Abstract Literature research has suggested that vaccination hesitancy (VH) has been increasing in recent years across migrant populations. It poses various challenges, including risks of outbreaks, and re-emergence of diseases. Nevertheless, only few studies provide an insight of the interventions successful in tackling VH among these groups. This presentation will describe a multi-country research enterprise that aims to respond to these needs. The “RIVER-EU - Reducing Inequalities in Vaccine uptake in the European Region - Engaging Underserved communities” project (funding from EU’s Horizon 2020, Grant Agreement No. 964353) involves several underserved communities, such as Ukrainian migrants (UMs) in Poland, migrants and refugees in Greece, females with Turkish/Moroccan migration background in the Netherlands and the marginalized Roma communities in Slovakia. The project aims at improving child and adolescent HPV/MMR vaccination among these communities by implementing tailored interventions, suitable for each target country. The focus is on removing health system barriers to vaccination. The data regarding VH are an important by-product of the RIVER-EU project, and shows the connection between perceived access barriers and VH. A project-based approach to combat VH among UMs in Poland will serve as a case study to demonstrate the complex process of development of adequate interventions, that could be used to support this migrant group, with disturbingly low vaccination uptake. However, this presentation will go beyond this case study findings - a review of the existing evidence will help generalize our results. Overall, this presentation will show to how investigating various drivers of under-immunization helps to assess what approaches, strategies and interventions are effective to combat VH among marginalized migrant groups.