Influenza Vaccination Coverage among Children Aged 6–23 Months — United States, 2008–09 Influenza Season

Infants and children aged <5 years, but especially younger than 2 years old, are at increased risk for severe complications from influenza, including emergency department visits and hospitalization.(1) The Advisory Committee on Immunization Practices (ACIP) first encouraged seasonal influenza vaccination of all children aged 6-23 months in 2002 and then, in 2004, recommended vaccination for this age group.(1,2) The recommendation for vaccination was expanded in 2006 to include all children 6-59 months. Using data from the 2009 National Immunization Survey (NIS), this report examines the proportion of children aged 6-23 months with provider-reported influenza vaccination for the 2008-09 influenza season. National and state-specific estimates for receipt of ≥1 dose and for children fully vaccinated are shown.

Key findings

• 41.5% (95% confidence interval [CI] 39.9-43.2) of children aged 6-23 months in the United States received 1 or more doses of influenza vaccine during September through December 2008.
• 24.7% (95% CI 23.4-26.1; range among areas, 8.7% to 45.9%, Table) were fully vaccinated against influenza.
• Substantial variability in influenza vaccination coverage was observed among states and local areas (Table). The estimated percentages of children who were fully vaccinated with influenza vaccine exceeded 40% in six states (Massachusetts 45.9%, Minnesota 41.1%, New Hampshire 43.3%, Rhode Island 43.9%, Vermont 43.6%, and Wisconsin 41.2%). Full vaccination coverage was less than 15% in three states (Arkansas 14.2%, California 14.9%, and Mississippi 8.7%) and two local areas (Los Angeles County, California 9.0% and El Paso County, Texas 7.5%).
• Nationally, the percentage of children receiving at least one dose of influenza vaccine and the percentage of children fully vaccinated was similar to the previous (2007-08) influenza season (3) Figure 1.
• Over one third (40.4%) of children who received at least 1 dose of influenza vaccine required, but did not receive, a second dose by January 31 (or by date of interview, if interviewed in January).
• The first (or only) dose of influenza vaccination was most often given in weeks 42-46 (i.e., October 19-November 22) with a drop in doses administered during week 47 (i.e., November 23-29, the week including the holiday of Thanksgiving; Figure 2). Among children requiring 2 doses, the second dose was most often administered in weeks 46-50 (i.e., November 16-December 20).

Table: Influenza vaccination-coverage levels for September-December 2008 among children aged 6-23 months*, by state and local area–National Immunization Survey (NIS), United States

* n=11,023 (unweighted) The influenza vaccination-coverage measures are based upon a subset of children included in the 2009 NIS. Only those children who were aged 6-23 months during the entire period of September-December 2008 and who had provider-reported immunization records are included.

†Fully vaccinated is defined as: 1) receipt of 2 doses from September 1, 2008, through the date of interview or January 31, 2009 (whichever was earlier) among influenza vaccine naïve children and children who received 1 dose for the first time in the previous influenza season, or 2) received 1 dose of influenza vaccine from September 1, 2008 through December 31, 2008 among all other children.

¶ Confidence Interval

** Estimate may not be reliable; confidence interval width >20.0.

• Nationally, the percentage of children receiving at least one dose of influenza vaccine or in the percentage of children fully vaccinated was similar to the previous (2007-08) influenza season (3) Figure 1.

Figure 1: Percentage of children aged 6-23 months who received influenza vaccination during September-December, by influenza season and vaccination status–National Immunization Survey, United States, 2002-03 through 2008-09 influenza seasons

* Advisory Committee on Immunization Practices.

† Full vaccination: 1) receipt of 2 doses, from September 1, 2008 through the date of interview or January 31, 2009 (whichever was earlier), among influenza vaccine naïve children and children who received 1 dose for the first time in the previous influenza season; or 2) receipt of 1 dose of influenza vaccine during September 1-December 31, 2008 among all other children.

