National, State, and Local Area Vaccination Coverage Among Children Aged 19--35 Months --- United States, 2007

The National Immunization Survey (NIS) provides vaccination coverage estimates among children aged 19--35 months for each of the 50 states and selected urban areas.* This report describes the results of the 2007 NIS, which provided coverage estimates among children born during January 2004--July 2006. Healthy People 2010 established vaccination coverage targets of 90% for each of the vaccines included in the combined 4:3:1:3:3:1^† vaccine series and a target of 80% for the combined series ( 1). Findings from the 2007 NIS indicated that >90% coverage was achieved for most of the routinely recommended vaccines ( 2). The majority of parents were vaccinating their children, with less than 1% of children receiving no vaccines by age 19--35 months. The coverage level for the 4:3:1:3:3:1 series remained steady at 77.4%, compared with 76.9% in 2006. Among states and local areas, substantial variability continued, with estimated vaccination coverage ranging from 63.1% to 91.3%. Coverage remained high across all racial/ethnic groups and was not significantly different among racial/ethnic groups after adjusting for poverty status. However, for some vaccines, coverage remained lower among children living below the poverty level compared with children living at or above the poverty level. Maintaining high vaccination coverage and continued attention to reducing current poverty disparities is needed to limit the spread -preventable diseases and ensure that children are protected.

To collect vaccination information on age-eligible children (i.e., those aged 19--35 months), NIS uses a quarterly, random-digit--dialing sample of telephone numbers for each survey area. When respondents grant permission to contact providers, the telephone interview is followed by a mail survey of the children's vaccination providers to validate immunization information. NIS methodology, including how the responses are weighted to represent the population of children aged 19--35 months, has been described previously ( 3). During 2007, the household response rate ( 4) was 64.9%; a total of 17,017 children with provider-verified vaccination records were included in this report, representing 68.6% of all children with completed household interviews. Statistical analyses were conducted using t-tests. Differences were considered statistically significant at p<0.05. A poverty status variable^§ was added to the logistic regression models to control for racial/ethnic differences among children living at or above the poverty level and children living below the poverty level. This report describes coverage levels for vaccines that have been included in the routine childhood vaccination schedule recommended by the Advisory Committee on Immunization Practices (ACIP) since 2000 or before ( 2).

In 2007, national coverage with the 4:3:1:3:3:1 series was 77.4%; this coverage has been stable since 2004. Coverage with the combined 4:3:1:3:3:1:4 vaccine series (i.e., the 4:3:1:3:3:1 series plus >4 doses of 7-valent pneumococcal conjugate vaccine [PCV7]) is being reported for the first time and was 66.5%. National coverage was >90% for each of the vaccines included in the 4:3:1:3:3:1 series except for >4 doses of DTaP (84.5%); coverage with >3 doses of DTaP was 95.5% (Table 1). Coverage with >1 dose of varicella vaccine (VAR) reached 90% for the first time. VAR coverage among American Indian/Alaska Native (AI/AN)^¶ children increased significantly, from 85.4% in 2006 to 94.9% in 2007. National vaccination coverage estimates for PCV7 continued to increase, from 86.9% in 2006 to 90.0% in 2007 for >3 doses and from 68.4% to 75.3% for >4 doses. Among AI/AN children, coverage with the fourth dose of PCV7 increased significantly, from 62.7% to 80.4%.

Substantial differences were observed in vaccination coverage among states and local areas (Table 2). Estimated coverage for the 4:3:1:3:3:1 series ranged from 91.3% in Maryland to 63.1% in Nevada. Among the 14 local areas included in the 2007 NIS, coverage with the 4:3:1:3:3:1 series ranged from 82.2% in Philadelphia, Pennsylvania, to 69.6% in San Bernardino, California.

