Abstract
Maintaining COVID-19 vaccine demand was key to ending the global health emergency. To help do this, many governments used chatbots that provided personalized information guiding people on where, when and how to get vaccinated. We designed and tested a WhatsApp chatbot to understand whether two-way interactive messaging incorporating behaviourally informed functionalities could perform better than one-way message reminders. We ran a large-scale preregistered randomized controlled trial with 249,705 participants in Argentina, measuring vaccinations using Ministry of Health records. The behaviourally informed chatbot more than tripled COVID-19 vaccine uptake compared with the control group (a 1.6 percentage point increase (95% confidence interval, (1.36 pp, 1.77 pp)) and nearly doubled uptake compared with the one-way message reminder (a 1 percentage point increase (95% confidence interval, (0.83 pp, 1.17 pp)). Communications tools designed with behaviourally informed functionalities that simplify the vaccine user journey can increase vaccination more than traditional message reminders and may have applications to other health behaviours.