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Health Psychology
2013, Vol. 32, No. 9, 978 –985
© 2013 American Psychological Association
0278-6133/13/$12.00 http://dx.doi.org/10.1037/a0031590
Inviting Free-Riders or Appealing to Prosocial Behavior? Game-Theoretical
Reflections on Communicating Herd Immunity in Vaccine Advocacy
Cornelia Betsch, Robert Böhm, and Lars Korn
This document is copyrighted by the American Psychological Association or one of its allied publishers.
This article is intended solely for the personal use of the individual user and is not to be disseminated broadly.
University of Erfurt
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.
Keywords: public health, immunization, social dilemma, advocacy, communication strategies
Vaccination yields a direct effect by reducing infection. Moreover, vaccination against contagious diseases has an additional
indirect effect (Fine, Eames, & Heymann, 2011): The transmission
of a disease is reduced with an increasing number of vaccinated
individuals. An indirect effect of vaccination can have two major
implications: On the one hand, each vaccination reduces the transmission of an infection in the population (Anderson & May, 1991),
which protects other susceptible individuals (for instance, those
who are too young to vaccinate or are immunocompromised). With
a critical vaccination level, herd immunity and disease eradication
can be reached (e.g., 95% vaccine coverage will allow for eradication of the measles in Europe; Christie & Gay, 2011; Smith,
1970). Hence, vaccination yields a social benefit, as vaccine coverage above the critical level is optimal from the collective perspective. On the other hand, if everyone else is directly protected
by vaccination, free-riders (or free-loaders; see Fine et al., 2011)
can benefit from the indirect effects of vaccination, and henceforth
avoid individual costs of vaccination (e.g., money, time, vaccineadverse events, inconvenience). The indirect effect of vaccination,
therefore, also yields an individual benefit. The presence and
awareness of both individual and social benefit from herd immunity result in a mixed-motive situation that renders vaccination a
strategic interaction (Schelling, 1960). In order to reach public
health goals, high vaccination uptake is of major importance for
society. It is therefore a fundamental question how societies can
increase vaccination uptake.
We investigated how the communication of herd immunity may
affect vaccination uptake. More precisely, we examined how the
Vaccination is typically treated as an individual decisionmaking task. In addition to motivational factors, such as adherence
to social norms (Brown et al., 2011; Liao, Cowling, Lam, &
Fielding, 2011; Sturm, Mays, & Zimet, 2005; Ajzen & Fishbein,
1980), a vaccination’s (perceived) individual costs and benefits are
especially predictive of vaccination intention. This is proposed by
several theoretical models of preventive health behavior (for an
overview see Weinstein, 1993) and has been confirmed by empirical work (e.g., Brewer et al., 2007; Brewer & Fazekas, 2007;
Nguyen, Henningsen, Brehaut, Hoe, & Wilson, 2011). Costs (barriers) of vaccination can be monetary and nonmonetary, such as
the time needed to obtain a vaccination, but also include the risks
associated with vaccination such as the occurrence of vaccineadverse events. Benefits of vaccination vary according to the
vaccine’s effectiveness, as well as the likelihood and severity of
the disease the vaccine protects against.
Cornelia Betsch and Robert Böhm, Center for Empirical Research in
Economics and Behavioral Sciences (CEREB), University of Erfurt, Erfurt,
Germany; Lars Korn, Department of Psychology, University of Erfurt.
Cornelia Betsch and Robert Böhm contributed equally to this paper. The
authors are grateful to the master students of the fall 2011 term, who helped
set up the study and collect data. Tilmann Betsch, Wasilios Hariskos,
Fabian Kleine, Frank Renkewitz, Heather Fuchs, and the CEREB research
group made helpful comments on a draft of the paper.
Correspondence concerning this article should be addressed to Cornelia
Betsch, University of Erfurt, Nordhäuser Strasse 63, D-99089 Erfurt,
Germany. E-mail: [email protected]
978
COMMUNICATING HERD IMMUNITY IN VACCINE ADVOCACY
awareness of a vaccination’s individual benefit, social benefit, or
both, affects vaccination intention. According to the theory of
reasoned action and the theory of planned behavior, salient beliefs
determine an individual’s attitude toward a behavior and behavioral intentions (Ajzen & Fishbein, 1980). An individual’s awareness of herd immunity may therefore either increase or decrease
vaccination uptake, depending on the salience of individual and
social benefits that result from the indirect protection of herd
immunity.
