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Parental-Reported Full Influenza Vaccination Coverage of Children in the U.S.

Published:December 22, 2016DOI:https://doi.org/10.1016/j.amepre.2016.10.040

      Introduction

      Depending upon influenza vaccination history, children aged 6 months–8 years need one or two doses of influenza vaccine to be considered fully vaccinated. The objectives of this study were to quantify the percentage of children aged 6 months–8 years who were fully vaccinated against influenza based on parental report, overall, by state, and by sociodemographic characteristics, and to examine sociodemographic characteristics associated with being fully vaccinated.

      Methods

      Data from the National Immunization Survey-Flu for the 2012–2013 and 2013–2014 influenza seasons were analyzed in 2015 using the Kaplan–Meier method to produce vaccination coverage estimates. Wald chi-square tests were used to test for bivariate associations, and Cox proportional hazards models were used to test for demographic characteristics independently associated with the child being fully vaccinated.

      Results

      The percentages of children aged 6 months–8 years who were fully vaccinated during the 2012–2013 and 2013–2014 influenza seasons were 41.0% and 45.2%, respectively. Full vaccination varied widely by state and was more likely for children requiring only one dose. Based on the statistical models, children likely to be fully vaccinated were older, non-black, had a mother with an education >12 years, or lived in a high-income household.

      Conclusions

      Most children in the U.S. are not fully vaccinated against influenza. Reminder systems and interventions that reduce or remove barriers to children receiving their second doses of influenza vaccine may improve full influenza vaccination coverage among all children.

      Introduction

      Influenza infections are associated with significant morbidity and mortality in children.

      Centers for Disease Control and Prevention (CDC). Influenza-associated pediatric mortality. http://gis.cdc.gov/GRASP/Fluview/PedFluDeath.html. Accessed May 1, 2016.

      Furthermore, children have been found to play a large role in transmission of influenza into their household.
      • Longini I.M.
      • Koopman J.S.
      • Monto A.S.
      • Fox J.P.
      Estimating household and community transmission parameters for influenza.
      Household contacts of influenza-positive children have been found to have more missing work or school days and more medical visits.
      • Principi N.
      • Esposito S.
      • Marchisio P.
      • Gasparini R.
      • Crovari P.
      Socioeconomic impact of influenza on healthy children and their families.
      Influenza vaccination is the most effective strategy to prevent people from getting influenza and influenza-related complications.
      Centers for Disease Control and Prevention (CDC)
      Prevention and control of influenza with vaccines: recommendations of the Advisory Committee on Immunization Practices, United States, 2015-16 influenza Season.
      The Advisory Committee on Immunization Practices (ACIP) recommends annual influenza vaccination for all people aged ≥6 months.
      Centers for Disease Control and Prevention (CDC)
      Prevention and control of influenza with vaccines: recommendations of the Advisory Committee on Immunization Practices, United States, 2015-16 influenza Season.
      Two doses, administered a minimum of 4 weeks apart, is recommended for many children aged 6 months–8 years to ensure they obtain the optimum immune response and protection from influenza disease.
      • Ritzwoller D.P.
      • Bridges C.B.
      • Shetterly S.
      • Yamasaki K.
      • Kolczak M.
      • France E.K.
      Effectiveness of the 2003-2004 influenza vaccine among children 6 months to 8 years of age, with 1 vs 2 doses.
      • Allison M.A.
      • Daley M.F.
      • Crane L.A.
      • et al.
      Influenza vaccine effectiveness in healthy 6- to 21-month-old children during the 2003-2004 season.
      The ACIP definition of which children aged <9 years require two doses to be considered fully vaccinated has varied from season to season.
      Centers for Disease Control and Prevention (CDC)
      Prevention and control of influenza: recommendations of the Advisory Committee on Immunization Practices (ACIP).
      Centers for Disease Control and Prevention (CDC)
      Prevention and control of influenza. Recommendations of the Advisory Committee on Immunization Practices (ACIP).
      Centers for Disease Control and Prevention (CDC)
      Prevention and Control of Influenza: recommendations of the Advisory Committee on Immunization Practices (ACIP).
      Centers for Disease Control and Prevention (CDC)
      Prevention and control of influenza. Recommendations of the Advisory Committee on Immunization Practices (ACIP), 2007.
      Centers for Disease Control and Prevention (CDC)
      Prevention and control of influenza: recommendations of the Advisory Committee on Immunization Practices (ACIP), 2008.
      Centers for Disease Control and Prevention (CDC)
      Prevention and control of seasonal influenza with vaccines: recommendations of the Advisory Committee on Immunization Practices (ACIP), 2009.
      Centers for Disease Control and Prevention (CDC)
      Prevention and control of influenza with vaccines: recommendations of the Advisory Committee on Immunization Practices (ACIP), 2010.
      Centers for Disease Control and Prevention (CDC)
      Prevention and control of influenza with vaccines: recommendations of the Advisory Committee on Immunization Practices (ACIP), 2011.
      Centers for Disease Control and Prevention (CDC)
      Prevention and control of influenza with vaccines: recommendations of the Advisory Committee on Immunization Practices (ACIP)—United States, 2012-13 influenza season.
      Centers for Disease Control and Prevention (CDC)
      Prevention and control of seasonal influenza with vaccines. Recommendations of the Advisory Committee on Immunization Practices (ACIP)—United States, 2013-2014.
      Centers for Disease Control and Prevention (CDC)
      Prevention and control of seasonal influenza with vaccines: recommendations of the Advisory Committee on Immunization Practices (ACIP)—United States, 2014-15 influenza season.
      Two approaches were recommended by ACIP for the 2012–2013 and 2013–2014 influenza seasons.
      Centers for Disease Control and Prevention (CDC)
      Prevention and control of influenza with vaccines: recommendations of the Advisory Committee on Immunization Practices (ACIP)—United States, 2012-13 influenza season.
      Centers for Disease Control and Prevention (CDC)
      Prevention and control of seasonal influenza with vaccines. Recommendations of the Advisory Committee on Immunization Practices (ACIP)—United States, 2013-2014.
      According to the first approach, during the 2012–2013 and 2013–2014 seasons, children aged 6 months–8 years needed only one dose if they had received a total of two or more doses of seasonal vaccine since July 1, 2010, whereas children who had not received two or more doses of seasonal vaccine since July 1, 2010, required two doses. The second approach involves taking into account a child’s vaccination history prior to July 1, 2010, when it is available.
      Although parent-reported influenza vaccination coverage with one or more dose has been routinely reported for children aged 6 months–17 years since the 2010–2011 season using the National Immunization Survey Flu (NIS-Flu) and during the 2009–2010 season using the National H1N1 Flu Survey, national estimates of full vaccination coverage among all children aged 6 months–8 years have not been reported.

      Centers for Disease Control and Prevention (CDC). FluVaxView. www.cdc.gov/flu/fluvaxview/. Accessed May 5, 2016.

