title: Inviting free-riders or appealing to prosocial behavior? game-theoretical reflections on communicating herd immunity in vaccine advocacy.
abstract: OBJECTIVE
  Vaccination yields a direct effect by reducing infection, but also has the indirect effect of herd immunity: If many individuals are vaccinated, the immune population will protect unvaccinated individuals (social benefit). However, due to a vaccination's costs and risks, individual incentives to free-ride on others' protection also increase with the number of individuals who are already vaccinated (individual benefit). The objective was to assess the consequences of communicating the social and/or individual benefits of herd immunity on vaccination intentions. We assume that if social benefits are salient, vaccination intentions increase (prosocial behavior), whereas salience of individual benefits might decrease vaccination intentions (free-riding).
  
  
  METHODS
  In an online-experiment (N = 342) the definition of herd immunity was provided with one sentence summarizing the gist of the message, either making the individual or social benefit salient or both. A control group received no information about herd immunity. As a moderator, we tested the costs of vaccination (effort in obtaining the vaccine). The dependent measure was intention to vaccinate.
  
  
  RESULTS
  When a message emphasized individual benefit, vaccination intentions decreased (free-riding). Communication of social benefit reduced free-riding and increased vaccination intentions when costs to vaccinate were low.
  
  
  CONCLUSIONS
  Communicating the social benefit of vaccination may prevent free-riding and should thus be explicitly communicated if individual decisions are meant to consider public health benefits. Especially when vaccination is not the individually (but instead collectively) optimal solution, vaccinations should be easily accessible in order to reach high coverage.
authors.affiliation: Center for Empirical Research in Economics and Behavioral Sciences, Germany. cornelia.betsch@uni-erfurt.de
authors.forename: Cornelia
authors.initials: C
authors.lastname: Betsch
authors.forename: Robert
authors.initials: R
authors.lastname: Böhm
authors.forename: Lars
authors.initials: L
authors.lastname: Korn
mesh_terms: D000328:Adult; D005260:Female; D058015:Health Communication; D006801:Humans; D032521:Immunity, Herd; D033182:Intention; D008297:Male; D011582:Psychological Theory; D018570:Risk Assessment; D012919:Social Behavior; D014611:Vaccination / Q000009:adverse effects / Q000191:economics / Q000523:psychology*