Seasonal Influenza Vaccination Coverage Among Children Aged 6 Months--18 Years --- Eight Immunization Information System Sentinel Sites, United States, 2009--10 Influenza Season

Annual influenza vaccination was first recommended for children aged 6--23 months and 2--4 years by the Advisory Committee on Immunization Practices (ACIP) in 2004 and 2006, respectively (1,2). In August 2008, ACIP expanded its seasonal influenza vaccination recommendations to also include all children aged 5--18 years no later than the 2009--10 season (3,4). To update previous estimates (5) of seasonal influenza vaccination coverage among children aged 6 months--18 years, CDC analyzed data from the eight immunization information system (IIS) sentinel sites for the 2009--10 influenza season. Vaccination coverage with influenza A (H1N1) 2009 monovalent vaccine is not included in this report. Average (unweighted) vaccination coverage with ≥1 seasonal influenza vaccine doses was 26.3%, a 5.5 percentage point increase from the 2008--09 season (20.8%). Increases varied by age group, ranging from almost no increase among children aged 6--23 months (55.2% during the 2008--09 season to 55.7% during the 2009--10 season) to notable increases among children aged 2--4 years (from 33.0% to 38.4%), 5--12 years (19.0% to 27.1%), and 13--18 years (10.9% to 15.3%). Full vaccination coverage was low during the 2009--10 season, ranging from 34.7% among children aged 6--23 months to 15.3% among children aged 13--18 years. These findings highlight the need to identify varied strategies and venues for delivering influenza vaccine to different age groups of children to increase vaccination coverage.

An IIS is a confidential, computerized information system that collects and consolidates vaccination data from multiple health-care providers, generates reminder and recall notifications, and assesses vaccination coverage within a defined geographic area. For the 2008--2012 IIS sentinel site project period, CDC awarded supplemental funds to eight IIS sites to enhance data quality at the sites and to analyze data routinely to monitor immunization practices and vaccination coverage among children aged <19 years in the sentinel site geographic regions. The eight sites meet the following data quality criteria: 1) >85% of child vaccine provider sites were enrolled in IIS, 2) >85% of children aged <19 years who resided in the sentinel site region had ≥2 vaccine doses of any type recorded in IIS, and 3) >70% of administered doses were reported to IIS within 30 days of vaccination. IIS sentinel site results are not intended to be representative of and generalizable to vaccination practices in the United States; however, sentinel site data are complementary to other sources of influenza vaccination coverage (e.g., National Immunization Survey [NIS] and National H1N1 Flu Survey [NHFS]) because data 1) are provider-verified, 2) can track vaccination patterns throughout the entire influenza vaccination season, 3) can assess fully vaccinated status of children, and 4) can monitor vaccination among children and adolescents longitudinally. The six sentinel site areas in Arizona, Colorado, Michigan, Minnesota, Oregon, and Wisconsin consist of subsets of the entire state; the other two sentinel sites consist of the entire state of North Dakota and all of New York City.

To reflect ACIP recommendations for the 2009--10 influenza season (4), full vaccination with seasonal influenza vaccine for children aged 6 months--8 years was defined as 1) receipt of 2 valid vaccine doses (i.e., separated by at least 4 weeks) in the current season among previously unvaccinated children, 2) receipt of 2 valid doses among children who received only 1 dose for the first time during August 1, 2008--March 31, 2009, or 3) receipt of 1 vaccine dose in the current season among all other children. Children aged 9--18 years were considered fully vaccinated with receipt of 1 vaccine dose. Vaccination coverage was calculated for children aged 6--23 months, 2--4 years, 5--12 years, and 13--18 years who resided in a sentinel site area during the 2009--10 influenza season. Analyses included only children who were in the specified age groups during the entire influenza season to ensure that all children evaluated had the same opportunity for vaccination; children who aged into or out of an age group during the season were excluded because they would have had less time within the period to be vaccinated as part of the specified age group. As of March 31, 2010, a total of 5,361,835 children aged 6 months--18 years had been evaluated for this investigation (range: 37,061 in Colorado to 2,518,553 in New York City). Vaccination coverage at each sentinel site was calculated by dividing the number of children vaccinated by the total number of children in each specified age group during the entire influenza season, based on U.S. Census counts for the specific surveillance counties.* Vaccination coverage for the 2008--09 influenza season, which previously was published using IIS-based denominators (5), was recalculated in this report using U.S. Census denominators to provide comparable groups. The unweighted average for the eight sites (i.e., average site-specific coverage) was calculated by summing the percentages of children vaccinated at each site and dividing by the total number of sites (eight).

