Vaccination Coverage Among Children Aged 19–35 Months — United States, 2017

The Advisory Committee on Immunization Practices (ACIP) recommends routine vaccination by age 24 months against 14 potentially serious illnesses (1). CDC used data from the 2017 National Immunization Survey-Child (NIS-Child) to assess vaccination coverage at national, state, territorial, and selected local levels among children aged 19-35 months in the United States. Coverage remained high and stable overall, exceeding 90% for ≥3 doses of poliovirus vaccine, ≥1 dose of measles, mumps, and rubella vaccine (MMR), ≥3 doses of hepatitis B vaccine (HepB), and ≥1 dose of varicella vaccine. Although the proportion of children who received no vaccine doses by age 24 months was low, this proportion increased gradually from 0.9% for children born in 2011 to 1.3% for children born in 2015. Coverage was lower for most vaccines among uninsured children and those insured by Medicaid, compared with those having private health insurance, and for children living outside of metropolitan statistical areas* (MSAs), compared with those living in MSA principal cities. These disparities could be reduced with greater awareness and use of the Vaccines for Children† (VFC) program, eliminating missed opportunities to vaccinate children during visits to health care providers, and minimizing interruptions in health insurance coverage.

providers are mailed an immunization history questionnaire to record dates and types of vaccines administered; data from responding providers are combined to create a synthesized vaccination history for each child. NIS-Child methods, including weighting procedures, have been described. ¶ In 2017, the overall response rate** to the telephone interview portion of the survey was 26.1%. Adequate provider-reported vaccination data † † were available for 53.9% of children with a completed household interview, resulting in a sample size of 15,333 children. T-tests on weighted data were used to evaluate differences in coverage estimates by sociodemographic characteristics; differences were considered statistically significant for p-values <0.05. CDC assessed changes in survey accuracy, estimated components of difference between the 2016 and 2017 NIS-Child estimates, and estimated linear trends in vaccination coverage by month and year of birth using weighted linear regression. § § No evidence for change in survey accuracy from 2016 to 2017 was detected (2).

Vaccination Coverage
Coverage was >90% for vaccination with ≥3 doses of poliovirus vaccine (92.7%), ≥1 dose of MMR (91.5%), ≥3 doses of HepB (91.4%), and ≥1 dose of varicella vaccine (91.0%) ( Table 1). Children were least likely to be up-to-date with ≥2 doses of hepatitis A vaccine (HepA) (59.7%), the combined 7-vaccine series ¶ ¶ (70.4%), and rotavirus vaccination (73.2%). Coverage with HepB birth dose was also low (73.6%). ¶ Details regarding the statistical methodology of NIS-Child are available in the NIS-Child Data User's Guide 2016. https://www.cdc.gov/vaccines/imzmanagers/nis/datasets.html. ** The Council of American Survey Research Organizations (CASRO) household response rate is calculated as the product of the resolution rate (percentage of the total telephone numbers called that were classified as nonworking, nonresidential, or residential), screening completion rate (percentage of known households that were successfully screened for the presence of age-eligible children), and the interview completion rate (percentage of households with one or more age-eligible children that completed the household survey

Vaccination Coverage by Selected Characteristics
Coverage was lower (range = 2.6-6.9 percentage points) for children living in non-MSAs than among those living in MSA principal cities for most vaccines (Table 2). Children living in non-MSAs had a higher prevalence of having received no vaccinations (1.9%) compared with children in MSA principal cities (1.0%).
Coverage among children insured by Medicaid was lower (2.5-15.0 percentage points, depending on vaccine) than that among those with private insurance for all vaccines assessed except the HepB birth dose ( Table 2). The same pattern was observed among uninsured children: coverage was substantially lower (14.7-30.3 percentage points) than that among those privately insured. Prevalence of uninsured children in the 2017 NIS-Child was 2.8%. This lower vaccination coverage among the uninsured, Medicaid-insured, and those living outside of MSAs was especially evident for diphtheria and tetanus toxoids and acellular pertussis vaccine (DTaP), the full series of Haemophilius influenzae type b conjugate vaccine (Hib), and pneumococcal conjugate vaccine (PCV), that require a booster dose in the second year of life. In addition, the proportion of uninsured children who had received no vaccinations (7.1%) was higher than that among those with private insurance (0.8%). The proportion of unvaccinated children was similar among children insured by Medicaid and those with private insurance. Among unvaccinated children in the 2017 NIS-Child, 17.2% were uninsured. Differences in vaccination coverage by race/ethnicity and poverty status in 2017 were similar to those observed in previous years (Supplementary Table 1

Trends in Vaccination Coverage
Coverage by month and year of birth remained stable during January 2012-January 2016 for most vaccines (Figure) (2). Coverage by age 2 years over 12 consecutive birth months declined by 0.5 percentage points for ≥3 HepB doses and increased by 1.1 percentage points for ≥2 HepA doses (2). Coverage with ≥2 HepA doses was higher by age 35 months than by age 24 months (e.g., 75.3% versus 39.6% for children born January 2012) (2).