      Full vaccination coverage based upon provider-reported vaccinations among children aged 6–23 months has been published using data from NIS-Child.
      • Santibanez T.A.
      • Grohskopf L.A.
      • Zhai Y.
      • Kahn K.E.
      Complete influenza vaccination trends for children six to twenty-three months.
      These estimates, however, cannot be produced for one influenza season before the start of the following influenza season owing to the time required to obtain provider data.
      • Santibanez T.A.
      • Grohskopf L.A.
      • Zhai Y.
      • Kahn K.E.
      Complete influenza vaccination trends for children six to twenty-three months.
      Relying upon parental report of influenza vaccination status allows for timely production of estimates, as is done for the estimates of influenza vaccination coverage with one or more dose reported annually on FluVaxView.

      Centers for Disease Control and Prevention (CDC). FluVaxView. www.cdc.gov/flu/fluvaxview/. Accessed May 5, 2016.

      Some estimates of full influenza vaccination coverage based upon pediatric outpatient visits and on an HMO population have been reported.
      • Jackson L.A.
      • Neuzil K.M.
      • Baggs J.
      • et al.
      Compliance with the recommendations for 2 doses of trivalent inactivated influenza vaccine in children less than 9 years of age receiving influenza vaccine for the first time: a Vaccine Safety Datalink study.
      • Bhatt P.
      • Block S.L.
      • Toback S.L.
      • Ambrose C.S.
      Timing of the availability and administration of influenza vaccine through the vaccines for children program.
      Likewise, estimates have been published based upon Immunization Information Systems (IIS) for select states and local areas that have well-developed registries, and at least one study has examined full influenza vaccination coverage using a hospital-based immunization registry.
      Centers for Disease Control and Prevention (CDC)
      Influenza vaccination coverage among children aged 6 months–18 years—eight immunization information system sentinel sites, United States, 2008-09 influenza season.
      • Pabst L.J.
      • Fiore A.E.
      • Cullen K.A.
      Completion of the 2-dose influenza vaccine series among children aged 6 to 59 months: immunization information system sentinel sites, 2007-2008 influenza season.
      • Hofstetter A.M.
      • Natarajan K.
      • Martinez R.A.
      • Rabinowitz D.
      • Vawdrey D.K.
      • Stockwell M.S.
      Influenza vaccination coverage and timeliness among children requiring two doses, 2004-2009.
      However, full vaccination coverage among children aged 6 months–8 years by state and sociodemographic characteristics has not yet been evaluated using a national sample. Identifying differences in full influenza vaccination coverage would be useful to immunization programs for targeting interventions and could help guide public health action to improve full influenza vaccination coverage. The objectives of this study were to quantify the percentage of children aged 6 months–8 years who were fully vaccinated against influenza, based on parental report, overall, by state, and by sociodemographic characteristics and to examine sociodemographic characteristics associated with being fully vaccinated.

      Methods

      Data Sample

      Data from NIS-Flu for the 2012–2013 and 2013–2014 influenza seasons were analyzed in 2015. NIS-Flu is a national list-assisted, random-digit-dialed, dual-frame (landline and cellular telephones) survey of households with children aged 6 months–17 years.
      Centers for Disease Control and Prevention (CDC)
      Surveillance of influenza vaccination coverage—United States, 2007-08 through 2011-12 influenza seasons.
      NIS-Flu includes three components: NIS-Child for children aged 19–35 months, NIS-Teen for children aged 13–17 years, and the NIS-Child Influenza Module for children aged 6–18 months and 3–12 years who are identified during screening households for NIS-Child and NIS-Teen.
      Centers for Disease Control and Prevention (CDC)
      Surveillance of influenza vaccination coverage—United States, 2007-08 through 2011-12 influenza seasons.
      Centers for Disease Control and Prevention (CDC)
      National, regional, state, and selected local area vaccination coverage among Adolescents aged 13-17 years—United States, 2013.
      Centers for Disease Control and Prevention (CDC)
      National, state, and selected local area vaccination coverage among children aged 19-35 months—United States, 2013.
      • Jain N.
      • Singleton J.A.
      • Montgomery M.
      • Skalland B.
      Determining accurate vaccination coverage rates for adolescents: the National Immunization Survey-Teen 2006.
      • Smith P.J.
      • Battaglia M.P.
      • Huggins V.J.
      • et al.
      Overview of the sampling design and statistical methods used in the National Immunization Survey.
      • Smith P.J.
      • Hoaglin D.C.
      • Battaglia M.P.
      • Khare M.
      • Barker L.E.
      Centers for Disease Control and Prevention. Statistical methodology of the National Immunization Survey, 1994-2002.
      The Council of American Survey Research Organizations response rates for the 2012–2013 and 2013–2014 influenza seasons ranged from 52.2% to 63.4% for landline and 18.1% to 33.5% for cellular telephones.
      • Frankel L.
      The report of the CASRO task force on response rates.
      NIS-Flu included a total of 107,550 and 130,409 children with completed interviews for the 2012–2013 and 2013–2014 seasons, respectively. This study restricted analyses to children aged 6 months–8 years (as of November 1 for each season, consistent with how age is defined for official Centers for Disease Control and Prevention estimates of one or more dose) and excluded children with missing vaccination status (3.7% in 2012–2013 and 2.7% in 2013–2014). The resulting sample sizes were 51,620 children for the 2012–2013 season and 65,848 children for the 2013–2014 season.

      Measures

      Vaccination coverage estimates were based on interviews conducted October–June and included vaccinations received July–May of the respective influenza season. The main outcome measure was full vaccination. Estimates of coverage with one or more dose were also calculated.
      Influenza vaccination status was assessed by asking the parent or guardian if the survey-selected child in the household had received an influenza vaccination since July 1 and, if so, how many vaccinations and in which month and year the child received the first dose and in which month and year the child received the second dose if applicable. To assess history of vaccination, required to determine whether one or two doses were needed to be fully vaccinated, additional questions were asked. For the 2012–2013 season survey, the parent was asked, Did [selected child] receive any flu vaccinations during the last two flu seasons? This would be from July 1, 2010 to end of June 2012. If yes was selected, the following question was asked, How many flu vaccinations did [selected child] receive in the past two flu seasons, which were from July 1, 2010 to end of June 2012? Similar questions were asked for the 2013–2014 season. Note that for the 2013–2014 season the survey question was in reference to the last two influenza seasons (back to July 2011) and not the last three seasons (back to July 2010). Children for whom the number of doses needed to be fully vaccinated could not be determined owing to missing responses to the vaccination history questions (11.3% of vaccinated children in 2012–2013 and 9.7% of vaccinated children in 2013–2014) were considered as needing two doses to be fully vaccinated. This approach reflects ACIP recommendations, which have a vaccination history response of don’t know equated to no in the vaccination decision algorithm.
      Centers for Disease Control and Prevention (CDC)
      Prevention and control of seasonal influenza with vaccines: recommendations of the Advisory Committee on Immunization Practices (ACIP)—United States, 2014-15 influenza season.
      The full vaccination estimates are based upon the Kaplan–Meier analyses with the event being time (in months) of receipt of the dose that would make the child fully vaccinated (Dose 1 for those who needed only one dose and Dose 2 for those who needed two doses). For children reported to have been vaccinated but had a missing month and year of vaccination for their first (or only) dose (12.5% for the 2012–2013 season and 16.3% for the 2013–2014 season), month and year were imputed from donor pools matched for week of interview, age group, region of residence, and race/ethnicity using hot deck imputation. Likewise, for children reported to have received two doses of influenza vaccine but had a missing month and year of vaccination for their second dose (2.6% for the 2012–2013 season and 2.9% for the 2013–2014 season), the month and year were imputed. For children reported to have not been fully vaccinated, they were censored on the month of interview. For all vaccination outcome measures, Kaplan–Meier survival analysis was used to obtain the cumulative vaccination coverage estimates as detailed in previous reports.
      • Lu P.J.
      • Ding H.
      • Black C.L.
      H1N1 and seasonal influenza vaccination of U.S. healthcare personnel, 2010.
      Respondent-reported demographic characteristics included in this study were child’s gender, age, and race/ethnicity; mother’s education level; household poverty level; number of children aged <18 years in the household; and Metropolitan Statistical Area (MSA). The income/poverty level variable was defined based on total family income in the past calendar year, and the U.S. Census poverty thresholds for that year specified for the applicable family size and number of children aged <18 years.

