Free-Riding Behavior in Vaccination Decisions: An Experimental Study

Citation: Ibuka Y, Li M, Vietri J, Chapman GB, Galvani AP ( Free-Riding Behavior in Vaccination Decisions: An Experimental Study Yoko Ibuka 0 Meng Li 0 Jeffrey Vietri 0 Gretchen B. Chapman 0 Alison P. Galvani 0 Maciej F. Boni, Tohoku University, Japan 0 1 Graduate School of Economics, Tohoku University , Sendai, Miyagi , Japan , 2 Department of Health and Behavioral Sciences, University of Colorado Denver, Denver, Colorado, United States of America, 3 Health Economics and Outcomes Research, Kantar Health, Princeton, New Jersey, United States of America, 4 Department of Psychology, Rutgers University , Piscataway , New Jersey, United States of America, 5 Department of Epidemiology and Public Health, Yale University School of Medicine , New Haven, Connecticut , United States of America Individual decision-making regarding vaccination may be affected by the vaccination choices of others. As vaccination produces externalities reducing transmission of a disease, it can provide an incentive for individuals to be free-riders who benefit from the vaccination of others while avoiding the cost of vaccination. This study examined an individual's decision about vaccination in a group setting for a hypothetical disease that is called ''influenza'' using a computerized experimental game. In the game, interactions with others are allowed. We found that higher observed vaccination rate within the group during the previous round of the game decreased the likelihood of an individual's vaccination acceptance, indicating the existence of free-riding behavior. The free-riding behavior was observed regardless of parameter conditions on the characteristics of the influenza and vaccine. We also found that other predictors of vaccination uptake included an individual's own influenza exposure in previous rounds increasing the likelihood of vaccination acceptance, consistent with existing empirical studies. Influenza prevalence among other group members during the previous round did not have a statistically significant effect on vaccination acceptance in the current round once vaccination rate in the previous round was controlled for. - Funding: This study was supported by the National Science Foundation SBE-0624117 (http://www.nsf.gov/). The funder had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. Promoting vaccination is an important goal in public health policy. However, influenza vaccination coverage in the United States is still far below the public policy goal. Vaccination may be discouraged by the incentive to free-ride. Referred to herein as free-riders in vaccination, these individuals avoid the cost associated with vaccination while benefiting from other individuals vaccination [1] [2]. Vaccination for infectious diseases produces herd immunity, providing indirect benefit to unvaccinated individuals. As the result of herd immunity, the risk of infection for an individual depends on other individuals vaccination status; risk of infection generally decreases as the vaccination coverage in a community increases regardless of an individuals vaccination status. In economic theory, a free-riding problem occurs in the market of public goods that have two main characteristics: non-rivalry and non-exclusion of consumption. Vaccination holds both characteristics. Non-rival consumption indicates that consumption of a good by one person does not affect the quantities consumed by other individuals. Goods involving non-exclusion are costly or sometimes impossible to restrict their benefits to certain individuals. The production of public goods results in positive externalities, and accordingly, herd immunity effects are described as positive externalities. Vaccination externalities have been theoretically analyzed in both a static [3] [4] and a dynamic framework [4] [5] [6]. As empirical analyses, previous survey studies have demonstrated some evidence of free-riding in vaccination decision-making. Based on hypothetical scenarios regarding vaccination against an infectious disease, free-riding incentives have been found to significantly influence vaccination decisions [7]. In another study on parents vaccination choice, parents answered that they would be less likely to vaccinate their children if most of their childrens contacts were vaccinated [8], a response pattern that is consistent with free-riding. In this study, we used a computerized interactive game to examine individuals decision making about vaccination for a simulated influenza infection in a group setting. Specifically, we determined the role of free-riding incentives as well as other potential factors in vaccination decision-making. Due to uncertainty regarding the outcome, the decision to get vaccinated is dependent on more factors than ordinary economic goods, including health status, individuals beliefs, and other psychological factors such as fears or regrets [9], [10], [11], [12], [13]. We attempted to examine how individuals make a decision in response to observed decisions made by other individuals, controlling for these effects using experimental approach. In the game, eight to ten participants formed a group. Participants simultaneously and independently made a decision on whether they would get vaccinated given a set of parameter conditions specific to each round, sequentially playing 24 rounds of the game. Each participant was granted 2,000 points initially and lost points when they spent points on buying the vaccine or when they were infected. Final points were connected to monetary payouts for participants. In addition to free-riding incentives, we examined whether an individuals influenza exposure in prior rounds of the game or the influenza prevalence in the group would influence further decisions about vaccination acceptance. The experimental design mirrored the dynamic nature of influenza transmission where individuals chance of infection depended on the realized proportion of vaccination in the group. Our results showed that individuals vaccination decisions were significantly influenced by observed vaccination rate during the previous round, suggesting free-riding behavior. Experimental Design This study was approved by Rutgers University Institutional Review Board. In the experiment, participants provided assent in the following way. The consent form appeared on the computer screen when participants first sat down, and they read the form and then advanced to the next screen only if they wished to participate in the study. Thus, the fact that they provided responses on later computer screens documented that they consented to participant. As the research was low risk, the Institutional Review Board ruled that signatures on consent forms were not necessary. The computerized game experiment was conducted between April and October in 2008 with 269 undergraduat (...truncated)