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Vaccination as a Strategic Interaction
The interactive structure of vaccination decisions has recently
been discussed in the literature (e.g., Bauch & Earn, 2004; Bhattacharyya & Bauch, 2010; Galvani, Reluga, & Chapman, 2007;
Manfredi et al., 2009). Based on this literature, we devise a simple
model of vaccination as strategic interaction, which is illustrated in
Figure 1. The expected costs of a disease (E[cD]) are typically
treated as the product of the severity of the disease and the
probability to contract the disease. Similarly, the expected costs of
a vaccination (E[cV]) are the product of its costs (monetary and
nonmonetary) and their probability (e.g., the likelihood of adverse
advents): E[cD] ⫽ SEVD ⫻ PROBD and E[cV] ⫽ SEVV ⫻ PROBV
(Weinstein, 1993). The expected utility from vaccination (EUV) and
nonvaccination (EU¬ V) results from the difference between the
expected costs of the disease and the expected costs of vaccination:
EUV ⫽ E[cD] ⫺ E[cV] and EU¬ V ⫽ E[cV] ⫺ E[cD]. Consequently,
small differences in expected costs of the disease and vaccination
yield larger differences in the expected utility of vaccination and
nonvaccination, because EUV - EU¬ V ⫽ 2(E[cD] ⫺ E[cV]). Furthermore, the probability of contracting a contagious disease, and therefore also the expected costs of the disease, overproportionally de-
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creases as a function of the number of vaccinated individuals, because
the lifetime incidence for unvaccinated individuals decreases (e.g.,
Fine et al., 2011). At the same time, the expected costs of the
vaccination remain constant, because, for instance, the probability and
severity of side effects are not affected by the number of vaccinated
individuals. Therefore, when the number of vaccinated individuals
(vaccine coverage) increases, the expected costs of the vaccination
will at some point exceed the expected costs of the disease (E[cD] ⫺
E[cV] ⬍ 0; cf. intersection in Figure 1; Chen, 1999). From an
individual perspective, nonvaccination then becomes the best response as EUV ⬍ EU¬ V. In contrast, it is collectively optimal to
vaccinate until a vaccination level is achieved that eradicates the
disease, as the expected cumulative incidence is zero if coverage is
maintained above the critical vaccination level (Vc; Fine et al., 2011).
Therefore, as soon as E[cD] ⫺ E[cV] ⬍ 0 the vaccination decision
contains a motivational conflict between the individual and collective
interest. If Vc is reached, nonvaccination is the best response from an
individual and collective perspective. Within these boundary conditions, vaccination constitutes a N-person prisoner’s dilemma in which
individuals may decide whether or not to contribute (i.e., vaccinate) to
a public good (i.e., herd immunity); see shaded area in Figure 1.
It is important to note that expected costs are subjective variables and can therefore deviate from objective cost parameters.
Today, wild forms of severe vaccine-preventable diseases are rare
and most individuals lack a concrete, vivid representation of the
disease (⫽ low perceived costs of disease). At the same time,
vaccination costs are more visible, vivid, and tangible due to high
vaccination rates and immediate effort and inconvenience (⫽ high
costs of vaccination; Chen, 1999). Many modern vaccination decisions (e.g., against polio or the measles) can therefore be framed
as a social dilemma.
Figure 1. Simplified General Model of Vaccination as a Strategic Interaction. Note: n refers to the number of
all other individuals in the population (N ⫺ 1) who decide to vaccinate. The gray dashed line indicates the
expected individual costs of the vaccination (E[cV]) and the gray solid line indicates the expected individual costs
of the disease (E[cD]). The black dashed line indicates an individual’s expected utility if he/she decides
to vaccinate (EUV), whereas the black solid line indicates the individual’s expected utility if he/she decides not to
vaccinate (EU¬ V). Vc refers to the critical vaccination level (herd immunity threshold) that must be achieved to
eradicate the disease, which is simplified 1 ⫺ 1/R0, with R0 being the basic reproduction number of the infectious
disease (Fine et al., 2011). The shaded area indicates a conflict between individual and collective interest,
transforming the vaccination decision into a N-person prisoner’s dilemma.
