Framing the Future: How Vaccination Messages Shape Childhood Immunization Beliefs and Intentions Among Urban Low-Income Communities — a Randomized Field Experiment

Background: Unvaccinated and partially vaccinated children are often found in complex environments, including urban poor settlements. Beyond typical factors affecting vaccination among low-income urban populations, such as affordability, trust remains a critical determinant of caregiver immunization decisions for their children. Drawing on the Theory of Planned Behavior, this study examined the impact of messaging approaches on the interplay across four factors: attitudes, subjective norms, perceived behavioral control, and vaccination intentions. Method: This was an experimental study that employed a between-participants, 2×2×2 design, taking into consideration message content (benefit or cost), message appeal (factual or emotional), and messenger type (layperson or expert). Data were collected from 366 caregivers of children under five residing in randomly selected urban informal settlements. The proposed model was assessed using a partial least squares structural equation model. Results: The study found that caregivers’ attitudes toward vaccination (b=0.215, p<0.001, f²=0.087), subjective norms (b=0.086, p=0.03, f²= 0.026), and perceived behavioral control (b=0.199, p<0.001, f²= 0.087) were positively associated with their intentions to vaccinate their children. Structural equation modelling identified varying moderating effects of the messaging, with the inclusion of the moderators leading to an 8% increase in the R². Exposure to benefit-based (b=0.097, p=0.016, f²=0.016) and fact-based messages (b = 0.162, p<0.001, f²=0.031) intensified the influence of attitudes on vaccination intentions. In contrast, emotionally appealing messages strengthened the impact of perceived behavioral control on intentions (b= −0.149, p<0.001, f²=0.033). The use of expert images did not have significant advantages over layperson images. Conclusion: Findings underscore the potential impact of messaging on the efficacy of vaccination promotion efforts. In developing communication strategies, policymakers should consider the interplay between messaging and its moderating influence on relationships between attitudes, subjective norms, and perceived behavioral control with vaccination intentions. Trial Registration: Pan African Clinical Trials Registry. Trial no: PACTR202303735426128. Registered on 22/03/2023.