National, State, and Selected Local Area Vaccination Coverage Among Children Aged 19–35 Months — United States, 2014

Holly A. Hill, MD, PhD1; Laurie D. Elam-Evans, PhD1; David Yankey, MS, MPH1; James A. Singleton, PhD1; Maureen Kolasa, MPH1

The reduction in morbidity and mortality associated with vaccine-preventable diseases in the United States has been described as one of the 10 greatest public health achievements of the first decade of the 21st century (1). A recent analysis concluded that routine childhood vaccination will prevent 322 million cases of disease and about 732,000 early deaths among children born during 1994–2013, for a net societal cost savings of $1.38 trillion (2). The National Immunization Survey (NIS) has monitored vaccination coverage among U.S. children aged 19–35 months since 1994 (3). This report presents national, regional, state, and selected local area vaccination coverage estimates for children born from January 2011 through May 2013, based on data from the 2014 NIS. For most vaccinations, there was no significant change in coverage between 2013 and 2014. The exception was hepatitis A vaccine (HepA), for which increases were observed in coverage with both ≥1 and ≥2 doses. As in previous years, <1% of children received no vaccinations. National coverage estimates indicate that the Healthy People 2020 target* of 90% was met for ≥3 doses of poliovirus vaccine (93.3%), ≥1 dose of measles, mumps, and rubella vaccine (MMR) (91.5%), ≥3 doses of hepatitis B vaccine (HepB) (91.6%), and ≥1 dose of varicella vaccine (91.0%). Coverage was below target for ≥4 doses of diphtheria, tetanus, and acellular pertussis vaccine (DTaP), the full series of Haemophilus influenzae type b (Hib) vaccine, hepatitis B (HepB) birth dose,† ≥4 doses pneumococcal conjugate vaccine (PCV), ≥2 doses of HepA, the full series of rotavirus vaccine, and the combined vaccine series.§ Examination of coverage by child's race/ethnicity revealed lower estimated coverage among non-Hispanic black children compared with non-Hispanic white children for several vaccinations, including DTaP, the full series of Hib, PCV, rotavirus vaccine, and the combined series. Children from households classified as below the federal poverty level had lower estimated coverage for almost all of the vaccinations assessed, compared with children living at or above the poverty level. Significant variation in coverage by state¶ was observed for several vaccinations, including HepB birth dose, HepA, and rotavirus. High vaccination coverage must be maintained across geographic and sociodemographic groups if progress in reducing the impact of vaccine-preventable diseases is to be sustained.

NIS employs a dual-frame landline and cell phone** random-digit–dialing (RDD) design to identify households with children aged 19–35 months in the 50 states, the District of Columbia, selected local areas, and, in 2014, Puerto Rico.†† Once households with age-eligible children are identified, a parent or guardian is interviewed and asked for consent to contact the child's vaccination provider. If consent is obtained, the providers receive a mail survey requesting the child's vaccination history, including dates of receipt of specific vaccine doses. This information is used to calculate comprehensive estimates of coverage (i.e., the percentage of children who are up-to-date as recommended by the Advisory Committee on Immunization Practices [ACIP]) (4). Data are weighted to be representative of the population of U.S. children aged 19–35 months and are adjusted for multiple telephone lines, mixed telephone use (i.e., landline and cellular), household nonresponse, and the exclusion of households without telephones. Details regarding NIS methodology, including methods for synthesizing provider-reported immunization histories and weighting, have been described previously.§§ National estimates for the 2014 NIS are based upon 14,893 children with adequate provider data.¶¶ The national Council of American Survey Research Organizations (CASRO) response rates were 62.6% for the landline and 33.5% for the cell phone frame.*** Coverage estimates for Hib††† and rotavirus§§§ take into account the type of vaccine used because the number of doses required differs, depending on the manufacturer. Logistic regression was used to assess differences among racial/ethnic groups, adjusting for poverty status. Statistical comparisons were made using t-tests on weighted data, taking into account the complex survey design. Statistical significance was defined as a p-value of <0.05.