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BETSCH, BÖHM, AND KORN
As discussed, herd immunity can have two potential effects. On
the one hand, if the individual benefit of herd immunity is communicated, the individual’s selfish/egoistic preferences might be
activated (Dawes, 1980; Hardin, 1968) and accordingly affect
behavior (Ajzen & Fishbein, 1980). Therefore, the first hypothesis
predicts:
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H1: If the individual benefit of herd immunity is salient (but
not the social benefit), participants will show lower vaccination intentions, as compared with when the individual benefit
is not salient (free-riding hypothesis).
On the other hand, by making the social benefit of one’s own
vaccination salient, individuals’ positive, other-regarding preferences might be activated (for an overview see Fehr & Schmidt,
2006). This leads to the second hypothesis:
H2: If the social benefit of herd immunity is salient (but not
the individual benefit), participants will show higher vaccination intentions, as compared with when the social benefit
is not salient (prosocial behavior hypothesis).
This study extends prior work (e.g., Hershey, Asch, Thumasathit, Meszaros, & Waters, 1994; Shim, Chapman, Townsend,
& Galvani, 2012) by orthogonally manipulating the salience of
individual and social benefits. We are, therefore, also able to
explore the interaction between the salience of individual and
social benefits.
In Figure 1, if the expected costs of vaccination increase or
decrease (gray dashed line moves up or down, respectively),
ceteris paribus, the intersection of E[cD] ⫽ E[cV] shifts further to
the left or right, respectively. Consequently, the difference in
expected utility between vaccination and nonvaccination, on the
right-hand side of this intersection increases, or decreases, respectively. This effect occurs due to the indirect effect of vaccination.
If individuals are unaware of this indirect effect (e.g., because herd
immunity is not communicated), the expected costs of the disease
will not depend on the vaccination of others and will therefore be
constant. As, in this case, the solid gray line would be parallel to
the dashed gray line, selfish or other-regarding preferences should
thus have no effect.
The (perceived) costs of vaccination should therefore interact
with the effects of communicated herd immunity: Free-riding
entails the benefits of vaccination (due to others’ immunization)
without carrying the costs. It follows that the incentives for freeriding are especially high if costs to vaccinate are high. Likewise,
individuals who vaccinate due to a prosocial motivation (to protect
the unimmune) take over costs for the society. Hence, if these costs
are high, prosocial behavior may be less likely. This leads to our
third hypothesis, which integrates the structurally equivalent subhypotheses of individual and social benefit salience:
H3: If the individual benefit of herd immunity is salient, participants are more inclined to free-ride when the costs of
vaccination are high than when the costs of vaccination are low.
Similarly, if the social benefit of herd immunity is salient,
participants are more inclined to prosocial behavior when the
costs of vaccination are low than when the costs of vaccination
are high (vaccination costs interaction hypothesis).
Experiment
The hypotheses were tested in an online experiment that
assessed vaccination intentions regarding hypothetical diseases
(for a similar methodological approach, see cf. Betsch &
Sachse, in press; Betsch, Renkewitz, & Haase, in press; Betsch,
Ulshöfer, Renkewitz, & Betsch, 2011; Vietri, Li, Galvani, &
Chapman, 2012). The definition of herd immunity was provided
along with one sentence summarizing the gist of the message
(Reyna, 2012), making salient either the individual benefit,
social benefit, or both. A control group received no information
about herd immunity. Additionally, we tested if the cost of
getting vaccinated, operationalized as the amount of effort
required to obtain the vaccine, interacts with the communicated
benefits.
Method
Participants and design. Participants were recruited via mailing lists and social network websites (e.g., Facebook). As compensation, all participants took part in a raffle for one of five gift
certificates (25 Euro; ⬃ $ 31). Three hundred seventy-one participants completed the questionnaire. Twenty-nine participants were
excluded from the sample due to excessively long (⬎30 min) or
short (⬍5 min) duration of participation, resulting in a mean (M)
duration of 12.76 minutes (standard deviation [SD] ⫽ 5.14).
Hence, the final sample consisted of 342 participants, both students and nonstudents. Eighty-eight percent of the sample had an
Abitur (German University entrance diploma) or higher level of
education. The mean age of the sample was 30.34 years (SD ⫽
12.5); 221 (64%) participants were female.