      Statistical Analysis

      Wald chi-square tests followed by post hoc pairwise comparison t-tests were used to test the associations between influenza vaccination status and demographic characteristics. Comparisons of influenza vaccination coverage estimates between the 2012–2013 and 2013–2014 seasons were performed with t-tests assuming large df. Cox proportional hazards models were used to determine demographic characteristics independently associated with the child being fully vaccinated, and hazard ratios are reported. Proportional hazard assumptions were tested using time dependent covariates. All analyses were weighted to population totals and to adjust for households having multiple telephone lines, unit non-response, and non-coverage of non-telephone households. All estimates, along with 95% CIs, were calculated using SAS, version 9.3, and SUDAAN, version 11.01, to account for the complex survey design. All tests were two-sided with a significance level of 0.05.

      Results

      Demographic characteristics of the study sample and the percentages requiring one versus two doses to be fully vaccinated are presented in Table 1. Overall, approximately two thirds of children required two doses to be fully vaccinated (2012–2013, 62.0%; 2013–2014, 58.3%). These percentages were much higher for younger children, with >80% of children aged 6–23 months (2012–2013, 83.6%; 2013–2014, 81.2%) requiring two doses (Table 1).
      Table 1Demographic Characteristics of the Study Population of Children 6 Months Through 8 Years
      Demographic characteristic2012–2013 influenza season2013–2014 influenza season
      Unweighted nWeighted % (±95% CI)Unweighted nWeighted % (±95% CI)
      Overall51,62010065,848100
      Gender
       Male26,59351.0 (0.9)33,78951.3 (0.9)
       Female25,02749.0 (0.9)32,05948.7 (0.9)
      Age
      Age as of November 1 of the respective influenza season.
       6–23 months10,63217.1 (0.7)13,41417.4 (0.6)
       2–4 years18,08233.6 (0.9)22,78233.5 (0.8)
       5–8 years22,90649.4 (0.9)29,65249.1 (0.9)
      Child’s race/ethnicity
       Hispanic9,75725.3 (1.0)12,82824.6 (0.9)
       Black, non-Hispanic5,34414.1 (0.7)6,77414.1 (0.7)
       White, non-Hispanic30,25050.7 (1.0)37,97051.4 (0.9)
       Other/multiple races, non-Hispanic6,26910.0 (0.6)8,2769.8 (0.5)
      Mother’s education level
       <12 years4,93812.7 (0.7)6,49913.1 (0.7)
       12 years9,70420.5 (0.8)12,03919.3 (0.7)
       >12 years, not college graduate13,07725.6 (0.9)16,66325.5 (0.8)
       College graduate21,94038.0 (0.9)28,07038.6 (0.8)
      Poverty status
      Income/poverty level was defined based on total family income in the past calendar year, and the U.S. Census poverty thresholds for that year specified for the applicable family size and number of children aged <18 years. Poverty thresholds are available at www.census.gov/hhes/www/poverty/data/threshld/index.html. MSA, Metropolitan Statistical Area; No., number.
       Above poverty, >$75,000/year18,27830.1 (0.9)23,59630.8 (0.8)
       Above poverty, ≤$75,000/year18,84535.2 (0.9)23,12233.3 (0.8)
       At or below poverty level9,71125.9 (1.0)12,52025.3 (0.9)
       Unknown4,7868.9 (0.5)6,61010.5 (0.5)
      No. of children <18 in household
       1 child17,57825.8 (0.8)21,00523.5 (0.7)
       2 children20,65341.4 (1.0)26,30140.9 (0.9)
       ≥3 children13,20632.7 (1.0)18,27635.4 (0.9)
      Household in MSA
       MSA, principal city18,06734.8 (0.9)18,33127.5 (0.8)
       MSA, not principal city22,31848.9 (1.0)34,46058.1 (0.9)
       Non-MSA11,23516.3 (0.6)13,05714.4 (0.5)
      Doses to be fully vaccinated
       1 dose20,80238.0 (0.9)28,51841.7 (0.9)
       2 doses30,81862.0 (1.0)32,63258.3 (0.9)
      6–23 months
       1 dose2,12816.4 (1.4)2,97418.8 (1.3)
       2 doses8,50483.6 (1.4)10,44081.2 (1.3)
      2–4 years
       1 dose7,98941.1 (1.6)10,67845.4 (1.6)
       2 doses10,09358.9 (1.6)12,10454.6 (1.6)
      5–8 years
       1 dose10,68543.3 (1.3)14,86647.3 (1.2)
       2 doses12,22156.7 (1.3)14,78652.7 (1.2)
      Source: National Immunization Survey-Flu (NIS-Flu), 2012–2013 and 2013–2014 influenza seasons.
      a Age as of November 1 of the respective influenza season.
      b Income/poverty level was defined based on total family income in the past calendar year, and the U.S. Census poverty thresholds for that year specified for the applicable family size and number of children aged <18 years. Poverty thresholds are available at www.census.gov/hhes/www/poverty/data/threshld/index.html.MSA, Metropolitan Statistical Area; No., number.
      The percentages of children aged 6 months–8 years who received one or more dose of influenza vaccination during the 2012–2013 and 2013–2014 influenza seasons were 65.9% and 67.2%, respectively, whereas the percentages of children fully vaccinated were 41.0% and 45.2%, respectively (Table 2). State variability in full vaccination coverage was large, ranging from 28.0% in Oklahoma to 60.5% in Rhode Island during the 2012–2013 season and 31.1% in Idaho to 64.9% in Rhode Island during the 2013–2014 season (Table 3).
      Table 2Influenza Vaccination Coverage Among Children 6 Months Through 8 Years, by Characteristics and Dose Recommendations
      Demographic characteristic2012–2013 influenza season, estimate
      Influenza vaccination coverage estimates were calculated using the Kaplan–Meier survival analysis procedure. For the 2012–2013 season, estimates include vaccinations received from July 2012 through May 2013 and are based on interviews conducted October 2012 through June 2013. For the 2013–2014 season, estimates include vaccinations received from July 2013 through May 2014 and are based on interviews conducted October 2013 through June 2014.
      (±95% CI)
      2013–2014 influenza season, estimate (±95% CI)
      ≥1 dose
      Any (≥1) dose defined as receipt of at least one dose of influenza vaccine.
      Fully vaccinated
      Fully vaccinated was defined as either (1) receipt of two doses of influenza vaccine for children who were previously unvaccinated or did not receive a total of two or more doses of influenza vaccine from July 1, 2010, to the end of June 2012 for the 2012–2013 influenza seasons or June 2013 for the 2013–2014 influenza season, or (2) receipt of one dose of influenza vaccine otherwise.
      ≥1 doseFully vaccinated
      Overall65.9 (1.2)41.0 (1.1)67.2 (1.1)45.2 (1.3)
      Statistically significant difference compared to the 2012–2013 influenza season estimate.
      Gender
       A. Male66.1 (1.6)41.7 (1.6)66.9 (1.4)45.1 (1.7)
      Statistically significant difference compared to the 2012–2013 influenza season estimate.
       B. Female65.6 (1.7)40.4 (1.6)67.5 (1.7)45.3 (1.8)
