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Childhood Influenza Vaccination Coverage --- United States, 2003--04 Influenza Season

Children aged <2 years are at increased risk for influenza-related hospitalizations (1,2). Beginning in 2002, the Advisory Committee on Immunization Practices (ACIP) encouraged that, when feasible, children aged 6--23 months and household contacts and out-of-home caregivers for children aged <2 years receive influenza vaccinations each year (1). Beginning with the 2004--05 influenza season, ACIP strengthened the encouragement to a recommendation (3). Other children recommended to receive influenza vaccination include children aged 6 months--18 years who have certain high-risk medical conditions, are on chronic aspirin therapy, or who are household contacts of persons at high risk for influenza complications (3). This report provides an assessment of childhood influenza vaccination coverage for the 2003--04 influenza season, the second year of the ACIP encouragement for influenza vaccination of children aged 6--23 months. The findings demonstrate that vaccination coverage increased from the previous influenza season but remained low, with substantial variability among states and urban areas.

This report is based on data from the 2004 National Immunization Survey (NIS), which provides estimates of vaccination coverage among noninstitutionalized children aged 19--35 months at the time of household interview. NIS is an ongoing, random-digit--dialed telephone survey of households, followed by a mail survey to all of the children's vaccination providers to obtain vaccination data. For the 2004 reporting period, NIS included children born during January 2001--July 2003. The survey is conducted in all 50 states and 28 selected urban areas (4). Entire influenza vaccination histories are obtained from children's immunization providers.

Two measures of childhood influenza vaccination coverage are reported: 1) receipt of 1 or more doses of influenza vaccine during September--December 2003 and 2) full vaccination (based on ACIP recommendations for 2 doses of influenza vaccine for previously unvaccinated children aged <9 years and 1 dose for previously vaccinated children aged <9 years) (3). Children were considered fully vaccinated if they had 1) received no doses of influenza vaccine before September 1, 2003, but then received 2 doses from September 1 through the earlier of the date of interview or January 31, 2004, or 2) received 1 or more doses of influenza vaccine before September 1 and then received 1 or more doses during September--December 2003. Analyses for both measures included only those children who were aged 6--23 months during the entire span of September--December 2003. Data were weighted to adjust for households having multiple telephone lines, unit nonresponse, nonassessment of households without telephones, and known population control estimates.

In the 2004 NIS, the overall response rate for eligible households was 67.4%, and 13,881 children (unweighted sample size) met the age criteria for this assessment. Of these, 17.5% (95% confidence interval [CI] = 16.5--18.7) received 1 or more doses of influenza vaccine, and 8.4% (CI = 7.7--9.3) were fully vaccinated (Table). In comparison, coverage estimates for the 2002--03 season were 7.4% (CI = 6.7--8.1) for 1 or more doses of influenza vaccine and 4.4% (CI = 3.9--4.9) for fully vaccinated (5). Substantial variability in influenza vaccination coverage was observed among states and selected urban areas (Table). Percentages of children receiving 1 or more doses of influenza vaccine ranged from 5.7% (CI = 2.8--11.2) in Miami-Dade County, Florida, to 47.6 (CI = 39.7--55.6) in Rhode Island.

Reported by: TA Santibanez, PhD, JA Singleton, MS, J Santoli, MD, G Euler, DrPH, CB Bridges, MD, National Immunization Program, CDC.

Editorial Note:

The findings in this report indicate that during the second season in which the ACIP encouraged childhood influenza vaccination, coverage increased from the previous year but remained low and varied substantially among states. This increase in coverage from the 2002--03 to the 2003--04 influenza season might reflect increased provider implementation and parent awareness of the ACIP encouragement and the early peak in disease that was reported in the media.

The 2003--04 influenza season was unusual in several respects. Influenza activity began earlier than most seasons, with peak activity occurring in December (6). A total of 153 influenza-associated deaths among U.S. children were reported to CDC (7). The publicity surrounding these deaths and the early onset of the influenza season led to a considerable increase in demand for influenza vaccine, exceeding demand in previous years. In addition, a suboptimal match between the vaccine strain and one of the widely circulating viruses was noted as the season progressed; however, studies have demonstrated some degree of vaccine effectiveness despite the mismatch (6,8).

Beginning with the 2003--04 influenza season, ACIP expanded the Vaccines for Children (VFC) program to include annual influenza vaccination for all VFC-eligible children aged 6--23 months and for VFC-eligible household contacts of children aged <2 years.* The VFC program enables providers to administer free influenza vaccine to children who are uninsured or Medicaid insured, American Indian or Alaska Native children, and children whose insurance does not pay for vaccine who are vaccinated at a federally qualified health center. By addressing economic barriers among vulnerable children, this VFC expansion also might have contributed to increased vaccination coverage during the 2003--04 season.

The substantial variability in influenza vaccination coverage by state might be attributed to several factors. First, in 2003--04, influenza vaccination was not yet fully recommended by ACIP but rather encouraged when feasible, which might have resulted in varying degrees of programmatic and provider implementation during the second year of the ACIP encouragement. Second, parental awareness, attitudes, and access to influenza vaccination services for their children also were likely to have varied. Third, the early peak of influenza activity and perception of the severity of local epidemics might have contributed to variability in coverage. For example, influenza vaccination coverage for Colorado, a state in which much publicity about influenza-related deaths of children was generated, was higher than national coverage. Further study is needed to understand the considerable variability in vaccination coverage among states.

