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http://mdm.sagepub.com/content/32/3/447The online version of this article can be found at:

 DOI: 10.1177/0272989X11427762

2012 32: 447 originally published online 29 November 2011Med Decis MakingJeffrey T. Vietri, Meng Li, Alison P. Galvani and Gretchen B. Chapman

Vaccinating to Help Ourselves and Others  

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Vaccinating to Help Ourselves and Others

Jeffrey T. Vietri, PhD, Meng Li, MS, Alison P. Galvani, PhD, Gretchen B. Chapman, PhD

Background. Many behaviors affect not only the self butalso others. The utility of a vaccination to each individualdepends on population immunity, the cumulative resultof individual vaccination decisions. However, little isknown about how the benefit to others influences vaccina-tion decisions. Methods. In a series of 3 experiments (N =292, 316, and 299) using hypothetical scenarios and collegestudent respondents, we tested whether the vaccination de-cisions of individuals were sensitive to the level of immu-nity in the population when it had implications for eitheraltruistic or free-riding vaccination behavior. Results. Ourfindings indicate that decisions of individuals were sensi-tive to opportunities both to free ride by refusing vaccina-tion and to vaccinate altruistically. Although individualswere most willing to get vaccinated when they were at

risk themselves, they were also sensitive to the amount ofgood they could do for others. This altruistic sensitivitywas strongest when individuals were not vulnerable to thedisease themselves. Conclusions. The most effective vacci-nation strategies, from a public health perspective, oftenentail vaccinating the disease transmitters rather thanthose who are most vulnerable. Consequently, those whobear the burden of vaccination and those who benefit arenot the same individuals. Thus, effective vaccination cam-paigns require that disease transmitters vaccinate evenwhen it is not in their self-interest to do so. Our results sug-gest that it may be possible to encourage vaccination byappealing to altruistic motives. Key words: vaccination;altruism; free riding; herd immunity. (Med Decis Making2012;32:447–458)

Human behaviors often have widespread reper-cussions throughout a population. Each indivi-

dual’s decision affects the outcomes experienced byother people, creating a social dynamic where deci-sion makers are influenced by the decisions theythink other people will make. As a case in point,vaccination is not a solely personal behavior: It bothinfluences other people, via changes in the dynam-ics of disease transmission, and is influenced byother people because an individual’s risk depends

on disease prevalence within the population. Whenindividuals get vaccinated, it confers protection onnonvaccinated individuals. This provides theopportunity for nonvaccinated individuals to freeride or benefit from the vaccination of others. Ifenough of a population is vaccinated against an infec-tious disease, transmission can be curtailed or eradi-cated, as was achieved for smallpox. Conversely,individuals can choose to get vaccinated not only toaccrue benefits for themselves but also to preservethe health of others. Indeed, the Centers for DiseaseControl and Prevention recommend that caregiversof small children vaccinate against influenza to con-fer indirect protection on this vulnerable age class.1

In addition, although elderly people suffer the great-est morbidity and mortality from influenza infection,young people are primarily responsible for spreadingthe disease. Consequently, epidemiological modelssuggest that the optimal approach to protect theelderly from influenza is by vaccinating the youngpeople who are most responsible for spreading theinfection.2,3 Thus, vaccination decisions can be con-sidered from the vantage point of game theory,4

where the behavior and outcomes of each individualare influenced by the behavior of others. Because vac-cination of one person confers benefits on others inthe community, some individuals may seek to getvaccinated in part to contribute to this community

Received 17 March 2011 from Kantar Health, Princeton, NJ (JTV);Department of Psychology, Rutgers, The State University of New Jer-sey, Piscataway, NJ (ML, GBC); and Department of Epidemiology &Public Health, Yale University School of Medicine, New Haven, CT(APG). This research was supported by collaborative National ScienceFoundation (NSF) grants (SBE-0624117 and SBE-0624098) to APGand GBC. The funding source had no involvement in the study design,data collection, or data analysis. Preliminary results were reported atthe 2007 annual meeting of the Psychonomic Society and at the2009 Judgment and Decision Making preconference of the Societyfor Personality and Social Psychology. Revision accepted for publica-tion 11 September 2011.

Address correspondence to Gretchen B. Chapman, PhD, RutgersUniversity, School of Arts & Sciences, Department of Psychology,152 Frelinghuysen Road, Piscataway, NJ 08854-8020; telephone:(848) 445-2640; fax: (732) 445-2263; e-mail: gbc@rci.rutgers.edu.

DOI: 10.1177/0272989X11427762

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benefit. In the current studies, we evaluate the role ofaltruism in vaccination decisions, where altruism isdefined as accepting a net cost to one’s self in orderto benefit others.5

Do individuals take vaccination decisions andimmunity status of others into account when makingtheir own vaccination decisions? Specifically, arethey willing to get vaccinated when a vaccinationwould not benefit them directly but would benefitothers indirectly? In a focus group study, some parentsvolunteered social responsibility as a reason to gettheir children vaccinated.6 A review of studies ofparental vaccination decisions, however, revealedthat vaccine decliners were disinclined to vaccinatefor the benefit of the community, and even vaccineacceptors viewed community protection as a bonusrather than a primary aim.7 In a hypothetical scenariostudy, Hershey and colleagues found that bandwagon-ing, altruism, and the motivation to free ride on herdimmunity all play a part in vaccination intentions.8

In their study, participants considered vaccinationscenarios that differed in the proportion of the peoplewho had already been vaccinated, whether vaccina-tion would protect others in addition to the self, andwhether the scenario was framed as an opportunityto free ride or to behave altruistically. Their analysisindicated that others’ vaccination behavior exertedmost of its influence through bandwagoning, or thedesire to behave the way others are behaving withoutregard to the consequences. After statistically control-ling for bandwagoning, however, free-riding and altru-ism effects were also demonstrated.

