ABSTRACT
Introduction: Low-income child populations remain under-vaccinated. Our objective was to determine differences in the relative importance of maternal health literacy and socio-demographic characteristics that often change during early childhood on up-to-date (UTD) immunization status among a low-income population. Methods: We performed secondary data analysis of a longitudinal prospective cohort study of 744 Medicaid-eligible mother-infant dyads recruited at the time of the infant's birth from an inner-city hospital in the United States and surveyed every 6 months for 24 months. Our primary outcome was infant UTD status at 24 months abstracted from a citywide registry. We assessed maternal health literacy with the Test of Functional Health Literacy in Adults (short version). We collected socio-demographic information via surveys at birth and every 6 months. We compared predictors of UTD status at 3, 7, and 24 months. Results: The cohort consisted of primarily African-American (81.5%) mothers with adequate health literacy (73.9%). Immunizations were UTD among 56.7% of infants at 24 months of age. Maternal health literacy was not a significant predictor of UTD immunization status. Instead, adjusted results showed that significant predictors of not-UTD status at 24 months were lack of a consistent health care location or “medical home” (OR 0.17, 95%CI 0.18–0.37), inadequate prenatal care (OR 0.48, 95%CI 0.25–0.95), and prior not-UTD status (OR 0.31, 95%CI 0.20–0.47). Notably, all upper confidence limits are less than 1.0 for these variables. Health care location type (e.g., hospital-affiliate, community-based, none) was a significant predictor of vaccine status at age 3 months, 7 months, and 24 months. Conclusions: Investing in efforts to support early establishment of a medical home to obtain comprehensive coordinated preventive care, including providing recommended vaccines on schedule, is a prudent strategy to improve vaccination status at the population level.
Introduction
Vaccination is a hallmark preventive care practice for children.Citation1 Among two-year-olds in the United States (US), however, up to 40% have not received their vaccinations at the recommended, age-guided intervalsCitation2 and there has been a resurgence of vaccine-preventable diseases in recent years.Citation3,Citation4 A child's immunization status at 24 months of age is an especially critical indicator that foreshadows future vaccination coverage rates in preschools and herd immunity at the population level in the long-term.Citation5,Citation6 Prior research of up-to-date (UTD) status (i.e., receipt of recommended, age-appropriate vaccines) and immunization schedules has revealed several contributory factors influencing adherence. These factors primarily center on maternal characteristics (e.g., race, marital status, education level, and income) and clinic type (e.g., community health centers, hospital clinics, and private practices), with more recent evidence about environmental influences (e.g., religious beliefs, language, social norms, and legal frameworks).Citation7-11 Taken together, these studies show children with low socioeconomic status and living in environments with high levels of community distrust are less likely to be UTD than other groups.Citation7-11
Little is known, however, regarding the relative importance of specific socio-demographic characteristics, including those that often change during early childhood, on immunization status by the end of the first 24 months of life. During this period, there is ample opportunity for changes in family structure (e.g., single vs. two-parent households, additional children in the household) that are often accompanied by altered parenting approaches or circumstances.Citation10,Citation12 Furthermore, parental health literacy may also influence UTD status early in life.Citation13 Given that contemporary debates about the risks and benefits of vaccinations are often highlighted in the media, the impact of parental health literacy and its potential interaction with social norms and health beliefs on vaccine receipt in early childhood merits investigation.Citation11,Citation14,Citation15 Understanding changes in the relative influence of these predictors of vaccination status through 24 months of age may advance our understanding of the potential impact and ideal focus of outreach and intervention campaigns, yielding a potential decreased need for catch-up vaccination regimens as well as improved herd immunity.
In this study, we aimed to compare the predictors of UTD status in the first 7 months of life to those at 24 months of age among a Medicaid-eligible inner-city birth cohort in the United States (US). We hypothesized that the predictors identified at 24 months of age may differ from those identified at earlier agesCitation9 and may include factors that change during the 24-month period.
Results
The study cohort consisted of 744 mother-infant dyads who were primarily African-American (81.5%) young mothers between ages 18–24 (, Appendix). Of those, 693 (93.1%) had complete immunization data. The only significant difference in socio-demographic characteristics between infants with and without immunization data was that African-American infants were more likely to have immunization data than other groups (data available upon request). Maternal health literacy was adequate, i.e., these mothers could read and interpret most health texts, among nearly three-fourth of mothers and 42.7% had more than a high school education. Only 56.7% of infants had UTD vaccines at 24 months of age. In comparison, 64.5% percent of infants had UTD immunizations at age 3 months. Of those infants UTD at 3 months of age, 69.4% were UTD at 24 months. Of those infants UTD by 7 months, 83.5% were UTD at 24 months.