The findings in this report reveal a low rate of full vaccination, which increased only slightly from that of the previous season. A recent study highlights the importance of 2 doses of influenza vaccine for previously unvaccinated children aged <9 years. In a study evaluating the vaccine effectiveness of 1 and 2 doses of the 2003--04 influenza vaccine in preventing medically attended influenza-like illness (ILI) or pneumonia and influenza (P&I) among children aged 6--23 months, vaccine effectiveness was found to be 25% and 49%, respectively, for fully vaccinated children. No statistically significant reduction in ILI or P&I was found for partially vaccinated children aged 6--23 months (8). The maximum benefit from influenza vaccination is obtained when all recommended doses are administered before the onset of influenza activity in the community, which might be particularly difficult to achieve among children requiring 2 doses.

Two decisions made during this analysis might have influenced, in opposite directions, the vaccination-coverage estimates. First, analysis was limited to those vaccinations administered during September--December for the measure of receipt of 1 or more doses of influenza vaccine and during September 1, 2003--January 31, 2004 (or date of interview if the interview occurred before January 31), for the fully vaccinated measure, although some vaccines might have been administered after these months and would not have been counted. This approach possibly reduced both measures of influenza vaccination coverage described in this report, particularly the estimate of fully vaccinated children, because difficulty in scheduling and returning for the second dose of influenza vaccine might have delayed receipt of the second dose until later in the influenza season. Second, measurement of vaccination coverage was restricted to children aged 6--23 months during the entire influenza vaccination period of September--December. Children in this age group were eligible for vaccination under the ACIP encouragement for the entire period of assessment, so their caregivers and providers all had an equal amount of time to ensure vaccination for all of the children in the sample. Therefore, the sample of children included in this assessment likely had higher vaccination coverage than children excluded who were aged 6--23 months during only a portion of the 4-month vaccination interval (i.e., those aged 21--23 months as of September 1, 2003, or who reached the vaccine-eligible age of 6 months after September 1, 2003).

The findings in this report are subject to at least three limitations. First, NIS is a telephone survey; although statistical adjustments compensate for nonresponse and households without telephones, some bias might remain. Second, NIS relies on provider-verified vaccination histories; incomplete records and reporting might result in biased estimates. Finally, because of sampling uncertainty and wide confidence intervals for many state and urban area estimates from NIS, these estimates should be interpreted with caution.

Influenza-vaccination coverage estimates increased during the second year of the ACIP encouragement but remained low. For the 2004--05 influenza season, ACIP replaced the encouragement with a recommendation (3). This change to a full recommendation appears to have resulted in increased vaccination coverage. February 2005 data from the Behavioral Risk Factor Surveillance System (BRFSS) indicated 48.4% coverage with at least 1 dose of influenza vaccine for children aged 6--23 months; this coverage also reflected effective prioritization of vaccine delivery despite an overall vaccine shortage during that influenza season. Any comparison between BRFSS and NIS data, however, must be made cautiously because of the differing birth cohorts and vaccination periods measured and because BRFSS is based on parental report, whereas NIS is based on provider-reported data (9). Analysis of immunization registry and enrollment data from one large health maintenance organization indicated that influenza vaccination coverage for the 2004--05 influenza season was 57.4% among children aged 6--23 months (10).

This report underscores the need to fully implement the new recommendation for children aged 6--23 months and household contacts of children aged <2 years to reduce the number of preventable influenza-related hospitalizations among young children (2). Complete recommendations for the 2005--06 influenza season have been published (3), and updates on the influenza season and vaccine supply are available at http://www.cdc.gov/flu.

References

  1. CDC. Prevention and control of influenza: recommendations of the Advisory Committee on Immunization Practices (ACIP). MMWR 2002;51(No. RR-3).
  2. Izurieta HS, Thompson WW, Kramarz P, et al. Influenza and the rates of hospitalization for respiratory disease among infants and young children. N Engl J Med 2000;342:232--9.
  3. CDC. Prevention and control of influenza: recommendations of the Advisory Committee on Immunization Practices (ACIP). MMWR 2005;54(No. RR-8).
  4. Smith PJ, Hoaglin DC, Battaglia MP, Barker LE, Khare M. Statistical methodology of the National Immunization Survey: 1994--2002. Hyattsville, MD: US Department of Health and Human Services, National Center for Health Statistics; 2005.
  5. CDC. Childhood influenza-vaccination coverage---United States, 2002--03 influenza season. MMWR 2004;53:863--6.
  6. CDC. Update: influenza activity---United States and worldwide, 2003--04 season, and composition of the 2004--05 influenza vaccine. MMWR 2004;53:547--52.
  7. Bhat N, Wright JG, Broder KR, et al. Influenza-associated deaths among children in the United States, 2003--2004. N Engl J Med 2005;353:2559--67.
  8. Ritzwoller DP, Bridges CB, Shetterly S, Yamasaki K, Kolczak M, France EK. Effectiveness of the 2003--2004 influenza vaccine among children 6 months to 8 years of age, with 1 vs 2 doses. Pediatrics 2005;116:153--9.
  9. CDC. Estimated influenza vaccination coverage among adults and children---United States, September 1, 2004--January 31, 2005. MMWR 2005;54:304--7.
  10. CDC. Rapid assessment of influenza vaccination coverage among HMO members---northern California, influenza seasons, 2001--02 through 2004--05. MMWR 2005;54:676--8.

* After ACIP votes on a vaccine recommendation, the committee votes a second time to determine if the vaccine recommendation will be covered under the VFC program.

For this study, vaccine effectiveness (%) was defined as (1-hazard ratio) x 100, where the hazard ratio compared the rate of influenza-like illness or pneumonia and influenza outcomes in vaccinated children to the rate in unvaccinated children.

Table

Table 1
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