Discussion
This study found that HPV vaccine awareness had a significant positive impact on vaccine acceptability, a result not reported in previous studies which treated awareness as an exogenous variable in the equation for acceptability. The deviation noted in this study suggests that disregarding the potential endogeneity of HPV vaccine awareness may possibly misinform policy makers in their efforts to promote HPV vaccine awareness. The statistically significant and negative correlation of the error terms (ρ = −0.689) indicates that unmeasured differences by HPV vaccine awareness (ε2) were correlated with unmeasured factors (ε1) that influenced HPV vaccine acceptability, rendering the estimation of coefficients in the probit model inconsistent. This negative correlation suggests that the unmeasured factors impact HPV vaccine awareness and acceptability in opposite directions. In other words, the marginal effectiveness of increasing awareness of the HPV vaccine on vaccine acceptability at the population level may be much greater than the observed difference between those who are aware of the HPV vaccine and those who are not.
Although caution must be exercised in interpreting the relationship between HPV vaccine awareness and the eventual initiation of the vaccination series in this study, awareness should still be recognized as an important starting point of a continuum that leads to adopting change. The four stages of HPV vaccine adoption described by Allen et al. (2009) include pre-contemplation, contemplation, preparation and action, fairly similar to the stages of behavior change constructs of the transtheoretical model earlier presented by Prochaska and colleagues (2008). Women in the pre-contemplation stage were generally unaware of the HPV vaccine; compared with women in the more advanced stages of change, women in pre-contemplation had the lowest median scores for knowledge of HPV, perceived severity of an HPV infection, and perceived benefits of the HPV vaccine - all of which are constructs of psychosocial models that predict vaccination behavior. The results of this study complement the findings of Allen et al. (2009) which suggested that raising HPV vaccine awareness can result in positive changes in attitudes towards vaccine, which can lead to vaccine uptake. The positive consequences of being aware urge policy makers to intensify efforts to raise HPV vaccine awareness at the population level, through the maximum involvement of relevant channels and influential motivators such as health care providers.
While low vaccination rates have been previously reported in women from low-income and minority groups who are most at risk of HPV infection and cervical cancer, this study found weak evidence of disparities in HPV vaccine acceptability across ethnicities and socioeconomic status. This suggests that there are presumably other factors that hinder women - who would be, in fact, receptive of the vaccine - from eventual vaccination. These factors present women from socioeconomically disadvantaged groups with a greater obstacle. Perceived barriers to HPV vaccination are essentially multidimensional and may be further understood within the context of the health behavior change constructs. Moreover, barriers to an individual's intention to adapt a health behavior (e.g., lack of awareness and low acceptability) may be different from the barriers preventing a person from truly carrying out an intention to perform a particular behavior (e.g., vaccine cost and accessibility). Thus, aside from increasing emphasis on raising vaccine awareness, policy makers also need to be mindful of other barriers that impede vaccine uptake (i.e., high vaccine costs and limited access to health care facilities that administer HPV vaccinations), and to eliminate such barriers especially amongst the socioeconomically disadvantaged population so as to reduce socioeconomic disparities in HPV vaccination rates.
Several limitations should be acknowledged in this study. First, the HINTS 2007 did not offer any insights on how respondents' acceptability of the HPV vaccine may change in view of perceived barriers such as the affordability of the vaccine. Second, while this study quantified the potential effectiveness of improved HPV vaccine awareness on acceptability at the population level, the results of this study cannot directly provide individual-level evidence of the effect of HPV vaccine awareness on acceptability. Third, the key results were sensitive to the definition of HPV vaccine acceptability; when acceptability was defined as including "Not sure/It depends" in addition to "Yes", the main results were not statistically significant. Fourth, the relatively low response rate in the HINTS may not have captured a representative population. Finally, the HINTS 2007 was conducted almost two years after FDA's approval of Gardasil®, and levels of vaccine awareness and acceptability may have changed over the past few years.