Table 1. Population characteristics and association with immunization status at age 24 months.
There were significant differences in specific socio-demographic characteristics between infants who were UTD at 24 months and those who were not. Based on bivariate chi-square association tests, we found that maternal marital status (p = 0.001), birth order (p = 0.02), frequency of prenatal care (p = 0.002), and child's usual health care location type (p < 0.0001) were significantly different between infants who were UTD at 24 months and infants who were not. Children who were first-born, had unmarried mothers, and whose mothers had consistent prenatal care were more likely to be UTD at 24 months than their counterparts. Maternal health literacy was not significantly associated with UTD immunization status at 24 months. Among the time-varying predictors (i.e., employment, income, health care location, and WIC status), only the child's usual health care location type demonstrated significant association with UTD status at 24 months, with those children attending hospital-affiliated sites having the highest proportion of UTD status at 85.0%
The final adjusted best model included 686 (99.0% of 693) dyads in the analysis and showed that the strongest predictors of UTD status at 24 months were frequency of prenatal care visits, child's health care location type and prior UTD status (). The model produced a c-statistic of 0.80. Infants whose mothers reported receiving infrequent prenatal care were less likely to be UTD at 24 months compare with those receiving consistent prenatal care. Similarly, infants whose mothers reported having an unstable or no consistent usual health care location for the child were less likely to be UTD at 24 months than those who reported always receiving health care in hospital-affiliated sites. Infants UTD at 7 months were much more likely to be UTD at 24 months than those UTD at 3 months only, while infants not UTD at 3 or 7 months were much less likely to be UTD at 24 months than those UTD at 3 months only. Furthermore, after prior UTD status was removed from the model, the child's health care location type and frequency of prenatal care remained the only characteristics significantly associated with UTD status at 24 months.
Table 2. Adjusted odds ratios for predictors of UTD immunization status at age 24 months- multivariable models.
We found that variables predicting UTD status at 3, 7, and 24 months are different (). As previously reported,Citation9 maternal education and regularly attending a hospital-affiliated practice were key predictors of UTD status at age 3 months. Notably, prior UTD status was the strongest predictor of UTD immunization status at 7 and 24 months and attending a hospital-affiliated practice for the child remained a key predictor of UTD immunization status at all three time points during the first 24 months of infancy. Maternal education, maternal age, birth order, and prenatal care were key predictors at a single time point but not the other two. Maternal health literacy was not a key predictor of UTD immunization status at any time point in the first 24 months of life.
Table 3. Key predictors of up-to-date (UTD) immunization status during first 24 months of infancy.
Discussion
We found that prior UTD status and having a consistent health care location for the child are key predictors of UTD status in the first 24 months of life. A methodological advantage of our study was the opportunity to capture and measure the effects of socio-demographic predictors that often change over time on infant immunization status during the first 24 months of life. In this urban, low socio-economic status cohort, having prenatal care, having a consistent health care location, and prior UTD status at 3 and 7 months of age were associated with UTD immunization status at 24 months of age. These findings complement previous work in this area, which have similarly demonstrated the importance of prenatal care utilization and timely initiation of immunizations in predicting future UTD status.Citation10,Citation16 Furthermore, our finding that 56.7% of this predominantly African-American urban cohort was UTD at 24 months is consistent with national and regional estimates of coverage rates for populations with similar characteristics.Citation17 Our finding that using a consistent health care location is associated with improved UTD immunization status is consonant with the literature that continuity of care can foster ongoing relationships between families and clinicians permitting children's health conditions to be monitored regularly to maximize health.Citation18 Furthermore, our finding that children attending a hospital-affiliated site improves the likelihood of having UTD immunization status at 24 months is consistent with other studies.Citation19 Taken in concert, our findings underscore the importance of early establishment of a “medical home,” namely a “home base” for any child's comprehensive and coordinated medical care, to ensure timely initiation of vaccines and maintenance of UTD vaccine status.
Importantly, our findings suggest that health-seeking behavior becomes less associated with socioeconomic or socio-demographic characteristics as the child ages. Previous research in this area has found maternal characteristics such as age and marital status along with household size to be associated with underimmunization.Citation7–9,Citation20 From our earlier work,Citation9 maternal education and attending a hospital-affiliated practice were the strongest predictors of UTD immunization status at age 3 months. By age 7 months, attending a hospital-affiliated practice remained a predictor of UTD status, while prior UTD status emerged an overwhelmingly strong predictor accompanied by maternal age and birth order.Citation9 Our study design enabled us to account for elements that change over time, such as household income and employment status, to effectively weigh whether changes in these candidate predictors, with time, differentially impact the outcome of immunization status. Though children of unmarried mothers were more likely to be UTD at 24 months in our bivariate analysis, this finding did not persist in our final adjusted model suggesting that this factor is potentially related to other predictors, such as prenatal care utilization or birth order. Nevertheless, in this assessment of the first 24 months of childhood, our study revealed a decrease over time in the ability of familial characteristics to predict UTD status and an increase in the importance of prior health seeking behavior, especially using a consistent health care location and the receipt of earlier immunizations. This further supports the importance of investing in efforts to promote early establishment of a medical home.
