this mandate focused their concerns on the right of parental autonomy. Recommendations from KI focused on: stricter policies to enforce this law in public schools, clear messages about the new mandate and HPV vaccine current coverage by health care insurers in PR. Moreover, KI expresses the need for more support from the DOH (technical and educational training to school principals and teachers, health promotion), and the need for increased education about the HPV vaccine and recommendations. Conclusions: Although school vaccination mandates are an evidence-based strategy for improving vaccination rates, several implementation barriers could affect the impact. Findings from this study can be used for improving policy procedures and implementation, and can inform states/territories considering adopting similar immunization policies. The information generated will help determine adaptations/modifications that may be needed for policy implementation in PR and other populations in the future. Citation Format: Roxana Soto-Abreu, Manuel E. Rivera-Encarnación, Vilnery Rivera-Figueroa, Glizette O. Arroyo-Morales, Diana T. Medina-Laabes, Olga L. Diaz-Miranda, Pamela C. Hull, Ana P. Ortiz-Martínez, Erick L. Suárez-Pérez, María E. Fernández, Vivian Colón-López. Human papillomavirus school-entry vaccination mandate in Puerto Rico: Barriers and facilitators from the perspective of key informants [abstract]. In: Proceedings of the Twelfth AACR Conference on the Science of Cancer Health Disparities in Racial/Ethnic Minorities and the Medically Underserved; 2019 Sep 20-23; San Francisco, CA. Philadelphia (PA): AACR; Cancer Epidemiol Biomarkers Prev 2020;29(6 Suppl_2):Abstract nr D047.