Motivation Mass adult immunization for COVID‐19, coupled with the urgency, is a challenge for any lower‐middle‐income country (LMIC) like Bangladesh. Our analysis focuses on demand‐side constraints early in the vaccination campaign to help gauge vaccine acceptability and potential contributing factors. Identifying registration and compliance challenges early on will help ensure a seamless immunization programme. Purpose We seek to identify subgroups who may need specific interventions by comparing willingness to be vaccinated and registration behaviour, and to understand how actual registration and take‐up decisions compare between rural and urban slum regions. Approach and Methods We use data from three surveys conducted between late January and early September 2021. The article includes a nationally representative survey on vaccine acceptability and a study on vaccination rollout behaviour in rural and urban slums. Findings Willingness was not an issue in Bangladesh, but the weak link was getting individuals to register. Once they did, compliance was very high. When the information gap regarding registration was addressed by campaigning, registration and take‐up increased. Confidence in public service delivery influenced favourable responses to mass immunization efforts. Women were falling behind initially in terms of both registration knowledge and completion. Online registration needed to be complemented with alternatives. Social networking was a vital source of information and encouragement. Policy implications Communication strategies are necessary to inform the public at an early stage, which should provide information about registration eligibility and detailed registration instructions. Ensuring and sustaining service quality will also be beneficial. In LMICs like Bangladesh, low‐tech intensive registration methods are required. Information campaigns about the registration procedure should specifically target rural communities and women. Community‐based mechanisms may reduce transaction costs and increase confidence.