      Statistically significant difference compared to the 2012–2013 influenza season estimate.
      Age
      Age as of November 1 of the respective influenza season.
       A. 6–23 months77.0 (2.3)
      The presence or absence of capital letters denotes whether that estimate was statically significantly different at p<0.05 from another row, and denotes which row it differed from (A, B, C, D), based on pairwise comparison t-test. For example, the 77.0% of ≥1 dose vaccination coverage estimate of 6–23 months (A) was statistically significantly different from the 65.9% of coverage estimates of 2–4 years (B) and 61.7% of 5–8 years (C) in the 2012–2013 season. MSA, Metropolitan Statistical Area; No., number.
      ,B,C
      36.8 (2.3)B,C74.4 (2.0)B,C39.3 (2.3)B,C
       B. 2–4 years65.9 (1.9)A,C42.8 (2.1)A68.1 (1.8)A,C45.6 (2.3)A
       C. 5–8 years61.7 (1.8)A,B41.3 (1.6)A63.9 (1.7)A,B47.0 (1.8)
      Statistically significant difference compared to the 2012–2013 influenza season estimate.
      ,A
      Child’s race/ethnicity
       A. Hispanic69.0 (2.8)B,C41.4 (2.8)B74.2 (2.9)
      Statistically significant difference compared to the 2012–2013 influenza season estimate.
      ,B,C
      47.9 (3.9)
      Statistically significant difference compared to the 2012–2013 influenza season estimate.
      ,B
       B. Black, non-Hispanic63.5 (3.6)A,D33.9 (3.0)A,C,D63.3 (3.0)A,D37.8 (3.2)A,C,D
       C. White, non-Hispanic64.0 (1.4)A,D42.5 (1.4)B64.2 (1.2)A,D45.9 (1.2)
      Statistically significant difference compared to the 2012–2013 influenza season estimate.
      ,B
       D. Other/multiple races, non-Hispanic69.7 (3.7)B,C42.5 (3.1)B70.9 (2.7)B,C46.3 (2.9)B
      Mother’s education level
       A. <12 years66.7 (4.1)C35.5 (3.6)D72.5 (4.4)B,C45.9 (5.4)
      Statistically significant difference compared to the 2012–2013 influenza season estimate.
      ,B,C,D
       B. 12 years63.5 (2.8)D36.8 (2.6)D64.0 (2.3)A,C,D37.8 (3.2)A,D
       C. >12 years, not college graduate61.4 (2.3)A,D37.8 (2.3)D60.2 (2.1)A,B,D39.5 (2.3)A.D
       D. College graduate69.6 (1.7)B,C47.9 (1.7)A,B,C71.6 (1.3)B,C53.2 (1.4)
      Statistically significant difference compared to the 2012–2013 influenza season estimate.
      ,A,B,C
      Poverty status
      Income/poverty level was defined based on total family income in the past calendar year, and the U.S. Census Poverty thresholds for that year specified for the applicable family size and number of children aged <18 years. Poverty thresholds are available at http://www.census.gov/hhes/www/poverty/data/threshld/index.html.
       A. Above poverty, >$75,000/year71.2 (1.9)B,C,D49.7 (1.9)B,C,D71.2 (1.4)B54.0 (1.6)
      Statistically significant difference compared to the 2012–2013 influenza season estimate.
      ,B,C,D
       B. Above poverty, ≤$75,000/year61.2 (2.0)A,C38.1 (1.9)A62.0 (1.7)A,B,D40.0 (1.7)A,D
       C. At or below poverty level67.0 (2.7)A,B36.4 (2.5)A68.6 (2.8)B42.2 (1.9)
      Statistically significant difference compared to the 2012–2013 influenza season estimate.
      ,A,D
       D. Unknown63.7 (3.9)A36.8 (3.6)A68.9 (3.7)B43.0 (4.1)
      Statistically significant difference compared to the 2012–2013 influenza season estimate.
      ,A,B,C
      No. of children <18 in household
       A. 1 child67.9 (2.1)C40.1 (2.1)B67.6 (1.7)41.3 (1.8)B
       B. 2 children67.8 (1.8)C44.1 (1.9)A,C68.5 (1.5)C48.2 (1.7)
      Statistically significant difference compared to the 2012–2013 influenza season estimate.
      ,A,C
       C. ≥3 children61.7 (2.3)A,B37.8 (2.0)B65.6 (2.2)
      Statistically significant difference compared to the 2012–2013 influenza season estimate.
      ,B
      42.2 (2.6)
      Statistically significant difference compared to the 2012–2013 influenza season estimate.
      ,B
      Household in MSA
       A. MSA, principal city67.3 (2.0)C40.4 (1.9)B,C69.6 (2.0)C45.2 (2.4)
      Statistically significant difference compared to the 2012–2013 influenza season estimate.
      ,C
       B. MSA, not principal city66.7 (1.9)C43.0 (1.8)A,C67.8 (1.5)A,C46.4 (1.7)
      Statistically significant difference compared to the 2012–2013 influenza season estimate.
      ,C
       C. Non-MSA60.1 (2.4)A,B37.0 (2.2)A,B60.2 (2.3)A,B40.4 (2.3)
      Statistically significant difference compared to the 2012–2013 influenza season estimate.
      ,A,B
      No. of doses to be fully vaccinated
       A. 1 dose90.6 (1.4)B90.6 (1.4)B93.2 (1.2)
      Statistically significant difference compared to the 2012–2013 influenza season estimate.
      ,B
      93.2 (1.2)
      Statistically significant difference compared to the 2012–2013 influenza season estimate.
      ,B
       B. 2 dose50.9 (1.6)A10.7 (1.0)A48.9 (1.3)A10.9 (1.2)A
      Source: National Immunization Survey-Flu (NIS-Flu), 2012–2013 and 2013–2014 influenza seasons.
      a Influenza vaccination coverage estimates were calculated using the Kaplan–Meier survival analysis procedure. For the 2012–2013 season, estimates include vaccinations received from July 2012 through May 2013 and are based on interviews conducted October 2012 through June 2013. For the 2013–2014 season, estimates include vaccinations received from July 2013 through May 2014 and are based on interviews conducted October 2013 through June 2014.
      b Any (≥1) dose defined as receipt of at least one dose of influenza vaccine.
      c Fully vaccinated was defined as either (1) receipt of two doses of influenza vaccine for children who were previously unvaccinated or did not receive a total of two or more doses of influenza vaccine from July 1, 2010, to the end of June 2012 for the 2012–2013 influenza seasons or June 2013 for the 2013–2014 influenza season, or (2) receipt of one dose of influenza vaccine otherwise.
      d Age as of November 1 of the respective influenza season.
      e Income/poverty level was defined based on total family income in the past calendar year, and the U.S. Census Poverty thresholds for that year specified for the applicable family size and number of children aged <18 years. Poverty thresholds are available at http://www.census.gov/hhes/www/poverty/data/threshld/index.html.
      f Statistically significant difference compared to the 2012–2013 influenza season estimate.
      g The presence or absence of capital letters denotes whether that estimate was statically significantly different at p<0.05 from another row, and denotes which row it differed from (A, B, C, D), based on pairwise comparison t-test. For example, the 77.0% of ≥1 dose vaccination coverage estimate of 6–23 months (A) was statistically significantly different from the 65.9% of coverage estimates of 2–4 years (B) and 61.7% of 5–8 years (C) in the 2012–2013 season.MSA, Metropolitan Statistical Area; No., number.