Of the various ways that the immune status ofothers can affect our vaccination decisions, free rid-ing has received the most attention. Parents who donot want to vaccinate their child cite the vaccina-tion of others as a reason their child need not bevaccinated.9 Parents who are wary of the pertussisvaccine showed an appreciation of herd immunity,with their willingness to have their children vacci-nated decreasing with the extent to which otherchildren are vaccinated.10 Another study alsofound evidence of free riding in a computerizedlaboratory task version of vaccination decisions.11

In the current article, we focus on altruisticmotives for vaccination. A series of experimentsusing hypothetical scenarios tested whether vacci-nation decisions of individuals are influenced byaltruism. That is, we evaluated the circumstancesunder which vaccination decisions are influencedby a potential benefit for others. We devised scenar-ios such that altruism and free-riding motives pre-dicted different response patterns. Specifically, we

included scenarios where the only benefit of vacci-nation was the protection of others (with no benefitto the self). In these scenarios, we manipulated thepercentage of the population already immune, andhence the percentage of the population that can ben-efit if the decision maker gets vaccinated, but keptthe risk to the self constant. We term these scenarios‘‘altruism scenarios’’ because they have the potentialto reveal altruism motives for vaccination. A patternwhere one is more likely to vaccinate when largernumbers of people stand to benefit from one’s vacci-nation constitutes evidence for altruistic motivation.We also included scenarios where the percentage ofthose immune had implications for self-interestedmotivations (the number of people who could infectthe individual) as well as altruistic motivations (thenumber of people who could benefit if the individ-ual gets vaccinated). We term these scenarios‘‘herd immunity scenarios’’ because they entailboth the self-interested and the altruistic motiva-tions that arise from herd immunity. Our studiesnot only explore whether vaccination decisions areinfluenced by altruism but also examine the condi-tions under which altruism operates.

STUDY 1

The first study concerned vaccination againsthuman papillomavirus (HPV), an infection thatcauses cervical cancer in women and genital wartsin men and women. As shown in Table 1, we pre-sented subjects with 2 series of hypothetical scenar-ios and measured their willingness to get the HPVvaccine. The first series was designed to measurealtruism and varied the proportion of the populationthat the participant’s vaccination could benefitwhile holding risk to the participant constant.Therefore, differences in intent to get vaccinatedacross levels of immunity should be due to altruismbecause self-interest does not differ. The secondseries of scenarios was designed to measure willing-ness to free ride on herd immunity. In these scenar-ios, both self-interest and benefit to others varied asa function of population immunity; as the percent-age of those immune increases, both risk to the selfas well as the benefit to others are reduced.

Methods

Participants. College students (N = 292; 124female) participated in exchange for partial comple-tion of a course requirement.

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Materials and procedure. The vaccination scenar-ios were presented within the context of a largerWeb-based questionnaire about HPV (Appendix).The questionnaire included some introductory infor-mation about HPV and HPV vaccination, so all partic-ipants were aware of HPV, the possible consequencesof HPV infection, as well as the availability of a vaccinebefore reading the scenarios. Although at the time ofthe study (spring 2007) no HPV vaccine had yet beenapproved by the Food and Drug Administration foruse in men, we told participants that men who werevaccinated could protect their sexual partners fromHPV. (They were informed during debriefing that noHPV vaccination had yet been approved for men.)

All participants responded to the same 2 series ofscenarios in the same order (Table 1). The first seriesof 4 scenarios, called hereafter ‘‘altruism scenarios’’,tested whether participants would be sensitiveto opportunities for altruism. These scenariosdescribed a situation where some proportion of thepopulation (excluding the participant) had been vac-cinated with an old version of the HPV vaccine,which conferred protection against warts and

cervical cancer but did not prevent transmission toothers. Therefore, the participant’s own risk of con-tracting HPV is not affected by the percentage ofthose immune. However, the percentage of thoseimmune changes the participant’s potential to bene-fit others by getting vaccinated because the partici-pant’s vaccination will decrease the risk forindividuals who have not been vaccinated. The per-centage already vaccinated was 10%, 60%, 90%,and 100% across scenarios that were otherwiseidentical. This construction dissociates self-interest(held constant because risk to the self is the sameacross scenarios) from altruism in vaccination (var-ied across scenarios because benefit to othersdepends on the percentage already vaccinated).

A second series of 4 scenarios, hereafter called‘‘herd immunity scenarios’’, was designed to examineindividuals’ willingness to free ride on the protectionoffered by the immunity of others. These scenariosdescribed a situation where a proportion of the popu-lation (excluding the participant) was naturallyimmune to HPV and explained that this immunitywould protect the individual from both cervical cancerand genital warts as well as prevent the immune indi-viduals from transmitting HPV (Appendix). (Describ-ing these individuals as naturally immune ratherthan previously vaccinated was intended to avoidbandwagoning effects that would covary with percent-age of the population already immune.) In this set ofscenarios, the participant’s own risk of contractingHPV depends on whether others are immune to HPV(only those who are not immune can transmit HPV),and the participant’s vaccination behavior will benefitonly those individuals who are not immune (HPV canonly be transmitted to unvaccinated individuals; thus,only unvaccinated individuals can benefit from thevaccination of others). Consequently, self-interestedand altruistic motives are perfectly linked in theseherd immunity scenarios. The proportion of the popu-lation naturally immune to HPV was the same as inthe first 4 scenarios (10%, 60%, 90%, and 100%).