During the 2009--10 influenza season, average coverage with ≥1 influenza vaccine doses among all children aged 6 months--18 years was 26.3%, a 5.5 percentage point increase from the 2008--09 season (20.8%). Coverage in the 2009--10 season and increases in coverage from the 2008--09 season to the 2009--10 season varied by age group. Coverage among children aged 6--23 months was similar during the 2009--10 influenza season (55.7%) compared with the 2008--09 influenza season (55.2%). Coverage among children aged 2--4 years was 38.4% during the 2009--10 season, a 5.4 percentage point increase from the 2008--09 season (33.0%). The largest increase in coverage, from 19.0% to 27.1%, was observed among children aged 5--12 years. Coverage increased from 10.9% to 15.3% among children aged 13--18 years (Table 1).

Average full vaccination coverage among all children aged 6 months--18 years was 22.5%, a 5.9 percentage point increase from the 2008--09 season (16.6%). As with ≥1 dose coverage, full vaccination coverage was similar during the 2009--10 influenza season (34.7%) compared with the 2008--09 influenza season (33.4%) among children aged 6--23 months. A 5.0 percentage point increase from the 2008--09 season (26.0%) to the 2009--10 season (31.0%) was observed among children aged 2--4 years. Full vaccination coverage increased 7.6 percentage points, from 16.2% to 23.8%, among children aged 5--12 years (Table 2).

Reported by

LJ Pabst, MPH, C Weinbaum, MD, Immunization Svcs Div; SS Chaves, MD, Influenza Div, National Center for Immunization and Respiratory Diseases, CDC.

Editorial Note

The data in this report underscore the minimal increase in vaccination coverage among children aged 6--23 months from the 2008--09 season to the 2009--10 season, the continued low coverage among older children despite larger increases observed in these age groups, and the low full vaccination coverage among young children, leaving many unprotected against influenza. Several factors likely affected seasonal influenza vaccination coverage in the 2009--10 season, including new ACIP recommendations for vaccination of children aged 5--18 years, the presence of two separate influenza vaccines (for seasonal influenza and for 2009 influenza A (H1N1) and vaccination recommendations for each, and increased demand for seasonal influenza vaccine early in the season that strained vaccine supplies. However, the individual effect of each of these contributing factors on seasonal influenza vaccination coverage during the 2009--10 season is unknown.

Seasonal influenza vaccination recommendations have been in place for children aged 6--23 months for six seasons and for children aged 2--4 years for four seasons. For the first seasons, ACIP recommendations were issued for routine influenza vaccination of children aged 6--23 months and 2--4 years; coverage with ≥1 doses in these age groups reached approximately 30% (6,7) and 20% (8), respectively. Although vaccine providers were encouraged to implement the ACIP recommendation to vaccinate routinely children aged 5--18 years during the 2008--09 season, 2009--10 was the first season the ACIP recommendation was expected to be implemented fully, and coverage with ≥1 doses was similar to early coverage among younger children (3,4).

The increase in coverage from the 2008--09 season to the 2009--10 season among older children and adolescents could reflect the usually observed increase in vaccination coverage with newly recommended vaccines, increased awareness of influenza vaccination because of the 2009 H1N1 pandemic, or other reasons. In a national survey, at least 35% of vaccinated school-aged children were reported to have received influenza A (H1N1) 2009 monovalent vaccine at school-located vaccination (SLV) sites, but less than 10% were reported to have received seasonal influenza vaccine at an SLV site (CDC, unpublished data, 2010). Thus, SLV sites likely had limited effect on seasonal influenza vaccination coverage in the sentinel sites during the 2009--10 season. Among the eight IIS sites, three (Arizona, Colorado, and Michigan) reported that some SLV sites included seasonal influenza vaccine during the 2009--10 season. The low vaccination coverage among children aged 13--18 years, which might reflect fewer visits to primary-care providers in this age group, might be improved by SLV programs for these children.