      Table 3Influenza Vaccination Coverage Among Children 6 Months Through 8 Years, by State and HHS Region
      State2012–2013 influenza season, estimate
      Influenza vaccination coverage estimates were calculated using the Kaplan-Meier survival analysis procedure. For the 2012–2013 season, estimates include vaccinations received from July 2012 through May 2013 and are based on interviews conducted October 2012 through June 2013. For the 2013–2014 season, estimates include vaccinations received from July 2013 through May 2014 and are based on interviews conducted October 2013 through June 2014.
      (±95% CI)
      2013–2014 influenza season, estimate
      Influenza vaccination coverage estimates were calculated using the Kaplan-Meier survival analysis procedure. For the 2012–2013 season, estimates include vaccinations received from July 2012 through May 2013 and are based on interviews conducted October 2012 through June 2013. For the 2013–2014 season, estimates include vaccinations received from July 2013 through May 2014 and are based on interviews conducted October 2013 through June 2014.
      (±95% CI)
      ≥1 dose
      Any (≥1) dose defined as receipt of at least one dose of influenza vaccine.
      Fully vaccinated
      Fully vaccinated was defined as either (1) receipt of two doses of influenza vaccine for children who were previously unvaccinated or did not receive a total of two or more doses of influenza vaccine from July 1, 2010, to the end of June 2012 for the 2012–2013 influenza seasons or June 2013 for the 2013–2014 influenza season, or (2) receipt of one dose of influenza vaccine otherwise.
      ≥1 doseFully vaccinated
      National65.9 (1.2)41.0 (1.1)67.2 (1.1)45.2 (1.3)
      Statistically significant difference compared to the 2012–2013 influenza season estimate. HHS, Health and Human Services.
      Region 179.6 (2.7)56.8 (3.2)79.9 (2.1)60.4 (2.7)
       Connecticut80.5 (4.4)59.1 (5.9)84.0 (3.7)62.6 (5.0)
       Maine65.3 (6.1)40.4 (5.9)68.7 (4.8)45.0 (5.0)
       Massachusetts83.0 (4.7)59.9 (5.6)81.8 (3.7)64.3 (4.8)
       New Hampshire71.5 (5.8)50.7 (5.8)71.3 (5.4)53.4 (5.6)
       Rhode Island87.6 (7.5)60.5 (8.1)84.0 (4.3)64.9 (5.3)
       Vermont69.6 (6.1)49.7 (6.3)66.6 (5.3)46.7 (5.4)
      Region 273.1 (3.1)48.9 (3.4)74.5 (2.5)52.2 (2.8)
       New York69.2 (3.6)46.2 (4.0)71.2 (3.3)49.3 (3.5)
       New Jersey81.3 (5.5)54.0 (6.2)81.4 (3.6)58.3 (4.6)
      Region 373.2 (3.5)49.2 (3.9)71.2 (2.9)49.3 (3.1)
       Delaware72.3 (9.6)48.5 (8.7)77.6 (4.8)49.9 (5.7)
       Maryland71.7 (7.3)50.0 (7.8)76.2 (5.9)52.4 (6.8)
       Pennsylvania77.8 (5.8)55.5 (6.8)69.8 (4.9)
      Statistically significant difference compared to the 2012–2013 influenza season estimate. HHS, Health and Human Services.
      51.6 (5.2)
       Virginia70.3 (6.8)42.8 (7.3)70.7 (6.1)45.6 (5.8)
       West Virginia60.9 (6.7)34.3 (5.7)62.2 (5.0)39.0 (4.9)
       District of Columbia82.2 (5.7)48.8 (7.1)75.5 (5.5)50.5 (7.3)
      Region 461.3 (2.9)37.5 (2.8)61.2 (2.1)38.7 (2.0)
       Alabama64.3 (7.1)47.1 (12.6)68.5 (6.5)45.0 (7.1)
       Florida57.2 (8.4)29.0 (7.0)57.2 (4.9)31.2 (4.4)
       Georgia61.3 (6.3)40.8 (6.9)58.9 (5.4)39.8 (5.1)
       Kentucky70.3 (7.8)43.3 (6.1)60.9 (6.3)42.3 (6.3)
       Mississippi56.0 (6.8)29.6 (5.7)49.8 (5.1)32.9 (5.0)
       North Carolina63.6 (5.0)41.4 (5.0)66.9 (5.1)44.6 (5.2)
       South Carolina58.3 (6.0)34.9 (5.6)62.5 (6.2)38.0 (5.0)
       Tennessee67.2 (6.1)44.4 (6.2)68.1 (5.2)45.7 (5.2)
      Region 563.1 (2.5)40.1 (2.3)62.9 (2.1)41.9 (2.1)
       Illinois64.1 (5.3)37.9 (4.8)61.2 (3.7)37.8 (3.7)
       Indiana61.0 (5.3)39.8 (4.9)63.0 (5.0)41.1 (4.4)
       Ohio61.5 (6.5)40.8 (5.4)62.4 (5.2)45.4 (5.5)
       Michigan60.4 (5.6)37.6 (5.6)60.3 (5.5)36.2 (4.9)
       Minnesota68.3 (6.1)45.3 (5.7)70.1 (5.4)51.0 (5.8)
       Wisconsin66.3 (5.8)44.3 (5.9)65.2 (5.3)47.5 (5.5)
      Region 665.3 (3.0)39.3 (2.7)70.2 (2.8)
      Statistically significant difference compared to the 2012–2013 influenza season estimate. HHS, Health and Human Services.
      45.4 (4.1)
      Statistically significant difference compared to the 2012–2013 influenza season estimate. HHS, Health and Human Services.
       Arkansas63.9 (7.5)41.6 (5.5)71.5 (5.0)47.0 (5.9)
       Louisiana61.8 (6.2)36.2 (4.9)62.7 (4.9)40.5 (5.2)
       New Mexico70.8 (6.2)45.9 (6.4)73.3 (5.1)51.2 (5.2)
       Oklahoma58.6 (6.4)28.0 (5.1)62.3 (5.4)41.6 (5.4)
      Statistically significant difference compared to the 2012–2013 influenza season estimate. HHS, Health and Human Services.
       Texas66.4 (4.2)40.7 (3.9)72.1 (3.8)
      Statistically significant difference compared to the 2012–2013 influenza season estimate. HHS, Health and Human Services.
      46.2 (5.9)
      Region 762.5 (3.1)39.2 (2.9)66.2 (2.8)45.3 (2.9)
      Statistically significant difference compared to the 2012–2013 influenza season estimate. HHS, Health and Human Services.
       Iowa65.5 (6.2)40.5 (6.1)66.4 (6.2)45.2 (6.4)
       Kansas56.2 (5.5)33.1 (4.8)68.6 (5.1)
      Statistically significant difference compared to the 2012–2013 influenza season estimate. HHS, Health and Human Services.
      48.1 (5.7)
      Statistically significant difference compared to the 2012–2013 influenza season estimate. HHS, Health and Human Services.
       Missouri61.7 (5.7)38.9 (5.4)62.3 (4.9)41.7 (4.7)
       Nebraska69.7 (5.9)47.1 (5.7)75.0 (4.6)52.0 (5.5)
      Region 865.5 (3.0)42.0 (3.1)65.6 (2.8)47.1 (2.8)
      Statistically significant difference compared to the 2012–2013 influenza season estimate. HHS, Health and Human Services.
       Colorado68.0 (4.7)43.2 (5.2)71.6 (4.4)54.6 (4.3)
      Statistically significant difference compared to the 2012–2013 influenza season estimate. HHS, Health and Human Services.
       Montana59.1 (6.2)36.9 (5.3)63.1 (6.7)43.5 (6.6)
       North Dakota68.9 (5.9)45.7 (6.1)66.2 (5.1)46.5 (4.8)
       South Dakota81.6 (9.5)51.7 (7.4)70.5 (6.4)50.0 (6.2)
       Utah60.8 (5.9)40.0 (6.0)58.9 (5.5)39.8 (6.8)
       Wyoming51.0 (7.3)30.8 (6.6)50.2 (7.9)33.5 (6.6)
      Region 964.8 (4.2)36.5 (3.9)69.0 (4.7)47.5 (5.6)
      Statistically significant difference compared to the 2012–2013 influenza season estimate. HHS, Health and Human Services.
       Arizona60.7 (5.9)36.1 (5.7)59.3 (6.1)36.6 (6.4)
       California65.6 (5.2)36.0 (4.8)71.3 (5.8)50.2 (7.0)
      Statistically significant difference compared to the 2012–2013 influenza season estimate. HHS, Health and Human Services.
       Hawaii68.2 (9.9)43.2 (9.5)73.9 (7.7)49.0 (7.5)
       Nevada61.2 (5.4)38.6 (5.9)57.0 (5.0)35.1 (4.7)
      Region 1063.3 (4.7)35.9 (3.7)63.4 (3.8)41.8 (3.6)
      Statistically significant difference compared to the 2012–2013 influenza season estimate. HHS, Health and Human Services.
       Alaska55.0 (6.7)29.8 (5.2)61.7 (5.7)35.6 (5.3)
