Experimental tests of hypothetical lottery incentives on unvaccinated adults' COVID-19 vaccination intentions.

Objective: In May 2021, U.S. states began implementing "vaccination lotteries" encouraging COVID-19 vaccination. Drawing from Prospect Theory and math cognition research, we tested several monetary lottery structures and their framing to determine which would best motivate unvaccinated adults.

Method: In two online experiments, U.S. adults were asked to imagine that their state implemented a vaccination lottery. In Experiment 1, participants ( = 589) were randomly assigned to 1 of 12 conditions varying the monetary amount and number of winners, holding constant a $5 million total payout. In Experiment 2, participants ( = 274) were randomly assigned to one of four conditions in a 2 (Message Framing: Gain versus Loss) by 2 (Numeric Framing: Big versus Small) factorial design; in all conditions, five people would each win $1 million. Participants rated their baseline vaccination willingness (1 = to 4 = ) and postmanipulation COVID-19 vaccination intentions "if their state offered this incentive" (0 = to 100 = .

Results: Intentions did not differ across conditions (Experiment 1: [11, 561] = 1.29, = .224, η² = .03; Experiment 2: Message Framing, [1, 266)] = .01, = .940, η² = .000; Numeric Framing, [1, 266] = 1.40, = .237, η² = .01; Interaction, [1, 266] = 1.40, = .238, η² = .01). When participants were shown a list of 12 lottery structures and asked which they preferred, participants on average preferred options that awarded less money to more people. However, 41.9% of participants across both experiments indicated they would not vaccinate for any lottery-based monetary incentive.

Conclusions: Multiple lottery structures could be equally (un)motivating for unvaccinated adults. Structures that distribute incentives across more people or alternative public health strategies should be considered. (PsycInfo Database Record (c) 2023 APA, all rights reserved).