Vaccination coverage levels were higher among AI/ANs compared with whites for measles, mumps, and rubella (MMR) vaccine, hepatitis B (HepB) vaccine, and VAR (Table 3). Coverage with the fourth dose of DTaP and the fourth dose of PCV7 among black children was not significantly lower than white children after controlling for poverty status. Vaccination coverage with the fourth dose of DTaP and the fourth dose of PCV7 was lower among children living below the poverty level compared with children living at or above the poverty level, but this difference declined from 6.1% in 2006 to 4.8% in 2007 for >4 doses of DTaP and from 9.4% in 2006 to 3.5% in 2007 for >4 doses of PCV7. Vaccination coverage levels were similar across all racial/ethnic groups for the 4:3:1:3:3:1 series. Coverage differed for this series among children living at or above the poverty level compared with children living below the poverty level, but this difference declined from 4.9% in 2006 to 3.2% in 2007. Coverage between white and black children with the 4:3:1:3:3:1:4 series was not significantly different after controlling for poverty status.

Reported by: N Darling, MPH, M Kolasa, MPH, KG Wooten, MA, Immunization Svcs Div, National Center for Immunization and Respiratory Diseases, CDC.

Editorial Note:

NIS is the only population-based, provider-verified survey to provide national, state, and local area estimates of vaccination coverage among children aged 19--35 months. The results of the 2007 survey indicate that vaccination coverage for vaccines recommended routinely by ACIP since 2000 and before ( 2) reached record high levels. Improvements in vaccination coverage for VAR meant that national coverage estimates for all individual vaccines in the 4:3:1:3:3:1 series were >90%, except coverage with >4 doses of DTaP. Coverage with >4 doses of PCV7 also was <90%. However, 3-dose coverage for both DTaP and PCV7 remained high. Coverage with >4 doses of PCV7 increased significantly to 75.3% in 2007, a substantial increase since PCV7 was first recommended in 2000 ( 5). However, coverage with >4 doses of DTaP has not changed during the past 5 years. Increasing coverage for the fourth dose of DTaP and the fourth dose of PCV7 would improve national coverage for the 4:3:1:3:3:1 series and the 4:3:1:3:3:1:4 series, which will be used to monitor the Healthy People 2010 immunization objectives beginning with 2009 NIS data. The vaccine shortage that ended in September 2004 ( 6) might have reduced coverage with the fourth dose of PCV7 among children in the 2007 NIS cohort (i.e., those born during January 2004--July 2006). Use of effective interventions, such as parent and provider reminder/recall, reducing out-of-pocket costs, increasing access to vaccination, and multicomponent interventions that include education might further improve overall coverage in areas where coverage is low ( 7). In addition, closing the coverage gap between areas with the highest and lowest coverage remains a priority. To achieve this, further collaborative efforts among CDC, state immunization coordinators, immunization programs, and other entities are essential.

Vaccination coverage among AI/AN children for VAR, MMR vaccine, and the fourth dose of PCV7 increased significantly in 2007 compared with 2006; in 2007, coverage levels among AI/AN children were higher for two of these vaccines (VAR and MMR vaccine) compared with white children. Improved exchange of data between the Indian Health Service information system and state immunization information systems and implementation of evidence-based strategies such as reminder/recall at Indian Health Service and tribal facilities, might have contributed to these increases in vaccination coverage (A. Groom, CDC, personal communication, August 2008). However, further monitoring is needed to determine whether these levels will be sustained.

As in 2006, the results of the 2007 NIS indicate that differences in poverty status accounted for the observed differences in coverage between white and black children for the fourth dose of DTaP and fourth dose of PCV7. In 2007, these differences in coverage between children living at or above the poverty level compared with children living below the poverty level were reduced by one percentage point for DTaP and by nearly six percentage points for PCV7. Continued efforts are needed to improve vaccination coverage among children of all racial and ethnic groups living below the poverty level.