       Idaho53.9 (6.3)34.1 (6.0)53.0 (6.2)31.1 (5.2)
       Oregon52.3 (5.2)31.4 (4.4)61.6 (5.6)
      Statistically significant difference compared to the 2012–2013 influenza season estimate. HHS, Health and Human Services.
      43.3 (5.6)
      Statistically significant difference compared to the 2012–2013 influenza season estimate. HHS, Health and Human Services.
       Washington73.7 (7.7)39.9 (6.6)67.0 (6.3)44.2 (5.8)
      Source: National Immunization Survey-Flu (NIS-Flu), 2012–2013 and 2013–2014 influenza seasons.
      a Influenza vaccination coverage estimates were calculated using the Kaplan-Meier survival analysis procedure. For the 2012–2013 season, estimates include vaccinations received from July 2012 through May 2013 and are based on interviews conducted October 2012 through June 2013. For the 2013–2014 season, estimates include vaccinations received from July 2013 through May 2014 and are based on interviews conducted October 2013 through June 2014.
      b Any (≥1) dose defined as receipt of at least one dose of influenza vaccine.
      c Fully vaccinated was defined as either (1) receipt of two doses of influenza vaccine for children who were previously unvaccinated or did not receive a total of two or more doses of influenza vaccine from July 1, 2010, to the end of June 2012 for the 2012–2013 influenza seasons or June 2013 for the 2013–2014 influenza season, or (2) receipt of one dose of influenza vaccine otherwise.
      d Statistically significant difference compared to the 2012–2013 influenza season estimate. HHS, Health and Human Services.
      In bivariate analyses, full vaccination status varied by sociodemographic characteristics (Table 2). In both seasons, the percentage of fully vaccinated children aged 6–23 months was lower than those of children aged 2–4 years and 5–8 years. Hispanic, non-Hispanic white, and other/multiple race children had higher full vaccination coverage than non-Hispanic black children. Children of mothers who were college graduates had higher full vaccination coverage than children of mothers with lower education levels. Children living in households above poverty with a household income >$75,000 had the highest full vaccination coverage. Children in households with two children had higher full vaccination coverage than children in households with one or three or more children and children living in a non-MSA had lower full vaccination coverage than children living in an MSA. Among children who needed only one dose to be fully vaccinated, 90.6% in 2012–2013 and 93.2% in 2013–2014 received one or more doses and therefore were fully vaccinated. Among children who needed two doses to be fully vaccinated, the one or more dose coverage rate was 50.9% in 2012–2013 and 48.9% in 2013–2014, whereas the corresponding full vaccination coverage rates were 10.7% and 10.9%. Additional details of the associations with full coverage as well as associations with one or more dose coverage are shown in Table 2.
      Results of the multivariable Cox proportional hazard models are shown in Table 4 and were similar to results from the bivariate analyses. Generally, the models showed that the children most likely to be fully vaccinated were those who were aged >6–23 months, non-black, had a mother with an education >12 years, or lived in a high-income household (Table 4).
      Table 4Demographic Characteristics Associated With Full Influenza Vaccination Coverage Among Children 6 Months Through 8 Years
      Demographic characteristicsFully vaccinated,
      Fully vaccinated was defined as either (1) receipt of two doses of influenza vaccine for children who were previously unvaccinated or did not receive a total of two or more doses of influenza vaccine from July 1, 2010, to the end of June 2012 for the 2012–2013 influenza seasons or June 2013 for the 2013–2014 influenza season, or (2) receipt of one dose of influenza vaccine otherwise.
      HR (95% CI)
      2012–2013 influenza season2013–2014 influenza season
      Gender
       Male1.04 (0.98, 1.11)0.99 (0.93, 1.05)
       Femalerefref
      Age
      Age as of November 1 of the respective influenza season.
       6–23 monthsrefref
       2–4 years1.50 (1.37, 1.64)1.52 (1.40, 1.65)
       5–8 years1.41 (1.30, 1.54)1.49 (1.38, 1.60)
      Child’s race/ethnicity
       Hispanic1.27 (1.11, 1.44)1.35 (1.20, 1.53)
       Black, non-Hispanicrefref
       White, non-Hispanic1.22 (1.09, 1.36)1.21 (1.10, 1.35)
       Other/multiple races, non-Hispanic1.21 (1.05, 1.40)1.25 (1.11, 1.42)
      Mother’s education level
       <12 yearsrefref
       12 years1.11 (0.96, 1.28)0.88 (0.77, 1.00)
       >12 years, not college graduate1.16 (1.00, 1.34)0.98 (0.85, 1.13)
       College graduate1.44 (1.25, 1.65)1.34 (1.18, 1.52)
      Poverty status
      Income/poverty level was defined based on total family income in the past calendar year, and the U.S. Census poverty thresholds for that year specified for the applicable family size and number of children <18 years. Poverty thresholds are available at www.census.gov/hhes/www/poverty/data/threshld/index.html. HR, hazard ratio; MSA, Metropolitan Statistical Area; No., number.
       Above poverty, >$75,000/year1.28 (1.18, 1.39)1.31 (1.22, 1.41)
       Above poverty, ≤$75,000/yearrefref
       At or below poverty level1.02 (0.91, 1.14)1.05 (0.94, 1.17)
       Unknown0.91 (0.80, 1.04)0.99 (0.88, 1.11)
      No. of children <18 in household
       1 child1.01 (0.92, 1.10)0.99 (0.92, 1.07)
       2 children1.13 (1.04, 1.22)1.12 (1.04, 1.20)
       ≥3 childrenrefref
      Household in MSA
       MSA, principal city1.08 (0.98, 1.18)1.07 (0.97, 1.17)
       MSA, not principal city1.16 (1.06, 1.26)1.06 (0.98, 1.15)
       Non-MSArefref
      Source: National Immunization Survey-Flu (NIS-Flu), 2012–2013 and 2013–2014 influenza seasons.
      Note: Boldface indicates statistical significance (p<0.05) compared to the referent group.
      a Fully vaccinated was defined as either (1) receipt of two doses of influenza vaccine for children who were previously unvaccinated or did not receive a total of two or more doses of influenza vaccine from July 1, 2010, to the end of June 2012 for the 2012–2013 influenza seasons or June 2013 for the 2013–2014 influenza season, or (2) receipt of one dose of influenza vaccine otherwise.
      b Age as of November 1 of the respective influenza season.
      c Income/poverty level was defined based on total family income in the past calendar year, and the U.S. Census poverty thresholds for that year specified for the applicable family size and number of children <18 years. Poverty thresholds are available at www.census.gov/hhes/www/poverty/data/threshld/index.html.HR, hazard ratio; MSA, Metropolitan Statistical Area; No., number.