The experiment used a 2 ⫻ 2 ⫻ 2 between-subjects design with
individual benefit of herd immunity (communicated vs. not communicated), social benefit of herd immunity (communicated vs. not
communicated), and costs of vaccination (low vs. high) as factors. It
was realized with an online software program (EFS survey), which
randomly allocated participants to the eight conditions.
Herd immunity. In the control condition (individual and social
benefits of herd immunity were not communicated), participants
received no information about herd immunity. In the remaining three
conditions, participants received the following definition of herd immunity: “Herd immunity denotes the effect that occurs when acquired
immunity against a pathogen, generated through infection or vaccination, within a population (the “herd”) has reached such a level that
nonimmune individuals in this population are also protected, because
the pathogen can no longer be transmitted.” Furthermore, depending
on condition, one additional sentence summarized the gist of the
message, manipulating the salience of the individual benefit, social
benefit, or both. Individual benefit was highlighted by the sentence
“The more people are vaccinated in your environment, the more likely
you are protected without vaccination.” Social benefit was highlighted
by the sentence “If you get vaccinated, then you can protect others
who are not vaccinated.”
Vaccination costs. Participants were either informed that they
could get vaccinated immediately (low cost) or that they would
have to set up an appointment with the local hospital and that this
appointment would take almost 3 hours (high cost).
COMMUNICATING HERD IMMUNITY IN VACCINE ADVOCACY
Measures
Dependent measure. Vaccination intention was assessed (“If
you had the opportunity to vaccinate against [the illness] next
week, what would you decide?”) on a 7-point Likert-type scale
ranging from 1 ⫽ definitely not vaccinate to 7 ⫽ definitely vaccinate.1
Manipulation check. As a manipulation check, one additional item assessed the perceived costs of the vaccination (“How
do you estimate your personal vaccination costs?”; 1 ⫽ very low to
7 ⫽ very high).
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Procedure
After giving informed consent, participants were informed that
all presented information is fictitious. The questionnaire began
with the measurement of demographic characteristics. They were
then asked to imagine a fictitious scenario: During a routine
physical examination, the general practitioner informs them about
the severe infectious disease Cornicoviszidosis. This recently discovered illness had been diagnosed increasingly often. The participants received additional information about the origin of Cornicoviszidosis, the name of the responsible virus (Cornicovi), the
path of infection (smear infection), and the symptoms of the
disease (severe vomiting and diarrhea, severe dehydration, and
high fever). Participants received a data sheet for a fictional
vaccine termed Macentat containing information about vaccineadverse events: hypersensitivity reaction of the skin with a probability p ⫽ .1; headache, p ⫽ .0001 to .001; and vomiting, vertigo,
and skin rash with a probability less than .0001. Information about
herd immunity was displayed afterward. Before participants indicated their intention to get vaccinated, they were informed about
the costs of the vaccination. Finally, the manipulation checks were
recorded and participants fully debriefed.
Results
We present eta-squared as an effect size indicator along with all
statistically significant results. All nonsignificant comparisons
have an F ⬍ 1 if not stated otherwise.
Manipulation check. As intended, the vaccination costs in
the high cost condition were perceived as significantly higher than
in the low cost condition: Mhigh ⫽ 4.67, SD ⫽ 1.69; Mlow ⫽ 2.20,
SD ⫽ 1.36; F(1, 340) ⫽ 222.06, p ⬍ .001, ␩2 ⫽ .40.
Table 1
Means (Standard Deviations) of the Intention to Vaccinate as a
Function of Communicated Individual and Social Benefit as
Well as Costs to Obtain the Vaccination
Individual benefit
Social benefit
Communicated
Not communicated
Communicated
Not communicated
Communicated
Not communicated
High vaccination cost
3.44 (1.86)
3.62 (1.92)
3.75 (1.91)
4.20 (1.63)
Low vaccination cost
4.54 (1.50)
4.40 (1.92)
3.30 (1.83)
4.42 (1.51)
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Vaccination intention. The mean values and standard deviations of vaccination intention by experimental condition are displayed in Table 1. Inspection of the means in Table 1 suggests that
the control condition (without any herd immunity information)
showed, on average, higher vaccination intentions than when herd
immunity was communicated in either manner. Indeed, a post hoc
simple contrast analysis (control group vs. all other conditions)
yields a significant difference, F(3, 363) ⫽ 3.69, p ⫽ .012, ␩2 ⫽
.030. Still, what are the specific effects of communicating individual versus social benefits on vaccination intentions? H1 and H2
predict lower vaccination intentions when the individual benefit is
salient (free-riding hypothesis), and higher vaccination intentions
when the social benefit is salient (prosocial behavior hypothesis).