As shown in Table 1, the altruism and herdimmunity scenarios were the same in terms of theproportion of those immune and the correspondingproportion who could benefit from the participant’svaccination; they differed on whether the already-immune population could infect the participant.Each of the 8 scenarios was accompanied by a piechart showing the proportion of the population sus-ceptible to the disease and the proportion of thepopulation that can transmit the disease to others.For each scenario, comprehension questions askedparticipants to identify what proportion of the

Table 1 Summary of Scenarios in Each Experiment

Scenario Order Already Immune,a %Could InfectRespondent, %

Experiment 1Altruism

1 10 1002 60 1003 90 1004 100 100

Herd immunity5 10 906 60 407 90 108 100 0

Experiment 2 (altruism)1 5 02 90 0

Experiment 3Herd immunity

1 95 52 60 403 10 90

Altruism4 95 0 or 100b

5 60 0 or 100b

6 10 0 or 100b

a. The percentage of the population who could benefit from therespondent’s vaccination is equal to 1 minus this percentage.b. Depending on the between-subjects experimental condition.

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population could possibly transmit HPV to them andwhat proportion could possibly benefit from theirvaccination. They were told to imagine that a newvaccine had been developed that would both preventthem from developing warts and cancer while alsopreventing them from transmitting HPV. Participantsrated how likely they would be to get vaccinated ifthey had to pay the $360 cost out of pocket. (The mar-ket price of the vaccine was specified to keep the sce-nario realistic.) Responses were made on an 11-pointscale that ranged from 0% to 100% in 10% intervals.

Results and Discussion

Two male participants answered the check ques-tions incorrectly and were excluded from furtheranalysis, leaving a final sample of 124 women and166 men. Because HPV infections are more danger-ous in women than men (only women can get cervi-cal cancer), we included gender in the analysis.Intentions to get vaccinated were entered into a 2(gender) 3 2 (type of scenario: altruism v. herdimmunity) 3 4 (percentage of those immune)ANOVA, with type of scenario and percentage ofthose immune as within-subjects factors.

Figure 1 shows the mean ratings as a function ofexperimental condition. Notably, the curves for theherd immunity scenarios are negatively sloped,indicating that rated likelihood to get vaccinateddecreases with the percentage of individuals whoare immune. In contrast, the curves for the altruismscenarios are essentially flat. This pattern indicates

that participants are motivated by free riding butnot by altruism. That is, in the altruism scenarios,interest in the vaccination is unaffected by the num-ber of people who can benefit from the vaccination,indicating no altruistic motivation. In contrast, inthe herd immunity scenarios, interest in the vaccina-tion declines when there are fewer people who canspread the virus, suggesting a free-riding motivation.We do not attribute this pattern in the herd immu-nity scenario to altruism (even though fewer peoplewho can spread the virus also means fewer peoplewho can benefit from one’s vaccination) becausethe altruism scenarios showed no effect on the num-ber of people who could benefit.

Table 2 shows the ANOVA results. As expected,there was a main effect of gender, with greater inter-est in vaccination among women (mean = 68%) thanmen (M = 57%). As can be seen in Figure 1, therewas also a main effect of the percentage of thoseimmune in the population, with participants givinghigher ratings of likelihood to get vaccinated whenfewer were immune to the virus. Scenario typealso had a main effect, with more interest in a vacci-nation in the altruism scenarios, where risk to theself is held constant at a high level (M = 70%),than in the herd immunity scenarios, where risk tothe self varies (M = 55%). However, of primary inter-est, the effect of scenario type was moderated by thepercentage of those immune in the population.Analysis of simple main effects indicated that indi-viduals were sensitive to the percentage of thoseimmune in the population when risk to the self

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Figure 1 Mean self-reported probability of human papillomavirus (HPV) vaccination in study 1 for women and men as a function of the

percentage of the population immune to HPV infection and scenario type. In the altruism scenarios, the immune can act as carriers;thus, only the benefit to others, but not to the self, decreases with the percentage of those immune. In the herd immunity scenarios,

the immune do not act as carriers; thus, both benefit to the self and benefit to others decrease with the percentage of those immune.

‘‘b’’ represents the regression coefficients for simple main effect linear contrasts. Error bars represent 1 SEM.

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depends on it (herd immunity scenarios) (F3,864 =124.65, P \ 0.0001), so that a lower percentage ofthose immune led to greater interest in vaccination.No such effect was found when self-risk was notaffected by the percentage of those immune (altru-ism scenarios) (F3,864 = 0.23, P = 0.87). Figure 1shows the linear slopes for each curve, which weresignificant for the herd immunity scenario curves(t1734 = 18.69, P \ 0.0001) but not for the altruismscenario curves (t1734 = 0.71, P = 0.48).

We also observed a small 3-way interactionamong type of scenario, percentage of those immunein the population, and gender. Polynomial contrastsrevealed that this interaction was due to an interac-tion among gender, scenario type, and the quadraticfunction of percentage of those immune (F1,288 =9.57, h2 = 0.03, P = 0.002). That is, the herd immu-nity scenario curve was more concave for womenthan for men, whereas the altruism scenario curveswere equally flat for both genders.