      Discussion

      The results of this study indicate that only approximately two of every five children aged 6 months–8 years in the U.S. were fully vaccinated against influenza in the two studied influenza seasons. Approximately 60% of these children required two doses of influenza vaccine to be considered fully vaccinated against influenza. Among those children who required two doses, about half received at least one dose in 2012–2013 and 2013–2014, but only about 10% were fully vaccinated with two doses. The finding that approximately half of children who required two doses of influenza vaccine received their first but not second dose suggests there are barriers to receipt of the second required dose. These findings are similar to what was found based upon a study using provider-reported vaccination information, finding that for the subset of children aged 6–23 months requiring two doses and who received one dose, only 64% went on to receive their required second dose.
      • Santibanez T.A.
      • Grohskopf L.A.
      • Zhai Y.
      • Kahn K.E.
      Complete influenza vaccination trends for children six to twenty-three months.
      A study based on IIS data reported estimates of full influenza vaccination coverage stratified by age for the 2012–2013 and 2013–2014 seasons; these estimates were 46.8% and 47.0% for children aged 6–23 months, 38.9% and 38.8% for children aged 2–4 years, and 30.9% and 32.7% for children aged 5–8 years, respectively.

      AIRA 2016 IIS National Meeting. Trends in compliance with two-dose influenza vaccine recommendations in young children, 2010–2015. www.immregistries.org/resources/iis-meetings/Trends_in_Compliance_with_Two-dose_Influenza_Vaccine_Recommendations_in_Young_Children_2010-2015.pdf. Published April 6, 2016. Accessed July 22, 2016.

      These IIS estimates are higher than the estimates from this study for children aged 6–23 months but lower for children aged 2–4 years and 5–8 years. Additionally, this IIS study found that for the subset of children aged 6–23 months requiring two doses and who received one dose, 65.6% (in 2012–2013) and 67.6% (in 2013–2014) went on to receive their required second dose.

      AIRA 2016 IIS National Meeting. Trends in compliance with two-dose influenza vaccine recommendations in young children, 2010–2015. www.immregistries.org/resources/iis-meetings/Trends_in_Compliance_with_Two-dose_Influenza_Vaccine_Recommendations_in_Young_Children_2010-2015.pdf. Published April 6, 2016. Accessed July 22, 2016.

      Barriers to receipt of the second dose might include inconvenience, difficulty making a doctor appointment, insurance/financial barriers, difficulty getting time off of work, lack of provider–parent discussions on the importance of returning for the child’s second dose, parental beliefs and attitudes about vaccine effectiveness and safety, or other unknown factors.
      • Hofstetter A.M.
      • Barrett A.
      • Stockwell M.S.
      Factors impacting influenza vaccination of urban low-income Latino children under nine years requiring two doses in the 2010-2011 season.
      Children aged 6–23 months had lower full influenza vaccination coverage than older children. Preventing influenza among infants and young children is a public health priority because of their high risk for influenza-related complications.
      Centers for Disease Control and Prevention (CDC)
      Prevention and control of influenza: recommendations of the Advisory Committee on Immunization Practices (ACIP).
      Centers for Disease Control and Prevention (CDC)
      Prevention and control of influenza: recommendations of the Advisory Committee on Immunization Practices (ACIP).
      The second dose is essential, as evidenced by at least one study, which found that receipt of two doses of influenza vaccine was 69% effective against office visits for influenza-like illness and 87% effective against office visits for influenza-related pneumonia, whereas receipt of one dose did not prevent office visits at all for children aged 6–21 months.
      • Allison M.A.
      • Daley M.F.
      • Crane L.A.
      • et al.
      Influenza vaccine effectiveness in healthy 6- to 21-month-old children during the 2003-2004 season.
      More widespread use of strategies known to increase second dose vaccination rates is warranted.

