The effect of mobile phone messaging to support COVID-19 vaccination in Colombia: A randomized controlled clinical...

During the COVID-19 pandemic, the speed required and the complexity of the information provided about vaccines highlighted the relevance of communicating evidence effectively during a sanitary emergency. We tested three interactive voice response (IVR) interventions, plus one control to increase COVID-19 vaccine uptake in an adult population in Colombia. We conducted a parallel-group randomized controlled trial with four arms in Colombia between September and December 2022. Participants (n = 2,409) were recruited via random-digit dialing and followed for 7 weeks. Eligible individuals were adults who were not fully vaccinated against COVID-19 at baseline. Participants were randomized to a control arm or to one of three IVR message interventions: factual, narrative, or mixed (factual + narrative). The outcome measured was vaccine uptake during the assessment period. The intervention was provided by the engageSPARK platform. We used logistic regression and Cox proportional hazards models to evaluate the effectiveness of the intervention. Total observed follow-up exceeded 5,775 person-contacts. Exposure to any intervention was associated with higher odds of vaccination among participants who were not fully vaccinated at baseline and correctly answered the comprehension test (odds ratio: 1.40; 95% CI: 1.05-1.87), but not among those with no prior vaccine doses. In Cox models, assignment to any intervention was associated with a significantly higher hazard of vaccination compared with control (hazard ratio [HR]: 1.50; 95% CI: 1.24-1.83). Stratified analyses showed a substantially higher vaccination rate among those with at least one vaccine at baseline (HR: 1.53; 95% CI: 1.25-1.88). All intervention arms showed similar effectiveness. This IVR-based intervention was effective in accelerating COVID-19 vaccination uptake among adults with at least one vaccine, highlighting the importance of prior acceptance to the vaccine. Message comprehension and intervention intensity determine intervention effectiveness. IVR-based interventions represent a scalable and potentially cost-effective tool for supporting vaccination in low- and middle-income countries.