H3 suggests an interaction between benefit salience and costs.
To test the hypotheses, we conducted a 2 ⫻ 2 ⫻ 2 analysis of
variance with vaccination intention as the dependent variable.
Results support the free-riding hypothesis (H1): when individual
benefit of herd immunity was communicated, vaccination intentions were significantly lower than when it was not communicated,
Mcomm ⫽ 3.72, SD ⫽ 1.84; M¬ comm ⫽ 4.18, SD ⫽ 1.77;
F(1, 334) ⫽ 4.33, p ⫽ .038, ␩2 ⫽ .012. There was no difference
in vaccination intentions whether the social benefit was communicated or not, Mcomm ⫽ 4.01, SD ⫽ 1.86; M¬ comm ⫽ 3.89, SD ⫽
1.78, yielding no evidence in support of the prosocial behavior
hypothesis (H2). However, there was a significant interaction
effect between the individual and social benefit communication
conditions, F(1, 334) ⫽ 3.90, p ⫽ .049, ␩2 ⫽ .011. As Figure 2A
shows, when social benefit was not communicated, vaccination
intentions varied as a function of the communicated individual
benefit: the vaccination intention was significantly lower when the
individual benefit of vaccination was communicated than when it
was not communicated, F(1, 177) ⫽ 9.38, Bonferroni-corrected
p ⫽ .006, ␩2 ⫽ .051, indicating the tendency to free-ride on others’
indirect protection. When the social benefit of herd immunity was
communicated, however, there was no such difference, regardless
of whether the individual benefit was additionally communicated.2
In general, vaccination intentions were lower when vaccination
costs were high than when they were low, Mhigh ⫽ 3.83, SD ⫽
1
Prior to intention we also assessed the perceived risk of the disease and
the vaccination (general risk [0-100], respectively). We did not expect any
effects of the manipulations on the assessed variables, as we did not
provide any information about vaccine coverage. We used the risk variables as proxies for the expected costs of the disease and vaccination. The
mean perceived risk of the disease was Mdisease ⫽ 41.18 (SD ⫽ 23.92); the
risk of the vaccination was Mvaccination ⫽ 26.65, SD ⫽ 22.14; t(341) ⫽
7.89, p ⬍ .001. This mirrors the experiment materials, as the disease was
described as having severe symptoms, while potential vaccination side
effects were rather moderate. As a consequence, 100 participants perceived
a higher risk of vaccination than of the disease, while the majority of 237
participants perceived higher disease than vaccination risks; for 5 participants the costs of disease were equal to the costs of vaccination.
2
Strictly speaking, the preconditions for considering vaccination as a
social dilemma are only given for those participants who perceived the
costs (risk) of the vaccination to be higher than the costs (risk) of the
disease (see Figure 1). In support of this, two separate analyses showed that
the effects were indeed consistently stronger if E[cD] ⫺ E[cV] ⬍ 0 than if
E[cD] ⫺ E[cV] ⬎ 0. The effect sizes if E[cD] ⫺ E[cV] ⬍ 0 were ␩2 ⫽ .014
for individual benefit communication (vs. ␩2 ⬍ .01 if E[cD] ⫺ E[cV] ⬎ 0)
and ␩2 ⫽ .055 (vs. ␩2 ⫽ .001) for the interaction between social and
individual benefit communication. We conclude that this demonstrates the
validity of the proposed model.
BETSCH, BÖHM, AND KORN
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982
Figure 2. Intention to Vaccinate as a Function of Communicated Social Benefit and Communicated Individual
Benefit (A) or Vaccination Costs (B).
1.87; Mlow ⫽ 4.10, SD ⫽ 1.79; F(1, 334) ⫽ 4.57, p ⫽ .033, ␩2 ⫽
.013. As expected, vaccination costs interacted with the social
benefit communication, F(1, 334) ⫽ 7.43, p ⫽ .007, ␩2 ⫽ .021. As
Figure 2B shows, when the social benefit of herd immunity was
communicated, vaccination intentions were higher when costs
were low than when costs were high, F(1, 163) ⫽ 11.07,
Bonferroni-corrected p ⫽ .002, ␩2 ⫽ .064, indicating conditional
prosocial vaccination behavior. There was no such difference
when social benefit was not salient. However, vaccination intentions under individual benefit salience did not differ between the
low- versus high-cost conditions. Taken together, results partially
confirm the vaccination costs interaction hypothesis (H3). The
three-way interaction was not significant, F ⬍ 1.6, ns.
