Information Nudges for Influenza Vaccination: Evidence from a Large-Scale Cluster-Randomized Controlled Trial

Abstract Background Vaccination is the most effective means to prevent the spread of infectious diseases. Despite the proven benefits of vaccination, complacency, constraints, and lacking confidence keep many people away from getting vaccinated. This study investigates how written reminders with varying information contents to address vaccine hesitancy affect influenza vaccination coverage in two large and representative samples of older adults. Methods We implemented a large-scale cluster-randomized controlled trial in Finland. The study included the entire elderly population (>= 65 years of age) in two culturally and geographically distinct regions with a historically low (31.8%, N = 7398) and high (57.7%, N = 40727) influenza vaccination coverage. Participants were randomized before the influenza season 2018 - 2019 into three treatments: (i) no information letter, (ii) a standard information letter, reminding recipients about the individual benefits of vaccination, and (iii) a tailored information letter, reminding recipients about the additional social benefits of vaccination due to herd effect. The impact of varying information treatments on influenza vaccination coverage was measured using individual-level administrative health records. Findings Our results showed that a low-cost and scalable information intervention relying on individually mailed letters increased influenza vaccination coverage by 6.4 percentage points (95% CI: 4.1 to 8.8). The effect was particularly large among individuals with no prior influenza vaccination (8.8 pp, 95% CI: 6.5 to 11.1). Moreover, we observed a substantial positive effect (5.3 pp, 95% CI: 2.8 to 7.8) among the most consistently non-vaccinated individuals who had not received any type of vaccine during the previous nine years. There were no cross-vaccine spillovers to other age-appropriate vaccines. Our results further suggest that there was no difference in influenza vaccination coverage between the standard letter and the tailored letter that informed individuals about the social benefits of vaccination (0.2 pp, 95% CI: - 0.1 to 1.3). Interpretation Sending information letters is an effective and easily scalable low-cost intervention strategy to increase vaccine uptake in an elderly population. Communicating the social benefits of vaccination in addition to individual benefits does not enhance influenza vaccine uptake. The effectiveness of behavioral interventions aiming to improve vaccination coverage crucially depends on the prior vaccination history of the target population. These findings have meaningful implications for public health authorities who implement vaccine communication strategies to enhance vaccine uptake and aim to curb the spread of infectious diseases.