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 (N = 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 (N = 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 = "not at all" to 4 = "very") and postmanipulation COVID-19 vaccination intentions "if their state offered this incentive" (0 = "definitely would not" to 100 = "definitely would"). Results: Intentions did not differ across conditions (Experiment 1: F[11, 561] = 1.29, p = 0.224, [partial eta squared] = 0.03; Experiment 2: Message Framing, F[1, 266)] = 0.01, p = 0.940, [partial eta squared] = 0.000; Numeric Framing, F[1, 266] = 1.40, p = 0.237, [partial eta squared] = 0.01; Interaction, F[1, 266] = 1.40, p = 0.238, [partial eta squared] = 0.01). 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.