Amid the global response to the COVID-19 pandemic, Cambodia emerges as a notable exemplar, boasting a remarkable vaccination rate of 91% of its population by the end of 2021. This success can be attributed to a multifaceted approach encompassing strong governmental leadership, robust vaccine management, and effective risk communication. Despite notable success, challenges persisted in reaching unvaccinated segments of the population. Rapid Coverage and Community assessments (RCCAs) supported by UNICEF were deployed in 2022 and 2023 to support Government efforts in expanding vaccination reach to missed populations and bolster COVID-19 booster doses. RCCAs were conducted across 8 provinces, assessing 13,547 individuals from 87 low-coverage communes to identify not vaccinated people and understand their barriers. Overall, only seven percent missed any dose of C-19 vaccine, Disabilities and lack of education correlated with higher rates of zero doses (20.8% and 17.8%, respectively), with concerns about personal health conditions (25%) and adverse effects (10%) cited as top reasons for non-vaccination. RCCAs, differing from routine monitoring, utilized purposive sampling to identify barriers to vaccine access and uptake. Findings informed adaptive strategies by government and partners to enhance outreach in low-coverage areas. The RCCAs played a crucial role at the local level in ensuring vaccinations reached overlooked populations, providing innovative solutions and supporting UNICEF's national strategies. By the end of 2022, Cambodia achieved a high subnational coverage, contributing to a national coverage of 95% for primary doses and nearly 70% for the first booster for everyone above three years old. RCCAs also fostered local coordination, facilitating resource sharing, and coordinated action for more effective local outreach and community trust. Integration of RCCAs into national immunization programs is recommended to develop targeted outreach for underserved populations, including zero-dose children and communities. Cambodia's adaptation of RCCAs during the pandemic offers valuable insights into identifying and engaging missed populations, highlighting the importance of community involvement and targeted interventions for future pandemic preparedness, and achieving more equitable health outcomes in the longer-term.