Our results suggest that individuals are sensitive toopportunities to free ride on the immunity of others,but we did not observe sensitivity to the potential ben-efit for others. This apparent lack of altruism in oursample may have been related to our use of HPV asthe infectious agent, as HPV is sexually transmitted.Given that people generally believe that they havea great deal of control over their sexual relations, per-ceived control may have mitigated the altruistic moti-vation to get vaccinated. It could be that participantswould be more altruistic if they could unwittinglytransmit the virus to someone during casual contact,making transmission seem more likely. The high costof the vaccine might also have been a deterrent.

STUDY 2

Our second study was based on influenza vacci-nation decisions. Because influenza is transmittedthrough coughing and sneezing rather than throughintimate contact, people may be more sensitive tothe proportion of the population susceptible toinfluenza than to HPV. The influenza vaccine isalso considerably less expensive than the HPV vac-cine, and so individuals may be more willing toincur the cost for the benefit of others.

For this study, we included only altruism scenar-ios. In addition, to conduct a more sensitive test ofaltruism in vaccination decisions, we eliminatedany benefit for the self, such that vaccination onlyprotected others. In study 1, vaccination in the altru-ism scenarios was in the individual’s self-interest;individuals protected themselves by vaccinatingwhile also helping to protect the vulnerable portionof the population they might otherwise infect. It ispossible we failed to detect an effect of altruism inthat study because a strong self-interested motivemasked potential altruistic motives. Thus, the study2 scenarios described a situation where participantswere already protected from developing the disease,but unless they got vaccinated, they could still trans-mit the infection to others. Therefore, the self-inter-ested motive was eliminated, and altruism invaccination could be tested in a more parsimoniousway.

Methods

Participants. College students (N = 316) com-pleted an Internet questionnaire in fall 2007 inexchange for partial completion of a course require-ment. Only the 291 who completed the entire ques-tionnaire are included in the analysis.

Materials and procedure. As in study 1, the scenar-ios were embedded in a larger questionnaire about theflu. Participants were presented with both verbaldescriptions of population immunity as well as witha pie chart for each scenario. Comprehension ques-tions were included to ensure that all participantscould correctly identify what proportion of the popu-lation was vulnerable to the disease and what propor-tion was capable of transmitting the disease to them.Participants had to answer these correctly beforethey could proceed. Each participant responded withtheir likelihood to get vaccinated under 2 different lev-els of population immunity: 5% immune (95% vulner-able) and 90% immune (10% vulnerable)a (Table 1).

Table 2 Analysis of Variance for VaccinationIntentions in Study 1

Source df F h2

Between subjectsGender (G) 1 16.38a 0.05Error 288 (0.24)

Within subjectsScenario type (T) 1 196.49a 0.40Percentage of those immune (P) 3 106.27a 0.27G 3 T 1 1.44 0.00G 3 P 3 0.11 0.00T 3 P 3 143.75a 0.33G 3 T 3 P 3 4.06a 0.01Error (T) 288 (1.06)Error (P) 864 (0.07)

Note: Values enclosed in parentheses represent mean square errors.a. P \ 0.01.

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The scenario stated that participants were alreadyimmune to influenza (Appendix). Participants wereinformed that they and the other 5% (90%) of thepopulation had received an old version of the vac-cine that would prevent them from developing influ-enza, but they could act as carriers and transmit tothose who had not been vaccinated. They wereinformed of a new vaccine that would prevent trans-mission, available to them for $30 (this price reflectsthe realistic market value of the flu vaccine). Eachparticipant indicated their intention to get vacci-nated in each scenario on an 11-point scale rangingfrom 0% to 100% in 10% increments.

Results and Discussion

Intentions to get vaccinated were analyzed witha repeated-measures t test. Many participants intendedto get vaccinated, even though they would not be sus-ceptible to influenza. Responses of 100% were com-mon, comprising 41% of responses in the scenariowhere 10% of the population could benefit and 47%of responses in the scenario where 95% of the popula-tion could benefit.

Of primary interest, intentions to get vaccinatedwere sensitive to the percentage of those immune(t290 = 5.71, P \ 0.0001, h2 = 0.10), with participantsindicating greater willingness to get vaccinated (M =81%, SD = 30%) when 95% of the population couldbenefit than when only 10% of the population could(M = 74%, SD = 27%). A Wilcoxon signed-rank testalso revealed a difference in willingness to vaccinatebetween conditions (T = 2460.5, P \ 0.0001). Thus,study 2 revealed that vaccination decisions are (orcan be) influenced by the opportunity to benefit others.

STUDY 3

Study 2 demonstrated that flu vaccination inten-tions were sensitive to the potential for altruism,in that vaccination intentions were greater in a situa-tion where the individual’s vaccination could helpmore people. Study 1, however, showed no altruismeffect. Studies 1 and 2 differed in a number ofrespects, including disease type and cost of vaccine,and so the purpose of study 3 was to identify the fac-tor that determined whether vaccination decisionsare influenced by altruism. We predicted that thekey difference between studies 1 and 2 was the pres-ence or absence of a self-interested motivation to getvaccinated. Individuals who are concerned withprotecting themselves may be insensitive to the

opportunities to protect others, while those whoare secure in their own safety may be more willingto take the well-being of others into account. There-fore, we tested the hypothesis that vulnerabilitymoderates the impact of population immunity,with the vaccination intentions of those who arenot vulnerable to the disease showing the greatestsensitivity to outcomes for others.