§ Unweighted sample sizes: 2002-03 (n=13831), 2003-04 (n=13881), 2004-05 (n=12056), 2005-06 (13546), 2006-07 (n=9710), 2007-08 (n=11964), and 2008-09 (n=11023).

• The first (or only) dose of influenza vaccination was most often given in weeks 42-46 (i.e., October 19-November 22) with a drop in doses administered during week 47 (i.e., November 23-29, the week including the holiday of Thanksgiving). Among children requiring 2 doses, the second dose was most often administered in weeks 46-50 (i.e., November 16-December 20; Figure 2).

Figure 2: Percentage of children aged 6-23 months receiving influenza vaccination September-December 2008, by week of vaccination and dose received — National Immunization Survey, United States, 2008-09 influenza season

* Among all age-eligible children (n=11023).

† Among age-eligible children who met the Advisory Committee on Immunization Practices recommendation to receive 2 doses during the current season (i.e., had received no influenza dose before September 1, 2008 or who had received only 1 dose for the first time during the 2007-08 influenza season; n=8880).

• Over one third (40.4%) of children who received at least 1 dose of influenza vaccine required, but did not receive, a second dose by January 31 (or date of interview if interviewed in January).

Summary and Public Health Implications

This report of the 2008-09 season, the fifth season since ACIP recommended routine vaccination for all children aged 6-23 months, shows that there was not an increase in vaccination coverage compared to the previous 2007-08 season. The percentage of children fully vaccinated remains low. Strategies that have been successful at improving influenza vaccination coverage among children include standing orders, vaccination-only visits for children requiring only immunization services, and reminder/recall systems (4). See also: Recommendations Regarding Interventions to Improve Vaccination Coverage in Children, Adolescents, and Adults [139 KB, 5 pages], Prevention and Control of Influenza with Vaccines

In 2007, the ACIP recommended that children aged 6 months-8 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 (5). The change in recommendation was based a study indicating that children aged 6 months-8 years who received only 1 dose during their initial year of vaccination and then received 2 doses the following season had better protection against influenza than children who receive only 1 dose in each of their first two seasons (6). For the 2010-11 season, as in previous recommendations, all children aged 6 months-8 years who are receiving seasonal influenza vaccine for the first time should receive 2 doses. Children who received only 1 dose of seasonal vaccine in the 2009-10 season and had no prior seasonal vaccines should also receive two doses. In addition, for the 2010-11 influenza season, all children aged 6 months-8 years who did not receive at least 1 dose of an influenza A (H1N1) 2009 monovalent vaccine should receive 2 doses of a 2010-11 seasonal influenza vaccine, regardless of the number of prior seasonal influenza vaccinations. Children aged 6 months-8 years for whom the previous 2009-10 seasonal or influenza A (H1N1) 2009 monovalent vaccine history cannot be determined should receive 2 doses of a 2010-11 seasonal influenza vaccine.(7) See the vaccination algorithm at: Additional Information about Vaccination of Specific Populations.

Beginning with the 2010-11 influenza season, universal recommendation for influenza vaccination of persons aged ≥6 months has been launched (Prevention and Control of Influenza with Vaccines). Providers are encouraged to begin offering vaccination as soon as vaccine becomes available and to continue vaccination efforts throughout the season. This is especially important for children requiring 2 doses within the season. Additional studies are needed to determine evidence-based strategies that increase the percentage of children who are fully vaccinated. Prevention of influenza and its complications among infants and young children is a public health priority because of their high risk for influenza-related complication.