Data from the NHFS and the Behavioral Risk Factor Surveillance System (BRFSS) were combined to report national and state-specific seasonal influenza vaccination coverage for the 2009--10 season among children aged 6 months--17 years (9). State-specific coverage was approximately 14 percentage points higher in the NHFS/BRFSS data compared with sentinel site data. Differences in coverage might reflect differences in geographic assessment areas in six of the eight sites, overreporting in the surveys, underreporting in IIS, or a combination of these factors. NHFS and BRFSS rely on self-report of vaccination and have been found to overestimate the number of seasonal influenza vaccine doses administered in the United States and to overestimate coverage by 10%--15% (9), likely because of nonresponse bias and recall bias. In contrast, IIS might underestimate coverage because of a lack of information from unenrolled providers (although IIS sentinel sites have >85% vaccine provider site participation), failure of enrolled providers to submit all vaccination data to IIS, and failure to include vaccinations administered at nontraditional locations, such as schools. During the 2008--09 influenza season, IIS-based coverage estimates were consistent with provider-verified coverage calculated by NIS for children aged 6--23 months (5),† suggesting that IIS likely captured complete data for children in that age group during 2008--09. Comparable data from NIS are not yet available for the 2009--10 influenza season.

The data in this report demonstrate the increase in seasonal influenza vaccination coverage among children in the eight sentinel sites during the 2009--10 influenza season. The lack of increased vaccination coverage among children aged 6--23 months, low full vaccination coverage among children aged <5 years who are at greatest risk for influenza-associated complications, and low coverage among older children and adolescents suggest that continued implementation of existing strategies and the development of new strategies to improve seasonal vaccination coverage are needed. Existing strategies include vaccinating later in the season (January--March) and use of standing orders, reminder/recall notifications, parental education, and SLV programs (4,10). The reporting of influenza vaccinations that are administered to children in all traditional and complementary settings (e.g., SLV) to IIS is an important means of providing rapid assessment of influenza vaccination coverage among children of varying age groups. Data can be used by state and local IIS sites to provide interim assessments of influenza vaccination coverage during the season.

Acknowledgments

The findings in this report are based, in part, on contributions provided by staff members at the eight IIS sentinel sites and by T Vogt, Immunization Svcs Div, National Center for Immunization and Respiratory Diseases, CDC.

References

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3. CDC. Prevention and control of influenza: recommendations of the Advisory Committee on Immunization Practices (ACIP), 2008. MMWR 2008;57(No. RR-7).
4. CDC. Prevention and control of seasonal influenza with vaccines: recommendations of the Advisory Committee on Immunization Practices (ACIP), 2009. MMWR 2009;58(No. RR-8).
5. CDC. Influenza vaccination coverage among children aged 6 months--18 years---eight immunization information system sentinel sites, United States, 2008--09 influenza season. MMWR 2009;58:1059--62.
6. CDC. Childhood influenza vaccination coverage---United States, 2004--05 influenza season. MMWR 2006;55:1062--5.
7. Zimmerman LA, Bartlett DL, Enger KS, Gosney K, Williams WG. Influenza vaccination coverage: findings from immunization information systems. BMC Pediatr 2007;7:28.
8. CDC. Influenza vaccination coverage among children aged 6--59 months---six immunization information system sentinel sites, United States, 2006--07 influenza season. MMWR 2007;56:963--5.
9. CDC. Interim results: state-specific seasonal influenza vaccination coverage---United States, August 2009--January 2010. MMWR 2010;59:477--84.
10. Task Force on Community Preventive Services. Recommendations regarding interventions to improve vaccination coverage in children, adolescents, and adults. Am J Prev Med 2000;18:92--6.

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