Methods

Participants. College students (N = 299) partici-pated in an online questionnaire in spring 2008 inexchange for partial completion of a courserequirement.

Materials and procedure. The procedure was sim-ilar to the previous 2 studies. Participants weregiven a series of hypothetical scenarios regardingthe influenza vaccination and were presented withinformation regarding the proportion of the popula-tion that was immune. As before, we included piecharts with each scenario, depicting the proportionof the population susceptible to the infection. Con-sistent with the preceding study, participants wererequired to correctly identify the proportion of thepopulation who could transmit influenza to themas well as the proportion they could possibly infect,before rating their likelihood to get vaccinated.

Study 3 included a total of 6 scenarios (Table 1and Appendix). The first 3 were herd immunity sce-narios and were the same for all participants. Thelast 3 were altruism scenarios and varied betweensubjects on whether there could be a self-interestedmotive to get vaccinated. The herd immunity scenar-ios were included to provide a baseline for how pop-ulation immunity affects vaccination decisionswhen both altruism and free riding drive a vaccina-tion. These scenarios were similar to the herd immu-nity scenarios in study 1, except there were 3 levelsof immunity (95%, 60%, and 10%, in that order).

The 3 altruism scenarios differed between sub-jects in order to investigate whether the influenceof altruism on vaccination decisions depends onwhether the disease poses any risk to oneself, thatis, whether a self-interested incentive was relevant.For one group of participants (zero risk to self),self-interest was irrelevant, while in the other group(high risk to self), self-interest was relevant and heldconstant across the 3 scenarios (this is in contrast tothe herd immunity scenarios, where self-interest isnot only always relevant but also varies across the3 scenarios, linked to the benefit to others). Similar

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to the altruism scenarios in studies 1 and 2, thealtruism scenarios in study 3 described a situationwhere some portion of the population had receivedan old version of the vaccine that protected the recip-ient from developing influenza but did not preventthe recipient from transmitting influenza. Half ofthe participants were informed that they, too, hadreceived the old vaccine (as in study 2) and so couldnot develop influenza but could transmit it to others(zero risk to self condition). The other half of the par-ticipants were informed that they had not receivedthe old vaccine (as in study 1) and could thereforebe infected by anyone (high risk to self condition).In the altruism scenarios, everyone is a potential car-rier of influenza, regardless of the percentage of thosesusceptible. The percentage of those immune in bothconditions was the same as in the first set of scenar-ios: 95%, 60%, and 10%.

Participants were informed that a new vaccine pre-vents the individual from infecting others in additionto protecting the recipient. Participants rated theirlikelihood to get vaccinated with this new vaccineon a 12-point scale, which ran from 0% likely to100% likely in 10% increments, with the additionof a 5% option. In this study, we specified a higherprice for the vaccine and also some possible sideeffects in an attempt to decrease the number of partic-ipants who responded at the top of the responsescale. Thus, participants were informed that vaccina-tion would cost $50, which was associated with someside effects, including risk of soreness, redness, or

swelling where the shot was given; low-grade feverand aches; and on rare occasions, serious problems,such as severe allergic reactions.

Results and Discussion

Individuals’ stated likelihood to get vaccinatedwere entered into a 3 (percentage of those immune:95%, 60%, and 10%) 3 2 (scenario type: herdimmunity v. altruism) 3 2 (subject group with zerov. high risk to self in the altruism scenarios)mixed-model ANOVA, with percentage of thoseimmune and scenario type as within-subjects fac-tors. Note that the between-subjects factor (zero orhigh risk to self) only affected the set of 3 altruismscenarios. The herd immunity scenarios were identi-cal for the 2 subject groups.

As shown in Figure 2 and Table 3, all main effectsand interactions were significant. There was a maineffect of the percentage of those immune, such thatthe rated likelihood to get vaccinated increaseswith the percentage of the population who couldbecome infected with or transmit the virus. Therewas also a main effect of scenario type, such thatindividuals rated themselves as more likely overallto get vaccinated in the herd immunity scenariothan in the altruism scenarios. Additionally, therewas a main effect of the between-subjects condition,such that individuals in the high risk to self groupgave higher ratings of likelihood to get vaccinatedthan those in the zero risk to self group. These

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Figure 2 Mean self-reported probability to get vaccinated as a function of experimental condition in study 3. In the herd immunity sce-

narios, immune members of the population cannot transmit the virus, and so both risk to the self and benefit to others decrease withincreasing percentage of those immune. In the altruism scenarios, risk to the self is held constant, so only benefit to others decreases

with increasing percentage of those immune. The left panel shows the zero risk to self condition, where the altruism scenarios hold

risk to self constant at a zero level (self is immune and so cannot get infected, but vaccination of self will benefit others). The right panel

shows the high risk to self condition, where the altruism scenarios hold risk to self constant at a high level (self is not immune and so canget infected from anyone, but vaccination of self benefits both self and others). ‘‘b’’ represents the regression coefficients for simple main

effect linear contrasts. Error bars represent 1 SEM.