Discussion
Overall vaccination coverage among young children remained high and stable in the United States in 2017. However, the findings from this survey highlight several opportunities for improvement. Coverage was lower for most vaccines among uninsured and Medicaid-insured children and among children living outside of MSAs. These disparities were larger for vaccines that require a booster dose in the second year of life (e.g., DTaP, Hib, and PCV). Although the number of children who have received no vaccinations by age 24 months has been gradually increasing, most children are still routinely vaccinated. Continued evaluation of prevalence and reasons for nonvaccination is needed, as are improvements in access to and delivery of age-appropriate vaccinations to all children. CDC continues to examine barriers to early childhood vaccination, including assessing obstacles to and parents' experiences with accessing vaccination services.
Vaccination coverage differences by insurance status are concerning, given that children insured by Medicaid and uninsured children are eligible for the VFC program, which was designed to remove financial barriers by providing free vaccines to program participants. However, other issues, such as unfamiliarity with the VFC program and how to access it, transportation, child care, and convenience of clinic hours might also need to be addressed if the goals of this important element of the immunization safety net are to be fully realized. Lack of geographic proximity to vaccination providers, including those who participate in the VFC program, can be a barrier to vaccination. The shortage of health care providers, especially pediatricians, might partially explain the lower coverage among children living in rural areas (4). Vaccination coverage could be increased and sociodemographic and geographic disparities reduced with increased administration of all recommended vaccines during provider visits. A study of potentially achievable coverage estimated that 90% coverage would have been attained many years ago for the recommended number of doses of DTaP, PCV, and Hib for children aged 19-35 months if missed opportunities for administration of the final doses of these vaccines had been eliminated (5). Reducing missed opportunities would promote timely receipt of all recommended vaccine doses and decrease the amount of time that children remain vulnerable to vaccinepreventable diseases.

FIGURE. Estimated linear trend in coverage with selected vaccines* by age 24 months, † by month and year of birth § -National Immunization
The percentage of children who have received no vaccines has increased, reaching 1.3% for children born in 2015, compared with 0.3% among those 19-35 months when surveyed in 2001 (6). Some children might be unvaccinated because of choices made by parents, whereas for others, lack of access to health care or health insurance might be factors. Unvaccinated children in the 2017 NIS-Child were disproportionately uninsured: 17.2% of unvaccinated children were uninsured, compared with 2.8% of all children. Evidence-informed strategies addressing parents' decisions about vaccinating their children could focus on both programs and individual patients, such as vaccine delivery through school programs, strong recommendations by providers to parents to vaccinate their children, and reinforcement of the importance of community protection through vaccination (7).
Variation in coverage by health insurance and MSA status and the increasing percentage of unvaccinated children raise concerns about possible pockets of susceptibility in which children are not as well protected as national coverage estimates might indicate. Measles was declared eliminated from the United States in 2000, yet outbreaks caused by imported cases continue to occur each year; 118 measles cases were reported in 2017 (https://www.cdc.gov/measles/cases-outbreaks.html) (8). The continued occurrence of measles outbreaks in the United States underscores the need to ensure high MMR coverage among all young children.
The findings in this report are subject to at least two limitations. First, low response rates and lack of access to phoneless households could result in selection bias, which might persist even with application of survey weights designed to minimize such bias. Second, vaccination histories might be incomplete if not all providers were identified or some of those identified chose not to participate. Bias in vaccination coverage estimates has been evaluated in a sensitivity analysis accounting for these potential errors, with results indicating underestimation of actual vaccination coverage by 4 to 5 percentage points (9).
Vaccination coverage among young children could be improved through higher participation by both children and providers in the Vaccines for Children program. Consistent access to health insurance is another important element of the immunization safety net. Barriers to participation in the VFC program should be identified and eliminated so that all eligible children have the opportunity to access recommended vaccines. A number of evidence-based strategies have also been described that could enhance these efforts to increase vaccination coverage, such as notifying parents when children are due for a vaccination, establishing standing orders or policies that allow nonphysician personnel to administer vaccines, and enhancing computerized immunization information systems for tracking vaccinations (https://www.thecommunityguide. org/topic/vaccination) (10). Continued vaccination coverage assessment using the NIS-Child can guide efforts to improve vaccination coverage and protect children from vaccine-preventable diseases and better understand the low but increasing prevalence of nonvaccination.

Summary
What is already known about this topic?
The Advisory Committee on Immunization Practices recommends routine vaccination by age 24 months against 14 potentially serious illnesses.
What is added by this report?
In 2017, coverage with most recommended vaccines among children aged 19-35 months remained stable and high but was lower in more rural areas and among uninsured or Medicaidinsured children. A small but increasing proportion of children received no vaccines by age 24 months.
What are the implications for public health practice?
Collaboration with state immunization programs, eliminating missed immunization opportunities, and minimizing interruptions in insurance coverage are important to understand and address coverage disparities among children eligible for the Vaccines for Children program and those in rural areas.