Our findings contribute to the body of knowledge showing that having a consistent health care location improves the likelihood of having complete vaccines in a Medicaid-eligible cohort. Our findings also demonstrate a departure from the more established socioeconomic or socio-demographic predictors of incomplete immunization status.Citation21 Nonetheless, the role of such predictors should not be readily marginalized. In particular, the perception of immunizations as a “social norm,” religious beliefs, language, and vaccine hesitancy are likely additional factors influencing immunization patterns.Citation11,Citation14,Citation15,Citation22,Citation23 Though our analysis accounted for a range of candidate predictors, additional research in this area is warranted to advance our understanding of the mechanism connecting early immunization status with later immunization status.
There are several limitations in our study. One limitation of this work is the composition of the sample, which was predominantly African-American, low-income and living in an urban area. Though our findings may not be readily generalized to other populations, this study population is often the focus of intervention efforts. Another potential limitation of this work is that immunization status was derived from a citywide registry as opposed to medical chart review. However, while vaccine registries generally are known to have incomplete data when compared with some types of medical charts, Philadelphia's registry is a documented robust source of immunization data for the city's children.Citation24 Finally, the S-TOFHLA was used to assess health literacy in this study and we did not find an association between health literacy and vaccine status at 24 months. Though the S-TOFHLA is widely used and has high reliability, this instrument does not capture other dimensions of health literacy, such as numeracy,Citation25 that may be directly relevant to parent/caregiver decision-making about vaccination. Further research in this area is warranted.
In summary, prior UTD is the strongest predictor of future UTD vaccine status during the first 24 months of life among a largely African American, low-income population. Moreover, using a consistent health care location is a significant predictor of UTD vaccine status—starting as early as 3 months of age. Taken in concert, these findings underscore the importance of continued investment in expanding efforts that support early establishment of a medical home, especially in practices not affiliated with hospitals, to provide complete pediatric care including recommended vaccines that will ultimately improve population health at the national level.
Methods
Study design and study population
The Health Insurance Improvement Project (HIP) was a longitudinal prospective cohort study of Medicaid-eligible mothers and their healthy infants that was primarily designed to evaluate determinants of child Medicaid retention.Citation26 Study participants were recruited from the well-baby nursery of a large urban US hospital (Philadelphia, Pennsylvania) shortly after the infant's birth. Enrollment occurred between June 15, 2005 and August 6, 2006. Inclusion criteria were maternal Medicaid eligibility and maternal English proficiency. Exclusion criteria were infant gestational age less than 36 weeks, birth weight less than 2,500 g, not sent to the well-baby nursery after delivery, and entry into foster care or adoption services. The study was approved by the Institutional Review Boards at the University of Pennsylvania and The Children's Hospital of Philadelphia.
Data sources
At enrollment, mothers completed a baseline survey including socio-demographic information as well as items about health care access and public benefits received. We included the following potentially immunization-related characteristics in the survey: maternal race, marital status, maternal age, education level, employment status, monthly income, receipt of prenatal care, WIC participation, infant birth order, and usual clinical care site (i.e., hospital-affiliated site, private office or community clinic/health center, or none).Citation7,Citation8,Citation27-31 We administered the short version of the Test of Functional Health Literacy in Adults (S-TOFHLA) to each mother. The S-TOFHLA is a well-validated measure of functional health literacy that uses specific health-related examples to assess reading comprehension.Citation32 Every 6 months, thereafter, until 24 months of age, study staff administered a telephone survey to assess changes in relevant characteristics compare with baseline. We performed a secondary data analysis of the HIP study (Appendix) to address our UTD immunization research question.