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main effects were qualified by all 2-way interac-tions. Of primary interest, a significant 3-way inter-action indicated that the effect of the percentage ofthose immune on the rated vaccination likelihooddiffered across the zero/high risk to self conditions,but only in the altruism scenarios. Specifically, inthe altruism scenarios, the percentage of thoseimmune more strongly influenced intentions to getvaccinated among participants who were immuneto influenza themselves (zero risk to self condition)than those who were not immune (high risk to selfcondition). In the herd immunity scenarios, how-ever, the percentage of those immune influencedintentions to get vaccinated to the same extent forboth groups because the herd immunity scenarioswere the same for both subject groups.

Contrast analyses indicated that the main effect ofthe percentage of those immune and all interactionswith this variable were linear. Analysis of simplemain effects showed that the percentage of thoseimmune had an effect for both the herd immunityand altruism scenarios in the zero risk to self condi-tion (F2,594 . 15.03, P\0.0001) (Figure 2). However,in the high risk to self condition, the percentage ofthose immune only had an effect for the herd immu-nity scenarios (F2,594 = 22.93, P \ 0.0001) but not forthe altruism scenarios (F2,594 = 1.03, P = 0.36).

These results indicate that individuals are willingto get vaccinated even when there is no benefit tothe self. Furthermore, individuals are sensitive tohow much their vaccination might benefit others,with greater willingness to get vaccinated the morethat others are susceptible to the infection and

thus the more they could benefit from one’s vaccina-tion action. The procedure of the study also ensuredthat participants knew that their own risk did notchange across the 3 altruism scenarios, so that theirpattern of vaccination intention is not due to a mis-understanding about their own risk but rather a con-cern for the health of others. Of primary interest,sensitivity to others’ disease outcomes depends onone’s own risk; when the risk of the self developingthe disease was high, sensitivity to others’ healthoutcomes was eliminated. That is, benefit to othersdid motivate vaccination behavior, but this occurredonly when the self was not at risk for infection.

We suggest that the value that individuals place onthe altruistic value of vaccination depends not onlyon the costs and benefit of the vaccine but also on thefunction that an individual ascribes to the vaccine. Inreality, all vaccines make an individual less susceptibleto infection from a particular pathogen and reduce thelikelihood that the vaccinated individual will infectothers. However, whether the individual needs protec-tion may influence whether vaccination is regarded asan act of self-interest, altruism, or a combination.When the self is vulnerable, the vaccine is perceivedas serving a self-protective function. In that case, sec-ondary functions, such as protecting others, are under-valued relative to the case where the vaccine wasperceived primarily as a way to protect others.

GENERAL DISCUSSION

Experiments from psychology and economicshave demonstrated that decision makers sometimesmake choices that imply that their utility increaseswith the well-being of other people.5 Our studiesindicate that this altruistic motivation manifests invaccination decisions. We found that individualswere sensitive to the positive impact their vaccina-tion could have on the health of others. Althoughindividuals are most willing to get vaccinatedwhen they are at risk themselves, those who werenot vulnerable to the disease were still interestedin vaccination, and their interest was positivelyrelated to the vulnerability of others. In study 1,we found that when participants’ own risk of infec-tion was held constant, vaccination decisions wereunrelated to the percentage of others in the popula-tion who could benefit from the participant’s vacci-nation and contribution to herd immunity. Incontrast, in study 2, we found that participantswere more likely to get vaccinated when such vacci-nation could benefit many (as opposed to few)

Table 3 Analysis of Variance for VaccinationIntentions in Study 3

Source df F h2

Between subjectsSelf-interest present/absent (S) 1 37.78a 0.11Error 297 (0.36)

Within subjectsScenario type (T) 1, 297 43.71a 0.09Percentage of those immune (P) 2, 594 152.77a 0.33S 3 T 1, 297 169.63a 0.33S 3 P 2, 594 6.29a 0.01T 3 P 2, 594 18.64a 0.06S 3 T 3 P 2, 594 11.60a 0.04Error (T) 297 (0.10)Error (P) 594 (0.03)Error (T 3 P) 594 (0.02)

Note: Values enclosed in parentheses represent mean square errors.a. P \ 0.01.

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members of the population, even when their own riskof infection was held constant. Thus, study 2 showsevidence of altruistic vaccination, while study 1 didnot. The 2 studies had a number of procedural differ-ences, however. Of primary interest, in study 1, thealtruism scenarios held risk to the self constant ata high level (everyone in the population could poten-tially infect the participant), whereas in study 2, thealtruism scenarios held risk to the self constant atzero (the participant was immune to infection).

Study 3 explored the reasons for the differingresults across studies 1 and 2. Specially, in study 3when we experimentally manipulated whether thealtruism scenarios held risk to the self constant ata zero level or a high level, we found that in thehigh risk to self condition (where risk to self washeld constant at a high level), participants werevery interested in vaccinating, but vaccination inter-est bore little relationship to the potential to benefitothers. That is, vaccination decisions were notdriven by altruism in this case. In contrast, in thezero risk to self condition (where risk to self washeld constant at zero), participants were less inter-ested in vaccinating overall, but vaccination inten-tion bore a robust relationship to the potentialbenefit to others. That is, in this condition, vaccina-tion decisions were driven by altruism. Thus, ourresults indicate that vaccination decisions can beinfluenced by altruistic motivation, but this is pri-marily the case when the self is not at risk.

