From our analyses, it is apparent that methodological differences likely account for the previous divergent findings from 2 prior analyses6, 7
that used NHIS data to look at the potential relationship between chiropractic care use and preventive vaccination (). The 2 studies6, 7
used different CDC criteria and different years of NHIS source data and differed in their selection of comparison groups, in their operationalization of covariate variables, and in their design of regression models. The study by Stokley et al6
used 2002 CDC criteria and selected CAM users based on the NHIS variable “ever used chiropractic care,” whereas Jones et al7
used 2007 CDC criteria and selected users based on whether the modality had been used within the previous 12 months. In addition, the categorization between these 2 studies different in their model design (Stokley et al aggregated all nonchiropractic CAM users as a comparison group, whereas Jones et al compared chiropractic user with nonusers). Although both approaches appear reasonable, the differences clearly impact the results of the logistic models and offer different insights toward understanding the relationship between vaccination and use of CAM or chiropractic services.
We presented a “thought experiment” based on our replicating the methods of the Jones et al study7
and presuming universal vaccination compliance among adult chiropractic users with comorbid medical conditions of hypertension, heart disease, stroke, emphysema, or cancer. For our speculative “sensitivity analysis,” we were particularly interested in the group of chiropractic users with medical comorbidities for a number of reasons. First, these generally sicker individuals are at higher risk for developing serious complications from influenza or pneumococcal disease; therefore, they are identified as a very high-priority population for receiving the pneumococcal and seasonal influenza vaccines (by CDC,10–13
Stokley et al,6
and Jones et al7
). Second, older or sicker individuals are more likely than their younger, healthier counterparts to report having a usual source of medical care.17–22
Because adult chiropractic users with medical comorbidities are more likely to already have an established relationship with a medical provider as their “usual source of care,” then they may also more readily follow through with their chiropractor's recommendation or their chiropractor's directed referral, for receiving adult vaccination from their regular medical provider. Third, because the risks of sicker comorbid patients developing serious complications from influenza far outweigh the small potential risk of their receiving the vaccine, then chiropractic patients with comorbidity may be more readily convinced that preventive vaccination will benefit them directly, immediately, and profoundly. Younger or healthier individuals may perceive that they are less likely to get the flu or that they are less likely to get seriously ill if they do get the flu. In contrast to their elders, younger adults have not directly experienced past American pandemics such as polio and tuberculosis and perhaps may also be less inclined to appreciate the importance of collective benefits such as “herd immunity.” Such perceptions could partly explain why younger people are significantly and substantially less likely to be vaccinated against influenza than their older counterparts.23
If perceptions differ between older, sicker patients vs younger, healthier patients, then effective strategies for encouraging vaccination compliance may also need to be “individualized” to specifically appeal to different types of patients with varying perceptions and motivations. For instance, younger, healthier individuals may be more responsive to information that is tailored to their particular lifestyle risk, such as reminding students and members of the active workforce of their inherent risk of exposure to vaccine-preventable disease, because of the global nature of modern commerce, education, and travel.
More research is needed to better understand underlying perceptions, motivations, and experiences that may help to explain vaccination attitudes and behaviors of both patients and their health care providers. For instance, Rose and Ayad24
found interesting differences among chiropractic students in their baseline attitudes before taking a public health course, in that older chiropractic students with poorer self-reported health status were already more likely to have positive attitudes toward vaccination, compared with younger students in better health. After completion of the public health course, the younger, healthier students also demonstrated significant and more substantial positive changes in their attitudes about public health. Chiropractors and chiropractic users have historically been perceived as having “antivaccination” tendencies; however, evidence is accruing of changes over time in the personal and professional attitudes of chiropractors toward vaccination,24–30
and more research should be directed toward better understanding vaccination attitudes and behaviors of all health care providers,30
their patients, and the general public. More research is needed as well to better understand the challenges and implications of changing recommendations for preventive vaccinations, especially because the most recent CDC guidelines for seasonal influenza recommend that all US adults should be considered “high priority” for receiving vaccination.31
There are several limitations of this study that must be acknowledged. First, the NHIS data are self-reported and collected retrospectively. Therefore, errors may exist especially in cases where respondents were asked to recall their use of health care services up to 12 months ago; nevertheless, previous studies have shown that adult self-reported influenza vaccination is a reliable measure.32, 33
Second, nonresponse is a potential limitation of the data used for our study. However, in 2007, the adult response rate was excellent (78% for the Adult Core), and of these adult respondents, 97% completed the CAM supplemental questionnaire. Third, because our study was a cross-sectional design as were previous reports, we cannot establish a cause-effect relationship between using chiropractic care and adult immunization.
In addition to the aforementioned limitations inherent to survey studies, our analyses (as did those of Jones et al) relied on identifying current chiropractic users as those adults who reported using “spinal manipulation” in the previous 12 months. Therefore, these include users of osteopathic spinal manipulation. Lastly, although we were able to very nearly replicate the findings of previous reports, because of subtle differences in operational definitions, data management strategies, and other methodological considerations, small differences may still exist.