Using an inferior decoy alternative to nudge COVID-19 vaccination.

Optimizing vaccine uptake is a public health challenge that requires the implementation of effective strategies. The asymmetric dominance (or decoy) effect describes the increasing likelihood of selecting an option when a clearly inferior alternative is offered. Therefore, we aimed to test the impact of offering decoy alternatives-less convenient vaccination appointments-on vaccination intentions. Participants aged 18-33 years, residing in England, and initially not intending to get vaccinated, completed three online experiments. Participants were randomly assigned to either a control or an experimental condition in each experiment. The asymmetrically dominated options were: an appointment in two weeks at a distant location (experiment 1); a later time at the participant's local GP, pharmacy, or community centre (experiment 2); and a later time at a distant location (experiment 3). The primary outcome was vaccination intention, while secondary outcomes included an active interest in reading additional information about the vaccination procedure, perceived difficulty and cognitive effort. Initial analysis revealed no evidence of an asymmetric dominance effect. However, further subgroup analysis, supported by formative research, indicated that ensuring decoy alternatives are clearly perceived as inferior could enhance the effectiveness of this approach for certain individuals.