Immunization Coverage Levels Among 19- to 35-Month-Old Children in 4 Diverse, Medically Underserved Areas of the United States

Target Population

The 4 participating sites in the CHN project were designated as HPSAs⁵ because each area had a critical shortage of primary health care providers. HPSAs are defined as areas with a population-to-provider ratio of ≥3500 to 1, areas adjoining these underserved areas, or areas in which residents must travel >30 minutes to the nearest provider.^7,8 Urban areas eligible to be considered in this project were census tracts with a population of at least 100 000 that included a designated HPSA and any contiguous census tracts in which a majority of residing children <2 years old were eligible for Medicaid. Eligible rural areas were defined as nonmetropolitan or rural areas that were beyond a metropolitan statistical area⁶ and in which most residents were members of traditionally underserved minority populations: African Americans, Alaska Natives, American Indians, Asian Americans, Latinos/Hispanics, and Pacific Islanders.

Household Surveys

Surveys in the 4 areas were based on stratified cluster probability sample designs in which clusters of dwelling units were selected and all households in selected clusters were screened for the presence of children 12 through 35 months old.^9,10 The surveys were approved by the Institutional Review Board of the CDC in Atlanta, Georgia.

Household Interviews

Standardized interviews were conducted with primary caretakers of the age-eligible children in the sample households to obtain information on sociodemographic characteristics, utilization of immunization health services, utilization of federal programs, parental knowledge and beliefs about vaccination, and perceived barriers to accessing the immunization health care system.^11–13

For each age-eligible child, the information collected on utilization of immunization health services included a listing of all clinics or offices ever used for the child’s well-child care and/or for obtaining immunizations. Information was also collected as to whether the child currently had health insurance (public and/or private) and whether the child had a medical home. A child was classified as having a medical home if the survey respondent reported a source of well care that was the same as the source of sick care and that this place was not an emergency department.

The information collected about the utilization of federal programs encompassed aid received from the US Department of Agriculture Special Supplemental Nutrition Program for Women, Infants, and Children.

The questions asked about barriers to care included availability of transportation and vaccine costs incurred by parents. Because studies of access to immunization health services have shown that up-to-date (UTD) immunization status at 3 months is a good indicator of early access to the health care system, we also constructed this indicator for use as potential predictor variable.^3,5

Immunization Record

If a vaccination card was available in the home, the interviewer transcribed all immunization doses and dates to a standardized recording form. If an immunization record was not available, the interviewer completed the immunization section of the questionnaire based on the respondent’s recall. In such cases, respondents did not report specific dates for the immunizations and often responded “all shots,” or they reported that they did not know the child’s immunization history. All respondents were asked for permission to contact the child’s immunization providers. If this permission was granted, providers were sent a request for the child’s immunization record and information about the practice (including whether the facility was a Vaccines for Children Program [VFC] facility). All providers were surveyed. When 1 of a child’s providers supplied a complete record from birth, follow-up to collect data from other providers was discontinued.

Vaccination History

If a child’s parent-held records and provider records did not agree, we credited each child with all shots listed in either record. If no provider or parent-held record was available, the child’s vaccination status was classified as unknown; a report from recall was not accepted without confirmation from a provider.

Analytical and Statistical Methods

Community-wide immunization coverage levels were estimated for each area and measured as UTD for immunization at 19 to 35 months of age for comparability with the National Immunization Survey. UTD at 19 to 35 months of age was defined as receipt of 4 doses of diphtheria and tetanus toxoids and pertussis vaccine (DTP), 3 doses of poliovirus vaccine, 1 dose of measles/mumps/rubella vaccine (MMR), 3 doses of Haemophilus influenzae type b vaccine (Hib), and 3 doses of hepatitis B vaccine (HepB); these doses constitute the 4:3:1:3:3 series. We also measured UTD status for each antigen, ie, 4 doses of DTP; 3 of poliovirus vaccine, Hib, and HepB; and 1 of MMR. UTD at 3 months was defined as the receipt of 1 dose each of DTP, poliovirus vaccine, Hib, and HepB.

All estimates were weighted to represent the target populations in each of the selected sites. The weights account for unequal selection probabilities that occurred in the fieldwork in a few cases and also compensate for nonresponse by households and providers.