      Guide to Community Preventive Services. Increasing appropriate vaccination. www.thecommunityguide.org/vaccines/index.html. Accessed May 5, 2016.

      One study found that text reminders that educated the parent on the importance of the second dose were effective at increasing the percentage of children who received their needed second dose.
      • Stockwell M.S.
      • Hofstetter A.M.
      • DuRivage N.
      • et al.
      Text message reminders for second dose of influenza vaccine: a randomized controlled trial.
      This study found racial/ethnic differences in full vaccination coverage that were in contrast to one or more dose coverage. Non-Hispanic black children had lower full vaccination coverage compared to all of the other race/ethnicity groups in both the 2012–2013 and 2013–2014 seasons; however, black and white children had similar one or more dose coverage. This suggests there may be more barriers for black children to receive their second dose. Another recent study of full vaccination coverage of children aged 6–23 months based upon provider report likewise found that black children had lower full vaccination coverage than white children for all ten studied influenza seasons.
      • Santibanez T.A.
      • Grohskopf L.A.
      • Zhai Y.
      • Kahn K.E.
      Complete influenza vaccination trends for children six to twenty-three months.
      Minority children have also been found to have a longer interval between first and second doses.
      • Hofstetter A.M.
      • Natarajan K.
      • Martinez R.A.
      • Rabinowitz D.
      • Vawdrey D.K.
      • Stockwell M.S.
      Influenza vaccination coverage and timeliness among children requiring two doses, 2004-2009.
      Children of mothers with a college degree and children living in households with a higher annual household income consistently had higher full vaccination coverage. Studies of other childhood vaccinations had likewise shown that lower family income was associated with lower vaccination rates, perhaps because of numerous barriers faced by lower-income families such as lack of access to medical care, transportation issues, difficulty obtaining time off of work, and other barriers.
      • Smith P.J.
      • Santoli J.M.
      • Chu S.Y.
      • Ochoa D.Q.
      • Rodewald L.E.
      The association between having a medical home and vaccination coverage among children eligible for the vaccines for children program.
      Centers for Disease Control and Prevention (CDC)
      National, state, and selected local area vaccination coverage among children aged 19–35 months—United States, 2014.
      Children living in an MSA had higher full vaccination coverage than children residing in non-MSAs, suggesting there might be additional barriers to children in non-MSAs receiving their second required dose. As seen in publications on one or more dose influenza vaccination coverage among children, large variability in full influenza vaccination coverage by state was also found.

      Centers for Disease Control and Prevention (CDC). Flu vaccination coverage, United States, 2012-13 influenza season. www.cdc.gov/flu/fluvaxview/coverage-1213estimates.htm. Published 2013. Accessed September 27, 2016.

      Centers for Disease Control and Prevention (CDC). Flu vaccination coverage, United States, 2013-14 influenza season. www.cdc.gov/flu/fluvaxview/coverage-1314estimates.htm. Published 2014. Accessed September 27, 2016.

      It is unknown why this variability exists, but factors likely include differences in programmatic and provider implementation of influenza recommendations, medical care delivery infrastructure, population norms, state and local immunization program efforts and resources, and other factors.
      Centers for Disease Control and Prevention (CDC)
      Interim results: state-specific seasonal influenza vaccination coverage—United States, August 2009-January 2010.
      • Srivastav A.
      • Zhai Y.
      • Santibanez T.A.
      • Kahn K.E.
      • Smith P.J.
      • Singleton J.A.
      Influenza vaccination coverage of Vaccine for Children (VFC)-entitled versus privately insured children, United States, 2011-2013.
      Further study is needed to understand the variability in full influenza vaccination coverage between states.

      Limitations

      This study is subject to the following limitations. First, NIS-Flu is a telephone survey and selection and non-response bias is possible and may remain after weighting adjustments. Second, the Council of American Survey Research Organizations response rate was low, especially for the cellular sample, and non-response bias may remain even after weighting adjustments. Third, influenza vaccination status was based on parental report and was not validated with medical records. Validity studies have shown that parents may overreport their child’s influenza vaccination status and this may vary by race/ethnicity, income, and other sociodemographic characteristics.
      • Brown C.
      • Clayton-Boswell H.
      • Chaves S.S.
      • et al.
      Validity of parental report of influenza vaccination in young children seeking medical care.
      An analysis of full influenza vaccination based upon provider report among children aged 6–23 months found that full influenza vaccination coverage was 42.9% in the 2012–2013 season, higher than the 36.8% found in this paper based on parental report.
      • Santibanez T.A.
      • Grohskopf L.A.
      • Zhai Y.
      • Kahn K.E.
      Complete influenza vaccination trends for children six to twenty-three months.
      Although parent report may not be equally valid as provider report, estimates based on parental report can be obtained for one influenza season before the start of the next, and have been routinely reported since 2009.

      Centers for Disease Control and Prevention (CDC). FluVaxView. www.cdc.gov/flu/fluvaxview/. Accessed May 5, 2016.

      Fourth, the 2013–2014 NIS-Flu questionnaire did not allow the precise measure of full influenza vaccination because the survey asked for vaccination history since July 1, 2011, rather than 2010. This likely led to an overestimation of the number of children who needed a second dose of influenza vaccine and resulted in an underestimation of full vaccination coverage rates for the 2013–2014 influenza season. Additionally, using only one ACIP approach and treating missing responses to vaccination history as needing two doses may have also resulted in underestimation of full vaccination coverage. Fifth, when evaluating the proportional hazards assumption for the Cox model, age group did not satisfy the assumption but was kept in the final Cox regression because it was a primary variable of interest. The literature suggests that the proportional hazards model is robust to departures from this assumption.
      • Andersen P.K.
      • Gill R.D.
      Cox Regression-Model for counting-processes—a large sample study.
      • Binder D.A.
      Fitting cox proportional hazards models from survey data.
      • Lin D.Y.
      On fitting Cox’s proportional hazards models to survey data.

      Conclusions

      This study indicates that the percentage of children aged 6 months–8 years who are fully vaccinated against influenza remains low. These results demonstrate the need for continued emphasis of the importance of younger children receiving their second influenza dose when it is needed. Targeted interventions based on sociodemographic factors associated with low coverage, interventions recommended by the Community Guide such as expanded access in healthcare settings, intensive outreach and home visits, client reminder and recall systems, and client or community-wide education can help improve coverage.

      Guide to Community Preventive Services. Increasing appropriate vaccination. www.thecommunityguide.org/vaccines/index.html. Accessed May 5, 2016.

      Acknowledgments

      The findings and conclusions in this report are those of the authors and do not necessarily represent the official position of the Centers for Disease Control and Prevention.
      YZ and TAS conceived the study; YZ wrote the first draft of the manuscript and led revisions of all subsequent versions. YZ had access to all data and takes responsibility for their integrity. YZ also performed the statistical analyses. TAS, KEK, and AS participated in data interpretation and writing of the manuscript, and advised on the data analysis. All authors have reviewed and approved the submitted version of the manuscript.
      No financial disclosures were reported by the authors of this paper.

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