Influenza Vaccination Coverage Among Children Aged 6--23 Months --- United States, 2005--06 Influenza Season

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Children aged <2 years are at increased risk for influenza-related hospitalizations, and those aged <5 years have more influenza-related health-care visits than older children (1). In 2004, the Advisory Committee on Immunization Practices (ACIP) recommended annual influenza vaccination of children aged 6--23 months (2). Two doses, at least 4 weeks apart, were recommended to fully vaccinate children aged <9 years who were receiving influenza vaccination for the first time. To assess influenza vaccination coverage among children aged 6--23 months during the 2005--06 influenza season, data from the 2006 National Immunization Survey (NIS) were analyzed. This report describes the results of that analysis, which indicated that 31.9% of children in this age group received at least 1 dose of influenza vaccine and 20.6% were fully vaccinated according to ACIP recommendations; however, results varied substantially among states. The results underscore the need to continue to monitor influenza vaccination coverage among young children, develop systems to provide childhood influenza vaccination services more efficiently, and increase awareness among health-care providers and caregivers about the effectiveness of influenza vaccination among young children.

NIS, which provides estimates of vaccination coverage among noninstitutionalized children aged 19--35 months (3), is an ongoing, random-digit--dialed household telephone survey that is followed by a mail survey of vaccination providers to obtain vaccination data. The annual NIS interviews are conducted from approximately January--December of each year; the 2006 NIS interviews were conducted from January 5, 2006, through February 6, 2007, and included children born during January 21, 2003--June 18, 2005. In 2006, the survey was conducted in all 50 states, the District of Columbia, and 29 local areas. Lifetime histories of influenza vaccination were obtained from children's vaccination providers.

Analyses included only children who were aged 6--23 months during the entire span of September--December 2005. The doses counted were restricted to those received during September--December 2005 because interviewing began in early January and could not account for children who received vaccination after the interview date. Two measures were used to assess vaccination coverage: 1) receipt of at least 1 dose of influenza vaccine during September--December 2005 and, within that group, 2) full vaccination according to ACIP recommendations. Children were considered fully vaccinated if they had 1) received no dose of influenza vaccine before September 1, 2005, but then received 2 doses from September 1, 2005, through the date of interview or January 31, 2006 (whichever was earlier), or 2) received at least 1 dose of influenza vaccine before September 1, 2005, and received at least 1 dose during September--December 2005. Data were weighted to adjust for households with multiple telephone landlines, noncoverage of households without landline telephones, household and provider nonresponse, and U.S. population estimates.

The household survey response rate was 64.5%, and provider-reported vaccination records were available for 70.4% of children with a completed household interview. Of the 21,044 children aged 19--35 months with provider-reported vaccination data, a total of 13,546 children (unweighted sample size) met the age criteria for this assessment (i.e., were aged 6--23 months during the entire span of September--December 2005). Of these, 31.9% received at least 1 dose of influenza vaccine during September--December 2005, and 20.6% were fully vaccinated (Table).

Substantial variability in influenza vaccination coverage was observed among states. Percentages of children receiving at least 1 dose of influenza vaccine ranged from 8.3% in Mississippi to 52.9% in Connecticut (Table). Compared with the 2004--05 influenza season, the percentage of children receiving at least 1 dose of influenza vaccine in 2005--06 increased in Nevada (by 11.9 percentage points) and Vermont (15.0 percentage points); decreased in Alabama (15.0 percentage points), Massachusetts (14.2 percentage points), Mississippi (14.4 percentage points), and Montana (13.1 percentage points); and did not change significantly in the remaining states. The percentage of fully vaccinated children increased in nine states (Arizona, by 7.7 percentage points; Connecticut, 14.2; Florida, 6.1; Indiana, 10.0; Nevada, 8.4; New Hampshire, 13.9; Oklahoma, 9.0; Vermont, 19.3; and Washington 9.7), decreased in Mississippi (7.9 percentage points), and did not change significantly in the remaining states.

For the 2005--06 season, 76% of the children aged 6--23 months had received no influenza vaccine dose before the start of the season and therefore needed 2 doses according to ACIP recommendations. Among these children, 11% received at least 2 doses, and 15% received only 1 dose. Among children who received at least 1 dose before the 2005--06 season, 49% received at least 1 dose in the 2005--06 season.

Receipt of dose 1 (i.e., the first or only dose) most frequently occurred during October 10, 2005--November 19, 2005 (Figure). Among children recommended to receive 2 doses (i.e., those who had not received a previous dose), the second dose was received most frequently during November 28, 2005--December 17, 2005.

Reported by: TA Santibanez, PhD, JM Santoli, MD, G Mootrey, DO, GL Euler, DrPH, A Fiore, MD, National Center for Immunization and Respiratory Diseases, CDC.

Editorial Note:

The findings in this report indicate that during the 2005--06 influenza season, the second season after ACIP recommended annual influenza vaccination for all children aged 6--23 months, coverage remained low and did not increase substantially from the 2004--05 season. The national estimate for fully vaccinated children increased by approximately 3 percentage points from the 2004--05 influenza season; however, no significant change was observed in the proportion of children who received at least 1 dose of influenza vaccine. During the 2004--05 season, coverage was estimated at 33.4% for children who received at least 1 dose of influenza vaccine, compared with 31.9% for 2005--06. Coverage for fully vaccinated was estimated at 17.8% for 2004--05, compared with 20.6% for 2005--06. For the 2005--06 season, only one in five children aged 6--23 months was fully vaccinated, and only one in 10 children needing 2 doses received both doses. Influenza vaccination coverage varied substantially among states; no state had more than 40% of children fully vaccinated.