We assessed the UTD coverage level of children at each site separately. We examined the association between UTD coverage level and each factor and performed χ² and t tests to determine whether differences observed within and between sites were significant. The statistical analyses accounted for the clustered sample designs. To minimize the probability of a type I error, only P values of <.01 were considered significant. Analyses were conducted by using SUDAAN, a family of statistical procedures for analysis of data from complex sample surveys.¹⁴

Response Rates

As shown in Table 1, 150 to 381 clusters were selected randomly in the 4 study sites, a total of 16 304 to 23 706 household units were screened in the sampled clusters in each area, and responding households yielded data for 771 to 1091 children 12 to 35 months of age. The overall enrollment response rate for eligible children ranged from 79.4% to 88.1%.

Across all 4 sites, 7% of children enrolled had no provider to contact for immunization information. The remaining children had an average of 2 providers. In total, 4905 provider questionnaires were fielded, of which 78.2% were returned. Provider reports were obtained for 88.2% of children for whom a provider was contacted.

As a result of missing hand-held records and missing provider information, immunization status was classified as unknown for some enrolled children. Table 1 shows that the overall response rate for vaccination status ranged from 70.5% to 81.5% across the 4 sites.

Population characteristics that might predispose children to have low immunization coverage varied from site to site (Table 2). Most study children in Detroit were African American, and most in New York, San Diego, and rural Colorado were Hispanic. Teen motherhood was lowest in Detroit. Maternal education was notably lower in San Diego. Most study families lived in poverty, with the exception or rural Colorado. In all sites, approximately three quarters of the families with an age-eligible child had ≥2 children.

Population characteristics that enable access to health care also varied. Income was highest in Colorado. Overall, most children were insured. In Detroit, New York, and San Diego, 48% to 64% of children were privately insured, whereas in Colorado, 52% of children were insured by Medicaid or Civilian Health and Medical Program of the Uniformed Services. All study sites had an appreciable proportion of children with no insurance (7%–22%). Most of the children had been or were enrolled at the time in WIC and did not have out-of-pocket expenses for vaccinations. The vast majority of children had a medical home, and children in these communities had received immunizations from private, public, and other providers (hospital outpatient clinics and a mix of private and public providers). A majority of children in Detroit and New York had only 1 immunization provider, whereas the majority of children in Colorado and San Diego had at least 2. Overall, a large proportion of children (80%–88%) received their immunization services from providers in VFC facilities. However, 11.6% and 9.6% of children in Detroit and San Diego reported problems accessing the health care system because of transportation problems. Out-of-pocket expenses were paid for immunization services for nearly 47% of children in Colorado.

Vaccination Coverage Rates in the Study Sites

In New York and San Diego, all estimates of immunization coverage for DTP, poliovirus vaccine, Hib, and MMR among preschoolers were at least 90% (Table 3). For single antigens, coverage for 4 doses of DTP in Colorado fell below 90%. In Detroit, coverage for these antigens lagged, with coverage as low as 72% for 4 doses of DTP. Of note, the lowest coverage for HepB was 84% for Detroit, well above the goal of 70% established for the year 2000.

Coverage for the 4:3:1:3:3 vaccine series (UTD at 19–35 months) was lower and varied significantly across sites (χ² = 44.51; degrees of freedom = 3; P < .0001), with coverage in Detroit (66%) being lower than in San Diego (86%), New York (84%), and Colorado (75%).

Table 4 displays the percentage of children UTD at 19 to 35 months by selected factors. Although differences in coverage levels by ethnicity varied by site, differences were not significant. In Colorado and New York, coverage was slightly lower among Hispanic than white children (71% vs 76% and 83% vs 91%, respectively). In San Diego, coverage was lower among whites, compared with Hispanics (76% vs 85%). Coverage was also lower for African American than white children only in New York (75% vs 91%).

In San Diego and Colorado, vaccination coverage levels varied significantly by type of health facility. In fact, children in San Diego who received all vaccinations from a private provider had lower vaccination coverage levels than children receiving their vaccinations from the “other” category of providers (78% vs 91%; t = −3.25; P < .005); in Colorado, coverage was lower among children receiving vaccines from private providers than among children receiving vaccines from the “mixed” category of providers (71% vs 82%; t = −3.04; P < .005).

In all 4 sites, children for whom respondents reported having an immunization card at the time of the interview were more likely to have higher series coverage levels than children for whom a parent-held card was not available. Also, children who were UTD at 3 months of age had significantly higher vaccination-series coverage levels than children who were not UTD at 3 months of age. In addition, the vaccination coverage was lower for children in Detroit, whose parents reported problems accessing the health care system because lack of transportation (46%), compared with those who did not report such problems (65%); however, this difference did not reach significance (χ² = 6.0; P = not significant). In Colorado, the small proportion of children in families without a phone had a lower vaccination coverage level (58%) than those in households with a phone (75%) (χ² = 6.3; P = not significant).