INTRODUCTION:Communication interventions are widely used to promote childhood vaccination and sustain vaccine acceptance, but communication's role in changing people's beliefs and behaviours is not well understood. To determine why these interventions work or where they fail, evaluations must measure a range of outcomes in addition to vaccination uptake. As part of a larger project to develop a preliminary Core Outcome Set for vaccination communication, we conducted a qualitative focus group study exploring how parents and health professionals perceive and experience communication encounters and what outcomes are relevant to them. METHODS:Focus group participants included parents and health professionals involved in vaccination communication (healthcare providers, researchers and policymakers). Participants discussed their experiences with communication for childhood vaccination, and what made the communication 'successful' or 'unsuccessful.' Our analysis involved two stages: first, we thematically analysed the discussions, identifying key parent and professional themes. In stage two, we used an interpretive analysis approach to translate the themes and quotes into measurable outcomes. We compared these outcomes with outcomes measured in vaccination communication trials (previously identified and mapped). RESULTS:We held three focus groups with parents (n = 12) and four with professionals (n = 19). In stage one, we identified six parent themes (primarily related to decision-making) and five professional themes (primarily related to intervention planning, delivery and evaluation). In stage two, we translated 47 outcomes from parents and 73 from professionals (91 total, de-duplicated). All stakeholders discussed attitudes or beliefs and decision-making outcomes most frequently. Most (66%) of the focus group-generated outcomes were not measured in vaccination communication trials. CONCLUSION:Consulting with stakeholders through focus groups allowed us to explore how parents and professionals experienced vaccination communication, identify those aspects of the experience that were important to them, and translate these into outcomes that can be prioritised into a Core Outcome Set and measured in intervention evaluations.