Using individual identifiers collected at enrollment, immunization data for study members were abstracted from the Philadelphia Department of Public Health's (PDPH) citywide immunization registry, the Kids Immunization Database System (KIDS), by PDPH staff. Because Philadelphia providers are required to report to the registry all vaccinations given to children under the age of 19 years, KIDS has been shown to hold immunization records of nearly 92% of children living in Philadelphia.Citation24 In addition, regular medical record audits serve to ensure and increase accuracy of records, yielding a reliable data source for previous studies.Citation27,Citation33
Primary outcome definition
Our primary outcome of interest in this study was infant UTD status at 24 months of age as established by the Advisory Committee on Immunization Practices.Citation1 At age 24 months, children were considered to be UTD if they had received the 4:3:1:3:3:1:4 immunization series. This series was defined as receiving (a) four or more doses of diphtheria and tetanus toxoids and pertussis vaccine (DTaP), (b) three or more doses of any poliovirus vaccine, (c) one or more doses of measles, mumps, and rubella (MMR) vaccine, (d) three or more doses of Haemophilus influenzae type b (Hib) vaccine, (e) three or more doses of Hepatitis B (HepB) vaccine, (f) one or more doses of varicella vaccine, and (g) four or more doses of pneumococcal containing vaccine. Furthermore, we created the following categories of prior immunization status: (a) UTD by both 3 and 7 months of age, (b) UTD at 3 months of age only, (c) UTD at 7 months of age only, and (d) not UTD at either 3 or 7 months of age. We validated all doses prior to analysis, excluding vaccines administered with incorrect timing.Citation34 Of note, the only recorded vaccine shortage during the specified study period was one of Hib vaccine in December 2007.Citation35 However, this study cohort was unaffected by this shortage because the subjects turned one year old between June 15, 2006 and August 6, 2007, prior to the shortage.
Statistical analyses
Our primary analytic goal was to determine the set of predictors significantly associated with immunization status at 24 months of age. We used chi-square and t-tests to compare characteristics between mothers of infants enrolled in the parent HIP study with and without immunization data to assess whether the infants with immunization data were representative of the entire cohort. All predictors from the survey questions except maternal age (continuous) were included as categorical variables. We identified four predictors (employment, income, health care location, and WIC status) that had substantial variation over the 24-month observation period and created categories for patterns that were derived by examining the distribution of the variable and grouping the numerous patterns into only a few categories for purposes of statistical modeling. For example, for healthcare location, patterns were ultimately grouped as: always ‘no consistent place’; always hospital-affiliated; always private office/community setting; changes in health care location.
We explored the association of predictors with immunization status using logistic regression models with immunization status as the outcome. Logistic regression explores the functional relationship between a binary outcome variable and explanatory variables.Citation36 Notably, our study's purpose was to identify these predictors but not to estimate relative risks of UTD status for the US pediatric population as a whole because our study sample is not representative of the US population. Therefore, relative risks from this study sample cannot be generalized to the US population and are not provided. We performed model selection using the best subsets regression approach using all mothers with complete data. In this approach, predictors that were associated with 24-month UTD immunization status (p < 0.05 in univariable tests) were further considered in the multivariable logistic regression models. The best subsets approach looks at all possible models with two, three and more predictors to find the best fitting ones, as opposed to a stepwise approach that relies on the previous step. The model selection process of adding a variable stopped when the new best fitting model was not significantly superior to the previous one with one less variable. We performed this process twice, once with prior immunization status as a variable and once after removing prior immunization status as a candidate predictor. All analyses were conducted in SAS® 9.3.
Abbreviations
| UTD | = | up-to-date |
| OR | = | odds ratio |
| CI | = | confidence interval |
| WIC | = | Special Supplemental Nutrition Program for Women, Infants and Children |
| TOFHLA | = | Test of Functional Health Literacy in Adults |
| DTaP | = | diphtheria and tetanus toxoids and pertussis vaccine |
| MMR | = | measles; mumps; and rubella |
| Hib | = | Haemophilus influenzae type b vaccine |
| HepB | = | Hepatitis B vaccine |
| PDPH | = | Philadelphia Department of Public Health |
| KIDS | = | Kids Immunization Database System |
Disclosure of potential conflicts of interest
No potential conflicts of interest were disclosed.
Acknowledgments
We thank Drs. Vic Spain and Barbara Watson at the Philadelphia Department of Public Health for supervising confirmation of immunizations from the Philadelphia electronic immunization registry and their support of this project. We also thank the network of primary care physicians, their patients and families for their contribution to clinical research through the Pediatric Research Consortium (PeRC) at The Children's Hospital of Philadelphia. We additionally thank Suraj Bhatt, Elizabeth Brooks, and Elizabeth Majzoub for their research assistance on this project.
Funding
Portions of this work were supported by K23 HD047655 and R03HD056363 awards to Dr. Pati. Dr. Cnaan was supported by UL1TR000075 award from the National Center for Advancing Translational Sciences.
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Appendix
Appendix. Study enrollment protocol.