Discussion
The goal of this paper was to assess if and how the communication of herd immunity may affect vaccination uptake. We devised a simple theoretical model of (non)vaccination utility as a
function of the perceived costs of the disease and the vaccination
contingent on the number of vaccinated individuals in the population. Furthermore, some of the model’s implications were tested
in an experiment. The data show that communicating the concept
of herd immunity can have two effects, depending on the gist of
the message: First, when a message emphasized the individual
benefit of indirect protection through the “herd,” individuals’
inclination to free-ride increased (H1). This was especially the case
when the social benefit of herd immunity was not communicated.
Second, communicating the social benefit did not result in a
general increase in vaccination intentions (contradicting H2).
However, communicating the social benefit reduced free-riding
and also had the potential to increase vaccination intentions when
the costs of vaccinating are perceived as low. Thus, depending on
which implication of herd immunity was made salient, vaccination
intentions differed.
The lacking overall positive effect of communicated social
benefit, along with the obtained free-riding effect pose the question
whether the communication of herd immunity is advisable at all.
Strong emphasis on the social benefit, however, still seems recommendable; even if it might not have an overall positive effect,
it might at least prevent free-riding.
This becomes even more clear when we consider that many cues
in the decision structure may invite free-riding: Recent gametheoretical models of vaccination uptake have shown that the level
of vaccination may decrease dramatically and fall below the social
optimum if the expected costs of vaccination increase (e.g., due to
a vaccine scare or antivaccination activism; Bhattacharyya &
Bauch, 2010) or if the costs of the disease decrease (Jansen et al.,
2003). This occurs mainly because of free-riding on the immunized herd (e.g., Bauch & Earn, 2004; Galvani, Reluga, & Chapman, 2007; Manfredi et al., 2010). Furthermore, individuals are
sensitive to different levels of immunity in the population if these
are varied in a within-subjects’ setting (Vietri et al., 2012): The
more people were vaccinated, the less likely participants were to
vaccinate themselves. Our results contribute to this literature by
showing that quite subtle communications are also sufficient to
suggest a free-riding opportunity. Again, this implies that strategies that prevent free-riding are needed. The current results suggest
that appealing to prosocial motives might be such a strategy to
reduce free-riding tendencies (see also Shim et al., 2012).
As previous research focused particularly on the negative effect
of indirect protection through herd immunity (free-riding; e.g.,
Bauch & Earn, 2004), more scientific attention should be directed
COMMUNICATING HERD IMMUNITY IN VACCINE ADVOCACY
to its positive effect (i.e., prosocial behavior), in order to assess the
boundary conditions under which the communication of the social
benefit of vaccination does increase vaccine intentions (for instance, as in the present experiment, under low perceived costs of
vaccination; or when the risk for the self is low as shown by Vietri
et al., 2012). This becomes particularly important if the direct
effect of vaccination is very small or even absent and the indirect
effect has important consequences for eradicating a disease (as in
the most extreme case of malaria control; Carter, Mendis, Miller,
Molyneux, & Saul, 2000).
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Limitations and Further Research
A number of possible limitations to the results should be noted.
First, the hypothetical scenario and the self-report data (intentions)
might limit the external validity of the results. In addition, online
experiments may be subject to self-selection bias. Indeed, the
participants in our sample were typically well educated. However,
including education in the analyses did not affect the pattern of
results. Moreover, the present study tests hypotheses derived from
a general game-theoretical model. We do not assume different
relations between utility functions for individuals with different
levels of education. We therefore conclude that external validity of
the results is given even though the sample might be skewed
toward higher educated participants.
