While all central cities improved their immunization coverage rates between 2000 and 2006, not all improved their coverage at the same pace as the rest of the state. Only six of the 26 central cities “beat the odds”, with immunization coverage rate improvements exceeding those of the rest of their state. The remaining cities, the large majority, only kept pace with or fell behind the immunization rate gains experienced in the rest of the state during this time period.
These differences in immunization rate gains relative to the rest of their state were not associated with the compositional differences of race or poverty, two individual risk factors associated with underimmunization. When we controlled for ethnicity and education, poverty was no longer a significant predictor of immunization disparities. This suggests that poverty alone is not a barrier to success in raising city immunization coverage rates. Similarly, we showed that the disadvantages often shown at an individual level, namely being African American, low levels of education, or single parent families, do not play a role in determining the citywide progression toward elimination of immunization disparities. This finding is important, because it underscores the importance of distinguishing between individual and community risk. While we continue to reach out to low-income, minority families wherever they live, there is no evidence to suggest that communities that are impoverished or have high concentrations of African American families cannot attain national immunization goals. The huge gains demonstrated by the cities of Baltimore and Chicago, both with large African American and poor populations, attest to the power of cities to help their families overcome barriers to timely immunizations.
Although nationwide Hispanic children have made gains in their immunization coverage,22
our analysis shows that Hispanic ethnicity plays a different role as a contextual or community-level factor. Central cities with higher concentrations of Hispanics made slower progress in eliminating the central city-state disparity. Few if any studies have documented the citywide disadvantage associated with large Latino populations. Many factors could explain this contextual relation. The cities with high proportions of Hispanic families could have more medically underserved neighborhoods; they might have health systems which, perhaps unintentionally, impose additional barriers on the delivery of immunizations. Cities with large Hispanic populations are likely to have more recent immigrants, for whom access and use of primary care is problematic.23
Children of immigrant parents have been shown to be less likely to have routine primary care.24,25
Finally, these cities might have health care systems which reflect hidden ethnic biases such as language barriers or cultural patterning of health care delivery, as documented in the Institute of Medicine report, whereby minorities receive a lower quality of care than nonminorities.26
The findings from our study indicate that central cities do differ in how effective they are at mobilizing their central cities for childhood immunizations. Some cities clearly were better able to do this than others. These analyses can be used to guide future investigations into the programmatic and other contextual factors which enabled the cities which “beat the odds” to mobilize to ensure that all children are immunized on time. A better understanding of how Cleveland, Chicago, Nashville, Indianapolis, and Milwaukee mobilized to increase their immunization rates by 17.5 percentage points or more, above the average of 10.4% gains in the rest of state areas may help identify better practices for others to emulate. At the same time, we also need to examine the difficulties experienced in Memphis, Jacksonville, El Paso, and Miami in order to identify the community-level barriers that kept immunization coverage gains at such a low level compared to the rest of the state. For example, examining whether expansion of Vaccines for Children coverage might have played a role or the impact of other early childhood programs such as WIC or Head Start may be helpful.
This study has several limitations. This is a contextual study, where individual differences were not controlled. The compositional analysis used in our study may not fully reflect the sociodemographic heterogeneity in central cities, which could further influence immunization coverage. Not all central cities were included in our study, which was limited to the 26 cities for which NIS reports separate immunization data as a subsample of the rest of the state. Likewise, immunization data were not available in 2006 for all cities surveyed in 2000. Because of the nature of the NIS sampling, we could not distinguish inner city areas within the NIS central city designation, although our analysis of variance did confirm that the areas we designated as central cities were in fact significantly more disadvantaged in terms of concentrations of poverty and low-income populations. The sample sizes of some of these central cities were small due to the design of the NIS, which increases the variance of the immunization coverage estimate for the central cities.16,27
Another limitation is the overlapping of confidence intervals when looking at absolute differences between central cities and the rest of states; this makes less of a difference in looking at the change ratio since the confidence intervals in 2000 and 2006 are for the most part the same. Caution is needed for interpreting the rankings of central city immunization differentials, because these rankings do not explicitly control for possible differences in sampling variability for the central city vs. rest of state areas.28
By nature of the NIS methodology, there is potential for lower response rates in the central city areas, such as is often found in household and telephone surveys in urban areas. A related problem is the possible exclusion of families who only rely on cell phones, who were not eligible for inclusion because the sampling frame did not include cellular telephones.16
Another factor which might affect response rate might be the proportion of undocumented families which is higher among Latino communities; they may be less likely to have landline phones and, if sampled, may be hesitant to take part in a national survey.29
Finally, we cannot account for potential confounders such as regional vaccine shortages, nor the impact of changes in Advisory Committee on Immunization Practices recommendations between 2000 and 2006. Although the 4:3:1:3:3 series itself did not change, the addition of new vaccines could have caused providers to postpone some vaccines if parents were concerned by the number.
Our findings suggest that community context plays a role in influencing the pace of immunization disparity reduction. The dynamics of immunization disparity reduction appear to differ for central cities and states, and what happens at the state level may not penetrate to the central city and vice versa. Concepts of social epidemiology and urban health have been used to explain contextual influences on a wide range of health behavior outcomes, and they could be useful for exploring the city-state dynamics of immunization delivery and disparity reduction.30
Our study included only very general contextual features, such as ethnicity and poverty distribution, but future studies could include a more diverse set of contextual variables that would better model the hypothesized dynamics of community influence on immunization rates. These analyses need to be informed by qualitative studies of the processes by which certain cities narrowed their immunization disparities while others were hampered in achieving immunization coverage gains. Did these two groups of cities differ in perceived access to care or ease of obtaining immunizations? Do minorities in these cities have a different quality of access whereby they feel “entitled” to be users? Answers to these and related questions could inform city and state-specific policies to level the playing field and make it possible for all children to receive their immunizations on time, regardless of where they may live. As is evidenced by some of the limitations we have encountered in our study, efforts also are needed to improve the systems for tracking immunization coverage among the communities with children at high risk for underimmunization, namely the poor and minority populations of the central cities.