Delay and Probability Discounting of HIV Vaccines*

This study investigated how delay and probability discounting impact the perceived value of hypothetical HIV vaccines under different monetary cost conditions. Using hypothetical purchase tasks, two experiments assessed how sexual and gender minorities (SGMs) made trade-offs between immediate versus delayed vaccine access and certain versus uncertain availability. In Experiment 1 ( N = 260), participants chose between a 60% effective vaccine available immediately and a 99% effective vaccine with varying delays. Experiment 2 ( N = 246) involved a choice between a 55% effective vaccine with certain availability and a 95% effective vaccine with uncertain availability. In both experiments, participants were randomly assigned to conditions where the vaccine was either free or available for a $150 fee. Hyperbolic discounting models effectively described delay ( R ² = 0.96 − 0.99) and probability discounting ( R ² = 0.95 − 0.99). While vaccine cost did not significantly affect discounting, individual characteristics emerged as significant predictors. Age ( β = 0.01, p < 0.05) and prior STI testing ( β = −0.19, p < 0.05) influenced delay discounting, while probability discounting was influenced by age ( β = 0.01, p < 0.05), identifying as gay or bisexual ( β = −0.15, p < 0.05), health insurance status ( β = 0.33, p < 0.05), prior STI testing ( β = 0.18, p < 0.05), and political liberalism ( β = −0.004, p = 0.05). This research is among the first to model HIV vaccine acceptance using behavioral economic methodology. When designing vaccine distribution programs, public health strategies may benefit from considering factors such as age, STI testing behaviors, health insurance, and political views. Future research should explore more vulnerable populations such as sex workers and people who inject drugs (PWIDs), and investigate how the perceived value of HIV vaccines may intersect with other health interventions.