Data source and methods

The estimates are based on data from the 2009 National Immunization Survey (NIS). The NIS is an ongoing, random-digit–dialed telephone survey of households with children who are aged 19–35 months at the time of interview, followed by a mail survey of all of the children's vaccination providers (identified by the household respondent) to obtain vaccination data (8). The 2009 NIS interviews were conducted during January 6, 2009–February 10, 2009 and included the 50 states, 13 local areas and the U.S. Virgin Islands (this report excludes the U.S. Virgin Island estimates). Histories of influenza vaccination since birth were obtained through a mail survey of children’s vaccination providers. NIS methodology, including how responses were weighted, has been described previously (8). During the 2009 NIS, the household Council of American Survey Research Organizations (CASRO) response rate was 63.7%. The CASRO response rate is the product of the percentage of telephone lines identified as residential or non-residential (82.8%), the percentage of known households with a completed screening interview (92.4%), and the percentage of eligible respondents who complete the interview (83.2%). Provider-reported vaccination records were obtained for 17,053 children (68.7%) aged 19–35 months for whom household interviews were completed (8). Among those with provider-reported records, 11,023 met the age criteria for this assessment.

Two measures of influenza vaccination coverage are reported for children who were aged 6–23 months during the entire span of September–December 2008: 1) Receipt of 1 or more doses of influenza vaccine during September-December 2008 and 2) Full vaccination, defined as: i) receipt of 2 doses from September 1, 2008, through the date of interview or January 31, 2009 (whichever was earlier) among influenza vaccine naïve children and children who received 1 dose for the first time in the previous influenza season, or ii) received 1 dose of influenza vaccine from September 1, 2008 through December 31, 2008 among all other children. Later-season vaccination was not assessed because data collection began in January 2009. Season-to-season comparisons of influenza vaccination coverage estimates were conducted using t-tests and were considered statistically significant at <0.05.

The limitations of vaccination coverage data obtained through the NIS have been discussed in previous reports (8;9). The findings in this report are subject to at least two additional limitations. First, because some NIS interviews were conducted during the influenza season and some children receive influenza vaccinations after the interview, the coverage estimates are likely underestimated. Second, coverage estimates might be higher among children included in this analysis, compared with all children who were in the 6–23 month age group at some point during September–December. Children who became eligible for influenza vaccination at six months of age after September 1, 2007 (and thus were excluded from the analysis) might be less likely to have been vaccinated because of shorter duration of time eligible during the vaccination period.

References

1. CDC. Prevention and control of influenza: recommendations of the Advisory Committee on Immunization Practices (ACIP). MMWR Morb Mortal Wkly Rep 2002; 51(RR-3).
2. CDC. Prevention and control of influenza: recommendations of the Advisory Committee on Immunization Practices (ACIP). MMWR Morb Mortal Wkly Rep 2004; 53(RR-6):1-39.
3. CDC. Influenza vaccination coverage among children aged 6-23 months–United States, 2006-07 influenza season. MMWR Morb Mortal Wkly Rep 2008; 57(38):1039-1043.
4. CDC. Prevention and control of seasonal influenza with vaccines: recommendations of the Advisory Committee on Immunization Practices (ACIP), 2009. MMWR Early Release 2009; 58:1-52.
5. CDC. Prevention and control of influenza: recommendations of the Advisory Committee on Immunization Practices (ACIP), 2007. MMWR Morb Mortal Wkly Rep 2007; 56(RR-6):1-54.
6. Allison MA, Daley MF, Crane LA, Barrow J, Beaty BL, Allred N et al. Influenza vaccine effectiveness in healthy 6- to 21-month-old children during the 2003-2004 season. The Journal of Pediatrics 2006; 149:755-762.
7. CDC. Prevention and control of influenza with vaccines: recommendations of the Advisory Committee on Immunization Practices (ACIP), 2010. MMWR Recomm Rep 2010; 59(RR-8):1-62.
8. Smith PJ, Hoaglin DC, Battaglia MP, Khare M, Barker LE. Statistical methodology of the National Immunization Survey, 1994-2002. 2(138). 2005. Vital Health Stat. Ref Type: Report
9. CDC. National, state, and local area vaccination coverage among children aged 19-35 months–United States, 2009. MMWR Morb Mortal Wkly Rep 20010; 59(36):1171-1177.