National Vaccination Coverage

Among U.S. children aged 19–35 months, changes in coverage between 2013 and 2014 were small and not statistically significant (Table 1). The only exception was an estimated 2.0 percentage point increase for ≥1 HepA dose and 2.8 percentage point increase for ≥2 HepA doses. Coverage with the combined series and the vaccines that comprise the series were similar to those in 2013.

Vaccination Coverage by Selected Demographic Characteristics

Non-Hispanic black¶¶¶ children had lower coverage levels than non-Hispanic white children for DTaP, the full series of Hib, PCV, rotavirus, and the combined series (Table 2). Except for rotavirus vaccination, coverage differences between non-Hispanic black and non-Hispanic white children were no longer statistically significant after adjustment for poverty status. DTaP coverage was lower for multiracial children compared to their non-Hispanic white counterparts. In some instances, coverage among other racial/ethnic groups exceeded levels among non-Hispanic whites. Poliovirus vaccination and HepB birth dose coverage were similar among racial/ethnic groups. With few exceptions, vaccination coverage was significantly lower for children living below the federal poverty level**** compared with those classified as at or above the poverty level (Table 2). As in 2013, lower coverage for children living below the poverty level was observed for DTaP, poliovirus vaccine, the primary and full series of Hib, PCV, rotavirus vaccine, and the combined series. In contrast to 2013, coverage was also lower for MMR and ≥2 HepA doses.

Vaccination Coverage by Geographic Area

Variation in vaccination coverage by geographic area was also evident (Table 3). For MMR, the highest state-level coverage was observed in Maine (97.2%), where coverage increased by 6.2 percentage points from 2013 levels. The lowest estimated MMR coverage was 84.1% (Arizona). Coverage with ≥4 doses of DTaP vaccine ranged from 93.1% (Maine) to 72.8% (Wyoming). HepB birth dose coverage ranged from 88.4% (North Dakota) to 48.4% (Vermont). Coverage with ≥2 HepA doses ranged from 69.0% (Connecticut) to 32.7% (Wyoming), for rotavirus vaccination from 88.8% (Rhode Island) to 59.2% (Michigan), and for completion of the combined series from 84.7% (Maine) to 63.4% (West Virginia). Increases in rotavirus vaccination coverage compared with 2013 levels were observed in Alabama, North Carolina, Arkansas, New Mexico, Oklahoma, and Wyoming.

Discussion

Based on results from the 2014 NIS, national coverage for ACIP-recommended vaccines among U.S. children aged 19–35 months remained largely stable compared with 2013. Healthy People 2020 coverage targets were met only for poliovirus, MMR, HepB, and varicella vaccination. Coverage with the combined series remained below target levels; this appears to be largely because of suboptimal coverage with DTaP, the full series of Hib, and PCV. Coverage with the penultimate dose of each of these vaccines exceeded 90%, indicating that efforts focused on ensuring receipt of the final dose are important. The final dose for these vaccines is often scheduled during the second year of life, when routine visits to health care providers occur less frequently, and thus, opportunities to vaccinate are fewer. Ensuring that providers are fully aware of appropriate catch-up vaccination practices could be an important strategy for achieving full coverage with these vaccines, as could encouraging increased use of combination vaccines.

Lower coverage for non-Hispanic black children relative to their non-Hispanic white counterparts appears to be largely explained by poverty status, except in the case of rotavirus vaccination. Reasons for the persistent disparity are unclear and merit further investigation. Disparities in vaccination coverage by poverty status were frequent and often sizeable. Children living below the poverty level had rotavirus coverage that was 14.1 percentage points lower than that of children at or above the poverty level. ACIP recommends that rotavirus vaccination be initiated at age 2 months (maximum age at first dose is 15 weeks) and completed by age 6 months (maximum age at final dose is 8 months) (4). Therefore, the window for administering rotavirus vaccine is narrow and could be missed because of transportation challenges, difficulty obtaining time off from work, or other logistical issues, situations that might occur more frequently in poorer families. Disparities by poverty status were also observed for PCV, the full series of Hib, and DTaP. Although these vaccines have longer catch-up periods relative to rotavirus vaccine, receiving 3 or 4 doses to be up-to-date appears to pose a greater challenge to families living below the poverty level. The Vaccines for Children program (2) was implemented to eliminate financial barriers and appears to have been successful in substantially reducing disparities by race/ethnicity.

