HOW QUANTITATIVE RISK-REDUCTION INFORMATION INFLUENCES VACCINATION INTENTIONS: THE MEDIATING EFFECT OF MESSAGE CREDIBILITY

Purpose:   First, to examine how young women’s intentions to receive the Human Papillomavirus (HPV) vaccine are influenced by quantitative information on the vaccine’s absolute reduction of cervical-cancer risk (ARR).  Second, to examine whether ARR information influences vaccination intention through a “central route” (mediated by estimates of risk-reduction benefit) or a “peripheral route” (wherein the mere presence of quantitative information enhances message credibility). 

Method:  A total of 1,456 young women ages 18-26, none of whom had received any HPV vaccine doses, were recruited from a national online survey panel to participate in the study.  All participants were given basic non-quantitative information on cervical cancer, HPV infection and HPV vaccination, drawn from the CDC website.  In addition, a subset of participants was randomly assigned to receive quantitative information on the absolute reduction in cervical cancer risk expected from HPV vaccination (i.e., for every 4,000 women receiving the HPV vaccine, about how many would be “saved” from cervical cancer) based on published clinical trial evidence.  Next, all participants completed a questionnaire, in which they reported their intentions to receive the HPV vaccine, their estimates of the vaccine’s absolute risk-reduction benefit, and their perceptions of the credibility of the information they received.

Result: Participants receiving quantitative ARR information, compared to those in the no-ARR control group, reported lower estimates of the HPV vaccine’s absolute risk-reduction benefit (t=-15.4, p<.001) but higher intention to get vaccinated (t=2.91, p=.004).  This paradoxical finding occurred because the mere presence of quantitative ARR information increased message credibility (t=4.91, p<.001).  The positive impact of ARR information on vaccination intention was completely mediated by perceived credibility (Sobel (1982) test of mediation: z=4.62, p<.001).  In contrast, participants’ estimates of ARR had no effect on vaccination intentions, even among those scoring highest on numeracy.

Conclusion:  ARR information increased vaccination intention, despite reducing estimates of risk reduction benefit.  While participants generally understood the ARR information, they appeared to find it hard to evaluate (Hsee 1996).  As noted by Halovorsen (2010) “…one may understand the calculus of risk reductions but still find it difficult to judge whether a particular risk reduction is good or bad.” Instead, the mere presence of quantitative information (rather than the specific numbers) appears to serve as a peripheral cue to increase message credibility and vaccination intentions.