Our results are consistent with the previous Her-shey and others study.8 Their hypothetical scenariostudy also demonstrated altruism in vaccinationdecisions; however, in that study, altruism co-occurred with and was in part overridden by free-riding and bandwagoning motivation. Although thescenarios in their study did separate to some extentthe ability of the hypothetical vaccines to protect theself versus to prevent transmission to others, unlikethe present studies, their study did not contain anyscenarios where the consequence of vaccinatingwas only the protection of others (because the selfwas already immune). In our study, this situationrevealed the strongest altruism.

The scenarios in the current studies each describedthe current state of population immunity—that is, thepercentage of the population that was susceptible toinfection and that could spread the disease—andasked participants to make a vaccination choice condi-tional on those facts. It is possible, however, that par-ticipants made inferences about how populationimmunity might change over time and that these infer-ences influenced their decision. For example, when

only some in the population are already immuneand I am offered a new vaccine, I might infer that othernonimmune individuals are also being offered thesame new vaccine and that some of them may acceptit. I then have the opportunity to free ride on thesenew vaccinators. When there are many nonimmuneindividuals, there is a large benefit to the populationfrom my vaccination, but there are also many individ-uals who, like me, have the opportunity to receive thenew vaccine and hence many individuals on whom Imight free ride. This free-riding motivation couldcounteract an altruism motivation and lead to no neteffect. When the risk to the self is zero, however (asin study 2 and the zero risk to self condition in study3), there is no motivation to free ride. This couldexplain why that is the condition in which we findevidence of altruistic vaccination.

When the people who bear the cost of vaccinationare not the same people who reap the benefits of vac-cination, one strategy for achieving the populationoptimal outcome is compulsory vaccination. Indeed,a recent editorial argues that the case of compulsoryvaccination, while always controversial, is strongerwhen the vaccine confers herd immunity.12 Althoughcompulsory vaccination programs can be successful insome settings,13,14 one questionnaire study found thatmany health care workers were not even aware thattheir workplace had a compulsory vaccination pro-gram.15 One way to avoid the need for compulsoryvaccination programs is to encourage altruistic vacci-nation, such that individuals voluntarily vaccinate inorder to promote the community good.

Limitations

The current studies had several limitations. Theyemployed hypothetical scenarios, and the order of sce-narios in each study was not counterbalanced. The stud-ies employed within-subjects designs that may makeparticipants especially sensitive to the experimentalmanipulations. The participants were college studentsexpressing hypothetical intentions, and the vaccinationintentions expressed in our studies were quite high—higher than actual influenza vaccination rates seen inreal life. Thus, it remains for future research to revealthe extent to which the benefit to others motivates actualvaccination decisions. The current results neverthelesssuggest that this may be a fruitful avenue.

Conclusions

Public health is a collaborative effort, with each ofus making decisions that impact not only our own

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well-being but that of those around us as well. Insome cases, including influenza vaccination, diseaseprevention is more effective when the disease car-riers are vaccinated rather than those who wouldsuffer most from infection.2 Consequently, effectivevaccination programs may depend on the sensitivityof vaccination decisions to the well-being of others.Benefit to others may also motivate additionalhealth behaviors, including testing for sexuallytransmitted infections,16 safe sex practices, andsmoking cessation or treatment for other addictions.

Our studies reveal that vaccination decisions canbe sensitive to the effects that vaccination has onother people’s health outcomes. This responsivenesscould be important for future vaccination cam-paigns. In some circumstances, a self-interested vac-cination strategy is a poor one for public health. Inthe case of the seasonal influenza, the elderly aremost at risk for serious consequences from infection,while influenza typically is merely bothersome inyounger people. However, influenza vaccination isless effective in the elderly than in the young, andthe young transmit influenza more. Therefore, themost effective way to protect the vulnerable is byvaccinating the disease transmitters rather than themost vulnerable. The most effective vaccinationstrategies from a public health perspective maytherefore rely on altruistic motivations, where thosewho bear the burden of vaccination and those whobenefit are not the same individuals. Our resultssuggest such strategies may be viable.

APPENDIX

HPV Background Information from Study 1

In this part of the questionnaire, we will give yousome information about a new vaccine for humanpapillomavirus (HPV). Please read the followingcarefully.

a. HPV is a virus that infects both men and women.b. Most people will have HPV at some time in their

lives.c. Most HPV infections cause no health problems.

Sometimes, HPV causes genital warts soon afterinfection. Very rarely, an HPV infection causescervical cancer many years later. Cervical canceris a type of cancer that only women get. Bothmen and women can get genital warts.

d. The Pap smear test can detect cell changescaused by HPV. It screens for cervical changes

so that the changes can be treated before theybecome cancerous.

e. A new vaccine has been developed to protectagainst HPV.

f. The Food and Drug Administration (FDA) hasapproved the vaccine.

g. The vaccine is effective against the majority ofHPV infections that cause cancer.

h. The vaccine works when given prior to infection.It will NOT treat or cure HPV.

i. Getting vaccinated involves 3 shots within 6months.

j. HPV is a sexually transmitted virus.k. Men who are vaccinated against HPV protect

their female sexual partners from HPV infectionand cervical cancer.

Altruism Scenario from Study 1

Suppose you found out that 10% (60%, 90%,100%) of all other college-age American adults hadalready received an HPV vaccine, but it was an oldtype that prevents the vaccinated person from get-ting warts and cervical cancer but does not preventthe vaccinated person from spreading the virus.You have not received this older vaccine. Thismeans you could still get HPV infection from them.

