The more the merrier? Two online experiments on how decoys can increase vaccine uptake.

Objective: During the COVID-19 pandemic, individuals in some countries had the option to choose from different vaccines, some of which were perceived as less favorable than others. Research on the decoy effect suggests that the preference for an option (target) increases when an inferior option (decoy) is added to the choice set. However, it is unknown whether the decoy effect occurs in vaccination decision making.

Method: Two preregistered online experiments were conducted-a vignette experiment assessing hypothetical vaccination intentions (N = 1,268) and a behavioral experiment using an incentivized interactive vaccination game (N = 1,216)-and manipulated whether people were offered one vaccine (target) or two vaccines (target + decoy). Experiment 2 further tested four different types of decoys: the decoy was (a) a clone of the target or was inferior to the target in terms of (b) the probability of vaccine adverse events, (c) the severity of vaccine adverse events, or (d) vaccine effectiveness. The preference for the target vaccine (vs. nonvaccination) and the overall vaccine uptake were the main outcome variables.

Results: Both experiments showed substantial decoy effects. In Experiment 2, decoys with more severe vaccine adverse events or reduced effectiveness increased the preference for the target vaccine and the overall vaccine uptake.

Conclusion: Taken together, the results suggest that health communication programs must be designed carefully, as multiple options serve as evaluative anchors and might induce preference shifts. (PsycInfo Database Record (c) 2025 APA, all rights reserved).