Further, we neither manipulated nor measured the individual
perception of vaccine coverage and could therefore not test its
impact on perceived costs of the disease. This should be a next step
in further research. For instance, an experimental public goods
setting of vaccination (see Chapman et al., 2012) may be a viable
framework to test the dynamic relationship between vaccination
uptake and the number of others vaccinated. Nevertheless, the
perceived risk of both the vaccination and the disease were assessed in this study and can serve as proxies for perceived costs
(see footnote 1). As expected, the effects were stronger if E[cD] ⫺
E[cV] ⬍ 0 than if E[cD] ⫺ E[cV] ⬎ 0 (see footnote 2). The
presence of the effects in the total sample suggests that, even if
only a subsample of the population perceives higher vaccination
than disease costs, communicating the social benefit can have
positive effects on vaccination intentions.
The present experiment was ambiguous regarding whether the
other protected individuals were not able to get vaccinated (because they are too young or immunocompromised) or simply not
willing to do so (free-riders) when the individual benefit of herd
immunity was communicated. A large amount of research in
economics has shown that people are willing to cooperate if others
are also expected to do so (conditional cooperation; e.g., Bolton &
Ockenfels, 2000; Fischbacher, Gächter, & Fehr, 2001; Fehr &
Schmidt, 1999). Moreover, in social psychology there is evidence
that prosocial behavior is more likely when uncontrollable factors
created the situation of need (Weiner, 1980). Thus, if it is communicated that others are explicitly not able to protect themselves,
the communicated social benefit of herd immunity should have a
larger effect.
Similarly, the way in which costs were manipulated represents only one out of several possibilities. As said before, costs
accrue due to time, money, side effects, inconvenience, and so
forth. Future studies should use different approaches to manipulate costs. The perception or fear of potential side effects (such
983
as elicited in vaccine scares) decreases vaccination intentions
and are among the most prominent reasons against vaccination
(Betsch, Renkewitz, Betsch, & Ulshöfer, 2010; Brown et al.,
2010a, 2010b). Thus, it is possible that if costs are manipulated
via potential side effects of the vaccine, the obtained effects
may be stronger.
Practical Implications
Overall, it seems advisable to stress vaccination’s social
benefit in vaccine advocacy. This is especially the case if the
concept of herd immunity is communicated to the public, such
as during the process of eradicating diseases, for example, the
measles and rubella in Europe until 2015 (Christie & Gay,
2011). If the indirect effects of vaccination become obvious,
free-riding might increase, as vaccine coverage is usually already high (but not high enough). Therefore, stressing the social
benefit may help to reach critical vaccination levels in order to
eradicate diseases.
Protection of others is especially important in contexts with
highly vulnerable individuals, such as immunocompromised patients in a hospital. For this reason, the World Health Organization
recommends vaccination against influenza for health care personnel (HCP). Despite the availability of an effective and welltolerated vaccine, low seasonal and pandemic influenza vaccine
acceptance among HCP is a major problem detailed in many
studies from all over of the world (Salgado, Giannetta, Hayden, &
Farr, 2004; Talbot et al., 2010). The perception of vaccination risks
in addition to other expected vaccination costs are major reasons
why HCP do not get vaccinated against influenza (Betsch &
Wicker, 2012; Wicker, Rabenau, Doerr, & Allwinn, 2009). One
could speculate that HCP may generally be more prosocially
oriented (e.g., Van Lange, 1999). Thus, building on the idea of
tailoring health messages (Noar, Benac, & Harris, 2007), appeals
to the social benefit of vaccination could be a viable strategy to
increase HCP’s vaccination rates.
Conclusions
We conclude that the social benefits of vaccination need to be
explicitly communicated if the individual decisions are meant
to consider public health benefits. Even if it does not generally
raise vaccination intentions, it can prevent free-riding and has
the potential to increase vaccination intentions when the costs
to vaccinate are low. A recent approach for vaccine advocacy
suggests that “vaccination adoption ⫽ access ⫹ acceptance”
(Thomson & Watson, 2012). Acceptance can be understood as
E[cD] ⫺ E[cV] ⬎ 0 or as prosocially oriented behavior under
E[cD] ⫺ E[cV] ⬍ 0. Access can be understood as low costs to
obtain the vaccine. The latter also proved to be an important
variable in our study. Especially when vaccination is not the
individually optimal solution and public health considerations
suggest a collective benefit of vaccination (e.g., cocooning
newborns against pertussis, preventing schoolchildren from
spreading influenza to the elderly), access should be very easy
in order to obtain high vaccination coverage.
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Accepted October 5, 2012 䡲
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