Additional interventions targeted at families living below the poverty level are needed to further reduce, and ultimately eliminate, these disparities. Evidence-based strategies can be adopted by providers and public health systems to maintain overall immunization coverage and improve coverage in vulnerable subpopulations. Actively adopting such strategies is key to improving coverage among children living in poverty and for increasing coverage with vaccine booster doses at and after age 12 months. The Guide to Community Preventive Services (5) recommends strategies to enhance access to vaccination services, including reduced out-of-pocket costs, home visits, and vaccination programs in child care centers, schools, and Special Supplemental Nutrition Program for Women, Infants, and Children (WIC) settings. The guide also recommends interventions aimed at increasing community demand for vaccination, such as client reminder and recall, client or family incentives, and vaccination requirements for child care and school attendance. Proven provider or system-based strategies include use of immunization information systems (6), provider assessment and feedback, provider education, provider reminders, and standing orders for vaccination.

Geographic variation in coverage can result in pockets of susceptibility even for vaccinations associated with high national coverage, such as MMR. During the first 3 months of 2015, a total of 159 measles cases from 18 states and the District of Columbia were reported to CDC (7). Four outbreaks were identified, and >80% of cases occurred among unvaccinated persons or persons with unknown vaccination status. The largest outbreak was associated with Disney theme parks in California, accounting for 111 (70%) of the cases reported before the beginning of April 2015 (8). Although the United States reported elimination of indigenous measles transmission in 2000,†††† about 20 million measles cases still occur worldwide. Importation of measles from other countries remains a risk for unvaccinated U.S. residents, emphasizing the need for continued vigilance and maintenance of high vaccination coverage. Increasing DTaP coverage should also be an area of enhanced effort. A total of 28,660 pertussis cases were reported to CDC during 2014, a slight increase over the final case count of 28,639 reported in 2013 (9). Because vaccine-induced immunity to pertussis is known to wane over time, it is important that children receive all recommended DTaP vaccinations and boosters.

The findings in this report are subject to at least two limitations. First, household interview response rates were only 62.6% for the landline sample and 30.5% for the cell telephone sample. Among all eligible children with completed household interviews, 59.8% had adequate provider-confirmed vaccination data. This creates the possibility of selection bias, even after use of sample weights to adjust for nonresponse, exclusion of households without telephones, and overlapping samples of mixed (landline and cell) telephone users. Although results are weighted to be representative of the population of children aged 19–35 months, such weighting does not guarantee there will be no bias. Analyses of total survey error for the NIS for 2010,§§§§ 2011, and 2012 (through June) indicated bias in estimates attributable to incomplete sample frame and selection bias was low, on the order of less than two percentage points (10). Second, NIS estimates of ≥2 HepA doses likely underestimate the proportion of children who ultimately reach complete vaccination levels. ACIP recommendations are that children receive a dose of HepA at age 12–23 months, with a second dose 6–18 months later (4). Therefore, a child could be on schedule but not receive the second dose until age 41 months; this second dose would not be captured by NIS, which does not assess coverage for children aged >35 months.

For approximately 20 years, NIS has monitored vaccination coverage levels among young children in the United States. The 2014 data indicate that coverage remains consistently high for most vaccinations, although variation by poverty status and geographic area was observed. For some vaccines and population subgroups, improvement in coverage is necessary to achieve optimal protection. For all vaccines, maintaining high coverage is critical to sustain progress in reducing the impact of vaccine-preventable diseases.

1Immunization Services Division, National Center for Immunization and Respiratory Diseases, CDC.

Corresponding author: Holly A. Hill, hah4@cdc.gov, 404-639-8044.

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