Now, a new HPV vaccine is available. It protectsyou from getting HPV and also prevents you fromspreading the virus to the 90% (40%, 10%, 0%) ofpeople who are unvaccinated.

Imagine you could get the new HPV vaccine fora cost of $360.

Without the new vaccine, you can catch HPVfrom both vaccinated (immune to HPV) and unvacci-nated individuals (susceptible to HPV).

The new vaccine prevents you from catching HPVand spreading it to unvaccinated individuals (thosesusceptible to HPV).

Herd Immunity Scenario from Study 1

Suppose you found out that 10% (60%, 90%,100%) of all college-age American adults were natu-rally immune to HPV. You do not have this naturalimmunity. You cannot catch HPV from any of thenaturally immune people but can get HPV from the90% (40%, 10%, 0%) who are susceptible.

Now, a new HPV vaccine is available. It protectsyou from getting HPV and also prevents you from

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spreading the virus to the 90% (40%, 10%, 0%) ofpeople who are susceptible.

Imagine you could get the new HPV vaccine fora cost of $360.

Without the new vaccine, you can catch HPVfrom individuals who are not naturally immune.The new vaccine prevents you from catching HPVand spreading it to susceptible individuals.

Altruism Scenario from Study 2a

Suppose you found out that 5% (90%) of the USpopulation had already received an old vaccineagainst the flu and acquired immunity to the fluvirus. However, the flu virus has evolved—now itcould be carried by anyone, even people who havereceived the old flu vaccine. Those who havereceived the old vaccine WILL NOT develop flu orexperience any adverse effect by carrying the fluvirus, but they CAN transmit the flu virus to unvac-cinated people, who will develop the flu.

A new flu vaccine has been developed to preventpeople from developing the flu AND from carryingand transmitting the flu virus.

You HAVE received the old flu vaccine and areimmune to the flu virus; that is, you will not developthe flu even if you carry the flu virus, but you cancarry and transmit the virus to unvaccinated people.However, the new vaccine can prevent you from car-rying and transmitting the flu virus to the 95%(10%) of the people in the US who are unvaccinatedand consequently would potentially develop the fluif they were exposed to the flu virus.

Imagine you could get the new flu vaccine fora cost of $30. How likely would you be to get vacci-nated with the new vaccine?

Herd Immunity Scenario from Study 3

Suppose you found out that 95% (60%, 10%) ofthe US population was naturally immune to theflu. You do not have this natural immunity. YouCANNOT catch the flu from those who are naturallyimmune, but you CAN still catch it from the 5%(40%, 90%) of the population who are susceptibleand transmit the flu to these people in the casethat you do get infected.

A new flu vaccine has been developed to preventpeople from developing the flu AND from carryingand transmitting the flu virus. In other words, thenew vaccine can prevent you from getting infectedwith the flu from the 5% (40%, 90%) of the

population AND from transmitting the flu virus tothis 5% (40%, 90%) of the population, who do nothave immunity against flu and consequently wouldpotentially develop the flu if they were exposed tothe flu virus.

The side effects of the new flu vaccine include sore-ness, redness, or swelling where the shot was given;fever (low grade); and aches. On rare occasions, fluvaccination can cause serious problems, such assevere allergic reactions. Imagine you could get thenew flu vaccine for a cost of $50. How likely wouldyou be to get vaccinated with the new vaccine?

Altruism Scenario from Study 3

Suppose you found out that 95% (60%, 10%) ofthe US population had already received an old vac-cine against the flu and acquired immunity to the fluvirus. However, the flu virus has evolved—now itcould be carried by anyone, even people who havereceived the old flu vaccine. Those who havereceived the old vaccine WILL NOT develop flu orexperience any adverse effect by carrying the fluvirus, but they CAN transmit the flu virus to unvac-cinated people, who will develop the flu.

A new flu vaccine has been developed to preventpeople from developing the flu AND from carryingand transmitting the flu virus.

[Self-interest irrelevant condition] You HAVEreceived the old flu vaccine and are immune to theflu virus; that is, you WILL NOT develop the flueven if you carry the flu virus, but you can carry andtransmit the virus to the 5% (40%, 90%) of the popu-lation who are unvaccinated and consequently wouldpotentially develop the flu if they were exposed to theflu virus. However, the new vaccine can prevent youfrom carrying and transmitting the flu virus.

[Self-interest relevant condition] You HAVE NOTreceived the old flu vaccine and are SUSCEPTIBLE tothe flu virus; that is, you CAN potentially catch theflu virus from anyone in the population and developthe flu. In addition, you CAN carry and transmit theflu virus to the 5% (40%, 90%) of the populationwho are unvaccinated and consequently wouldpotentially develop the flu if they were exposed tothe flu virus. However, the new vaccine can preventyou from getting infected, and it prevents you fromcarrying and transmitting the flu virus.

The side effects of the new flu vaccine includesoreness, redness, or swelling where the shot wasgiven; fever (low grade); and aches. On rare occa-sions, flu vaccination can cause serious problems,

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such as severe allergic reactions. Imagine you couldget the new flu vaccine for a cost of $50. How likelywould you be to get vaccinated with the newvaccine?

NOTE

a. There was also a between-subjects manipulation ofsocial distance, with half of the participants readingthe standard scenario and the other half reading a sce-nario specifying that the population was their smallcommunity where they had a great deal of contactwith their friends and neighbors. This manipulationhad no effect on vaccination intentions and so is notdiscussed further. The results presented combinedresponses from both social distance conditions.

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