A systems approach to understanding mechanisms underlying immunisation barriers: a participatory design study in Tanzania

Introduction The underlying causes of supply-side and demand-side challenges in immunisation are poorly understood, leading to symptomatic solutions. This study engaged stakeholders to develop model-based tools for understanding underlying mechanisms, addressing barriers and supporting the design of interventions and policies for immunisation services in Tanzania. Methods Between March 2023 and April 2024, we conducted a qualitative study involving eight in-depth interviews, 12 focus group discussions with 75 participants, and two participatory group model building workshops with 14–16 participants each. Immunisation stakeholders including vaccinators, vaccine coordinators, programme managers, community members and non-governmental organisations provided insights on barriers and facilitators to immunisation access, supply and demand. Their perspectives, combined with evidence from scientific and grey literature, informed the development of a causal loop diagram of immunisation in Tanzania, exploring potential leverage points for improvement. Results Several feedback mechanisms influencing vaccine uptake were identified, including vaccine confidence, risk-benefit perception, vaccine operations (planning, distribution and administration), health workforce, awareness campaigns, safety communication, service accessibility and service quality. Concerns about vaccine safety reduce willingness to vaccinate while limited accessibility and poor service quality diminish motivation to attend sessions. Despite early recovery efforts, the COVID-19 pandemic impacted these mechanisms, exacerbating misinformation, workforce and financial shortages, decreasing vaccine uptake and exposing weak system resilience. Barriers related to infrastructure, accessibility, workforce and service quality varied by region, with rural areas facing greater obstacles. Overall, immunisation resilience and sustainability remain vulnerable due to insufficient investment. Conclusion Linking demand and supply dynamics highlights potential leverage points for sustainable and resilient immunisation services, including vaccine acceptability and operational challenges. Addressing these requires adequate investments and accountability in vaccine safety surveillance and communication, awareness campaigns, vaccination sites, workforce capacity and effective vaccine operations. Quantitative modelling and scenario analysis are needed to confirm leverage points and design effective interventions and policies.