Navigating HPV vaccination challenges in low-resource settings: Practical strategies for estimating the size of out-of-school girls

Background: Achieving the 2030 target of 90% human papillomavirus (HPV) vaccination coverage among girls by age 15 requires effectively reaching out-of-school (OOS) girls, particularly in low- and middle-income countries (LMICs) where most vaccine delivery occurs in school settings. This study explored country definitions of OOS girls and strategies for estimating the size of this population. Methods: Between May and September 2024, we conducted semi-structured in-depth interviews with 32 HPV vaccine program stakeholders across six African and Asian LMICs with established HPV vaccination programs: Cambodia, Cameroon, Kenya, Malawi, Mozambique, and Uganda. Using hybrid inductive-deductive thematic analyses, we identified country approaches to defining OOS girls and to enumerating or estimating population size. Results: Administratively, countries commonly defined OOS girls as those not enrolled in schools under the Ministry of Education’s purview, but in practice, definitions varied across contexts. National-level data (e.g., census, school enrollment rates) and household-level approaches (e.g., headcounts) were commonly used to enumerate in-school and OOS girls. However, the distinction between in-school and OOS girls was often blurred during HPV vaccination implementation and evaluation, limiting the operational utility of these methods. To address these challenges, some countries adopted alternative approaches, including proxy-based population estimates using historical or approximate data and the prioritization of areas perceived to have a higher presence of OOS girls. These alternative approaches were perceived as pragmatic compromises between accuracy, feasibility, and resource constraints. Conclusion: Accurate enumeration of OOS girls is important for HPV vaccination but may not be feasible in resource-limited settings. Pragmatic, context-specific estimation methods can offer a feasible way to identify and reach OOS girls more effectively with HPV vaccination services.