Several factors can contribute to low vaccination coverage. First, the ACIP recommendation for annual vaccination in this age group is recent; additional education for providers and caregivers, vaccine promotion, and other measures will be needed to fully implement the recommendation. Second, influenza vaccinations are required annually and must occur during a specified period, which often necessitates provider visits solely for vaccination for certain children aged 6--23 months (4). This time constraint makes measures to reduce missed opportunities for vaccination especially important. Strategies that have improved influenza vaccination coverage in children include standing orders, vaccination-only health-care visits for children, and reminder-recall systems (including those based on immunization information system data) (5,6). Third, vaccine supply might have contributed to the low rate. The manufacturer of one vaccine experienced delays during the 2005--06 season (7). Although this manufacturer produces an influenza vaccine product that is indicated only for use in children aged >4 years, the delay could have affected the use of other products that were licensed for broader age groups.

Children aged <9 years who have not been vaccinated previously are recommended to receive 2 doses during their first year of vaccination, a recommendation that is based on multiple studies demonstrating lower vaccine antibody levels or effectiveness for a single vaccination dose among these children (8--10). In July 2007, ACIP began recommending that children aged <9 years who received only 1 dose in their first year of vaccination receive 2 doses the following year, with single annual doses in subsequent years (1). This recommendation will further increase the number of children in the United States who are recommended to receive 2 doses of influenza vaccine in one season.

One important strategy for increasing the number of children who are fully vaccinated against influenza is to continue vaccinations through December and beyond for children who require 2 doses of vaccine or those who remain unvaccinated after November. Peak influenza activity has occurred in January or later in approximately 80% of influenza seasons since 1976; in approximately 60% of seasons, the peak occurred in February or later (1). Thus, children vaccinated after December will benefit from influenza vaccination during most influenza seasons. Even for children in communities where influenza activity has already begun, vaccination later in the season can be beneficial because more than one strain of influenza often circulates in a given season.

The substantial variability in influenza vaccination coverage by state likely is attributable to several factors, including varying degrees of programmatic and provider implementation during the first few years of a recommendation and variation in parental knowledge and understanding of the recommendation. Continued monitoring of state-level vaccine coverage and trends over time is critical for vaccination program evaluation, particularly after a new vaccine recommendation has been issued.

The findings in this report are subject to at least five limitations. First, doses received after December 31, 2005 (for those who received at least 1 dose) and after January 31, 2006 (for those who received second doses) were excluded; therefore, these results represent early-season vaccination only and underestimate coverage for the entire season. Second, measurement of vaccination coverage was restricted to children aged 6--23 months during the entire influenza vaccination period of September--December; this ensured that children included in the analysis were recommended to receive vaccine during the entire assessment period and that caregivers and providers for all children had an equal amount of time to obtain and provide vaccination. Therefore, the sample of children included in this assessment likely had higher vaccination coverage than the children who were excluded, which might have resulted in an overestimation of vaccination coverage in this group. Third, NIS is a telephone survey; although statistical adjustments compensate for nonresponse and households without landline telephones, bias might remain. Fourth, NIS relies on provider-verified vaccination histories; incomplete records and reporting might affect estimates. Finally, because of sampling uncertainty and wide confidence intervals for many state and local area estimates, these estimates should be interpreted with caution.

This report underscores the need to continue monitoring influenza vaccination coverage among young children, including the extent to which first-time vaccinees aged <9 years receive 2 doses of influenza vaccine. Health-care providers and immunization programs should continue identifying missed opportunities for influenza vaccination during the fall and winter and develop systems that provide influenza vaccination services efficiently. Improving vaccination coverage will require measures to increase awareness among health-care providers and caregivers about the burden of influenza among young children and the effectiveness of influenza vaccination for this age group.

References

1. CDC. Prevention and control of influenza: recommendations of the Advisory Committee on Immunization Practices (ACIP), 2007. MMWR 2007;56(No. RR-6).
2. CDC. Prevention and control of influenza: recommendations of the Advisory Committee on Immunization Practices (ACIP). MMWR 2004;53(No. RR-6).
3. Smith PJ, Hoaglin DC, Battaglia MP, Khare M, Barker LE. Statistical methodology of the National Immunization Survey, 1994--2002. Vital Health Stat 2005;2(138).
4. Szilagyi PG, Iwane MK, Schaffer S, et al. Potential burden of universal influenza vaccination of young children on visits to primary care practices. Pediatrics 2003;112:821--8.
5. Kempe A, Daley MF, Barrow J, et al. Implementation of universal influenza immunization recommendations for healthy young children: results of a randomized, controlled trial with registry-based recall. Pediatrics 2005;115:146--54.
6. Zimmerman RK, Hoberman A, Nowalk MP, et al. Feasibility of influenza immunization for inner-city children aged 6 to 23 months. Am J Prev Med 2004;27:397--403.
7. Bardenheier B, Strikas R, Kempe A, Stokley S. Influenza vaccine supply, 2005--2006: did we come up short? BMC Health Services Research 2007;7:66.
8. Allison MA, Daley MF, Crane LA, et al. Influenza vaccine effectiveness in healthy 6- to 21-month-old children during the 2003--2004 season. J Pediatr 2006;149:755--62.
9. Englund JA, Walter EB, Fairchok MP, Monto AS, Neuzil KM. A comparison of 2 influenza vaccine schedules in 6- to 23-month-old children. Pediatrics 2005;115:1039--47.
10. 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.

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Date last reviewed: 9/19/2007