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The Impact of Ad Hoc Vaccine Risk Communication

The CCP Vaccine Perceptions and Ad Hoc Risk Communication study included an experimental component, the aim of which was to test the impact of ad hoc risk communication. 

For purposes of the study, “ad hoc risk communication” was defined as information that correctly advises members of the public about the safety of childhood vaccines but that misrepresents the extent, nature, and consequences of public concern over vaccine risks.

Ad hoc vaccine-risk communication consists in a large array of boilerplate tropes: that a “growing crisis of public confidence” has generated an “erosion in immunization rates”; that the “evidence is overwhelming that declining vaccination rates” have generated “the resurgence of diseases considered vanquished long ago”; that growing distrust of vaccinations is part of a creeping “anti-science” sensibility linked to climate skepticism and disbelief in evolution; and that one or another recognizable segment of the public—from the conservative don’t-tread-on-me crowd that distrusts all government recommendations” to “egalitarians, [who] oppose . . . big corporations and their products”—is to blame for this escalating hostility to universal immunization.  “From Taliban fighters to California soccer moms,” one source concludes, “those who choose not to vaccinate their children against preventable diseases are causing a public health crisis.”

The proposition that vaccination rates have “declined” is contrary to the best available evidence as interpreted by the U.S. Center for Disease Control.  According to CDC data, national immunization rates for recommended childhood vaccines have held steady at 90% or above, the public-health target, for over a decade. During the period, the proportion of children receiving no vaccines has remained below 1%.  Rather than a “large and growing number” of “otherwise mainstream parents” refusing to vaccinate their children, the CDC reports that“ ‘nearly all parents are choosing to have their children protected against dangerous childhood diseases.’ ”

The refrain that “low vaccination rates” are fueling a “resurgence” in childhood disease also is not factually supported. Eliminated in the United States, measles outbreaks can still occur when individuals contract the disease abroad.  Fortunately, “[h]igh MMR vaccine coverage in the United States (91% among children aged 19–35 months),” the CDC states, “limits the size of [such] outbreaks.” “[D]uring 2001–2012, the median annual number of measles cases reported in the United States was 60 (range: 37–220).”

The incidence of pertussis, or “whooping cough,” has increased dramatically in recent years. But “[p]arents refusing to get their children vaccinated,” according to the CDC, are “not the driving force behind the large scale outbreaks” of this disease. In addition to “increased awareness, improved diagnostic tests, better reporting, [and] more circulation of the bacteria,” the CDC has identified “waning immunity “from an ineffective booster shot as one of the principal causes” (ibid). Experets outside the CDC concur (Cherry 2012).

There are enclaves, the CDC cautions, in which vaccination rates are significantly lower than the national average, and in which the extent and duration of pertussis and mealses outbreaks are likely to be enlarged.

These enclaves thus pose a genuine public health concern. They merit study aimed at understanding the influences that generate them and at formulating risk-communication strategies that might be employed to counteract and contain them.

The CCP National Risk Perception Study was not itself geared to those objectives, but its findings do generate insight into the sorts of empirical methods that are.

What the Study was directly designed to assess was the impact of empirically uninformed, ad hoc risk communication on public beliefs about vaccine safety.  Its findings suggest that the exaggerations and miststatements of fact that dominate such communication themselves pose a threat to public support for universal immunization programs.


The herd immunity conferred by universal vaccination is a collective good.  Individuals tend to contribute to public goods, research suggests, when they believe others are voluntarily doing so but refrain from contributing when they think others are shirking or free-riding (Gintis et al. 2004).

This dynamic—which has been found to explain varying levels of tax compliance, charitable giving, voting, and other forms of behavior (Kahan 2004)—has been shown to influence the intention of people to get vaccinated as well (Hershey et al. 1994).


In the CCP study, it was found that exposure to typical forms of ad hoc risk communication tend to induce individuals to substantially overestimate the percentage of parents who refuse to vaccinate their children and to underestimate vaccination rates.

This is a troubling result, for it implies that exposure to the exaggerations that pervade ad hoc risk communications could in fact weaken popular motivations to contribute to the collective good of herd immunity.

Another theme of ad hoc risk communication attributes the asserted decline in vaccination rates to an “anti-science” sensibility linked to disbelief in evolution and climate-change skepticism.

The survey component of the study found that attitudes about vaccine are not meaningfully related to either belief in evolution or positions on climate change: individuals on both sides of both issues overwhelmingly perceive vaccine benefits to be high and vaccine risks to be low.

In the experimental component of the study, however, subjects exposed to a communication patterned on ones using the anti-science trope did show signs of polarizing along lines that reflect cultural contestation over other risks, including climate change, legalizing marijuana, and instructing high school students on birth control.

This finding too is disturbing. The entanglement of positions on risks and like facts interferes with the capacity of ordinary citizens to recognize valid science and interferes with constructive public deliberations (Kahan, Peters, Dawson & Slovic 2013). It was exactly this dynamic that generated political controversy and persistent public ambivalence over the HPV vaccine.

The premise of ad hoc risk communication—that there is growing resistance and controversy over the safety of childhood vaccines—is false.

Propogating this claim, however, creates the risk of exactly such resistance and conflict.


Cherry, J.D. Epidemic pertussis in 2012 — the resurgence of a vaccine-preventable disease. New England Journal of Medicine 367, 785-787 (2012).

Gintis, H., Bowles, S., Boyd, R.T. & Fehr, E. eds. Moral sentiments and material interests: The foundations of cooperation in economic life (MIT Press, Cambridge, Mass., 2004).

Hershey, J.C., Asch, D.A., Thumasathit, T., Meszaros, J. & Waters, V.V. The roles of altruism, free riding, and bandwagoning in vaccination decisions. Organ Behav Hum Dec 59, 177-187 (1994).

Kahan, D.M. The logic of reciprocity. in Moral sentiments and material interests: The foundation of cooperation in economic life (ed. H. Gintis, S. Bowler & E. Fehr) 339-378 (MIT Univ. Press, Cambridge, MA, 2004).

Kahan, D.M., Peters, E., Dawson, E. & Slovic, P. Motivated Numeracy and Enlightened Self Government. Cultural Cognition Project Working Paper No. 116 (2013).


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To download Vaccine Risk Perceptions and Ad Hoc Risk Communication: An Empirical Assessment, click here.