The 2007 NIS results confirm that the majority of parents are vaccinating their children, with less than 1% of children receiving no vaccines by age 19--35 months. Although vaccination coverage in this age group remains high, recent outbreaks of measles have occurred in certain communities ( 8). Several factors might explain this apparent paradox. Despite record high coverage with MMR vaccine, nearly 8% of children aged 19--35 months surveyed for the 2007 NIS remained unvaccinated. Measles is highly contagious, and clustering of unimmunized children within geographic areas can increase risk for measles and other vaccine-preventable disease transmission. Clusters of unimmunized children might not be detected by NIS methods and might not be visible in national and state rates. Furthermore, any changes in vaccination behaviors among parents of children born after July 2006 would not have been detected by the 2007 survey. Increased attention to parental concerns about vaccine safety has become apparent in recent years ( 9). The 2008 NIS is collecting information on parental concerns about vaccine safety to better assess parental attitudes and beliefs about vaccines. In addition, CDC and its partners are developing new educational materials that can assist parents in making fully informed decisions about immunizing their children.**

The findings in this report are subject to at least three limitations. First, NIS is a telephone survey, and statistical adjustments might not compensate fully for nonresponse and households without landline telephones. Second, underestimates of vaccination coverage might have resulted from the exclusive use of provider-verified vaccination histories because completeness of these records is unknown. Finally, although national coverage estimates are precise, annual estimates and trends for state and local areas should be interpreted with caution because of smaller sample sizes and wider confidence intervals.

Achieving and maintaining high vaccination coverage levels is important to further reduce the burden of vaccine-preventable diseases and prevent a resurgence of measles and other diseases that have been eliminated in the United States ( 10). Although vaccination coverage estimates were at record highs and above the Healthy People 2010 target for most of the routinely recommended vaccines in 2007, ongoing efforts through partnerships among national, state, local, private, and public entities are needed to sustain these levels and ensure that vaccination programs in the United States remain strong.

Acknowledgments

The findings in this report are based, in part, on contributions by PJ Smith, PhD, Immunization Svcs Div, and BP Bell, MD, Office of the Director, National Center for Immunization and Respiratory Diseases, CDC.

References

1. US Department of Health and Human Services. Healthy people 2010 (conference ed, in 2 vols). Washington, DC: US Department of Health and Human Services; 2000. Available at http://www.healthypeople.gov/document/html/objectives/14-24.htm.
2. CDC. Recommendations and guidelines: 2008 child & adolescent immunization schedules for persons aged 0--6 years, 7--18 years, and catch-up schedule. Atlanta, GA: US Department of Health and Human Services, CDC; 2008. Available at http://www.cdc.gov/vaccines/recs/schedules/child-schedule.htm.
3. CDC. Statistical methodology of the National Immunization Survey, 1994--2002. Vital Health Stat 2005;2(138). Available at http://www.cdc.gov/nchs/data/series/sr_02/sr02_138.pdf.
4. Ezzati-Rice TM, Frankel MR, Hoaglin DC, Loft JD, Coronado VG, Wright RA. An alternative measure of response rate in random-digit-dialing surveys that screen for eligible subpopulations. J Econ Soc Meas 2000;26:99--109.
5. CDC. Preventing pneumococcal disease among infants and young children: recommendations of the Advisory Committee on Immunization Practices (ACIP). MMWR 2000;49(No. RR-9).
6. CDC. Pneumococcal conjugate vaccine shortage resolved. MMWR 2004; 53:851--2.
7. Briss PA, Rodewald LE, Hinman AR, et al. Reviews of evidence regarding interventions to improve vaccination coverage in children, adolescents, and adults. The Task Force on Community Preventive Services. Am J Prev Med 2000;18:97--140.
8. CDC. Update: measles---United States, January--July 2008. MMWR 2008;57:893--6.
9. Cooper LZ, Larsen HJ, Katz SL. Protecting public trust in immunization. Pediatrics 2008;122:149--53.
10. CDC. Measles---United States, January 1--April 25, 2008. MMWR 2008; 57:494--8.

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Date last reviewed: 9/4/2008