Final estimates for 2009–10 Seasonal Influenza and Influenza A (H1N1) 2009 Monovalent Vaccination Coverage – United States, August 2009 through May, 2010.

Because the novel influenza A (H1N1) virus [2009 influenza A (H1N1)] was identified in April 2009, two separate influenza vaccines were distributed in the United States in 2009–10 season: a seasonal vaccine and an influenza A (H1N1) 2009 monovalent vaccine (2009 H1N1 vaccine) [1, 2]. To estimate national and state level influenza vaccination coverage from August 2009 through May 2010, CDC used combined data from the Behavioral Risk Factor Surveillance System (BRFSS) and the National 2009 H1N1 Flu Survey (NHFS). This report describes national findings from these surveys and updates previously published interim 2009–10 seasonal [3] and 2009 H1N1 [4] influenza vaccination coverage estimates. The interim estimates were based on vaccinations reported through the end of January 2010 using interviews conducted through February 2010. The final estimates in this report include vaccinations reported through May 2010 based on interviews through June 2010.

Key Findings

Seasonal Influenza Vaccination Coverage

• National 2009–10 seasonal influenza vaccination coverage among all persons aged ≥6 months in the United States was 41.2% (95% confidence interval [CI] 40.8–41.6%). (Table 1)
• Among children aged 6 months –17 years, national seasonal influenza vaccination coverage [43.7% (95% CI 42.8–44.6)] was higher than coverage for adults ≥18 years [40.4% (95% CI 40.0–40.8%)].
• For adults aged 18—49 years, national seasonal influenza vaccination coverage was higher for persons with high–risk conditions [38.2% (95% CI 36.9–39.5%)] compared to those without high–risk conditions [28.4% (95% CI 27.8–29.0%)].
• Nationally, seasonal influenza vaccination coverage was highest among adults aged ≥65 years [69.6% (95% CI 69.0–70.2%)]. Among adults aged 50–64 years, national seasonal influenza vaccination coverage was 45.0% (95% CI 44.4–45.6%).
• For children aged 6 months –17 years, healthy adults aged 18–49 years, adults aged 18–49 years with high–risk conditions and adults aged ≥65 years, 2009–10 final estimates were higher than estimates for the same groups in the 2008–09 season [2008–09 estimates: 30.2% (95% CI 28.4–32.0%), 19.5% (95% CI 18.2–20.8%), 33.0% (95% CI 29.8–36.5%) and 65.6% (95% CI 63.3–67.8%) respectively; CDC, unpublished data, 2009 National Health Interview Survey (NHIS)].
• One or more seasonal influenza vaccine doses were administered to 31.6 million (95% CI 30.9–32.3 million) children and 91.6 million (95% CI 90.7–92.5 million) adults, for an estimated 123.3 million (95% CI 122.1–124.5 million) seasonal influenza vaccines during August 2009 through May 2010.

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^* Coverage estimates are for persons with reported vaccination during August 2009—May 2010 who were interviewed during October 2009—June 2010.

^† Excludes U.S. territories

^§ Month of vaccination was imputed for respondents with missing month of vaccination data. Percentages are weighted to the U.S. population.

^¶ Confidence interval

^†† High risk includes asthma, other lung problems, diabetes, heart disease, kidney problems, anemia, weakened immune system caused by a chronic illness or by medicines taken for a chronic illness.

Influenza A (H1N1) 2009 Monovalent Vaccination Coverage

• Nationally, among all persons aged ≥6 months, 2009 H1N1 vaccination coverage was 27.0% (95% CI 26.6–27.4%). (Table 2)
• Among children aged 6 months — 17 years, national 2009 H1N1 vaccination coverage was 40.2% (95% CI 39.4–41.0%) and was higher compared to corresponding level among adults aged ≥18 years [22.7% (95% CI 22.3–23.1%)].
• For persons in the initial ACIP target groups, national 2009 H1N1 vaccination coverage was 34.2% (95% CI 33.6–34.8%). Among the subset of adults aged 25–64 years with high–risk conditions included in the target group, national coverage was 28.6% (95% CI 27.5–29.7%).
• Among persons not in the initial target groups, national coverage was significantly higher in adults aged ≥65 years [28.8% (95% CI 28.1–29.5%)] compared to adults 25–64 years [16.7% (95% CI 16.1–17.3%)].
• One or more doses of 2009 H1N1 vaccine were administered to 29.1 million children (95% CI 28.5–29.7 million) and 51.5 million adults (95% CI 50.6–52.4 million), for an estimated 80.8 million (95% CI 79.6–82.0 million) 2009 H1N1 vaccinees during October 2009 through May 2010.

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^* Coverage estimates are for persons with reported vaccination during October 2009—May 2010 who were interviewed during November 2009—June 2010.

^† Excludes U.S. territories

^§ Month of vaccination was imputed for respondents with missing month of vaccination data. Percentages are weighted to the U.S. population.

^¶ Confidence interval

^** Pregnant women, persons who live with or provide care for infants aged <6 months, health–care and emergency medical services personnel, children and young adults aged 6 months—24 years, and persons aged 25—64 years who have medical conditions that put them at higher risk for influenza–related complication

^†† High risk includes asthma, other lung problems, diabetes, heart disease, kidney problems, anemia, weakened immune system caused by a chronic illness or by medicines taken for a chronic illness.

Summary and Public Health Implications

Vaccination is the single most effective strategy for protection from influenza virus infection and its complications. For the 2009–10 season, trivalent influenza vaccination was recommended by the ACIP for all children aged 6 months—18 years, persons 18–49 years with high–risk conditions, persons 50–64 years and persons 65 years and over in the United States [2]. The higher overall estimated seasonal influenza vaccination levels for this unique season compared to previous seasons may be attributable to increased demand for the seasonal vaccine due to early concerns about the severity of the 2009 H1N1 pandemic and delay in availability of the 2009 H1N1 vaccine, increased media attention, and the collective efforts of CDC, state departments of health, immunization program managers and numerous other stakeholders. The expansion of ACIP influenza vaccination recommendations to include all children aged 6 months—18 years also likely contributed to the increased level of coverage reached in children compared to previous years. The coverage reported here somewhat overestimates actual vaccination coverage, since the estimated number of seasonal vaccine doses that would have been utilized to achieve this level of coverage exceeds the number of seasonal doses distributed (approximately 114 million).

Distribution of the influenza A (H1N1) 2009 monovalent vaccine began in the United States in October 2009 [5]. This vaccine was recommended by the ACIP for five initial target groups – pregnant women, persons who live with or provide care for infants aged <6 months, health–care and emergency medical services personnel, children and young adults aged 6 months—24 years, and persons aged 25—64 years who have medical conditions that put them at higher risk for influenza–related complications [1]. By January 2010, approximately 124 million doses of 2009 H1N1 vaccine had been distributed [4]. By May 31 2010, an estimated 90 doses had been administered. The gap between doses distributed and administered may have been the result of the late availability of the 2009 H1N1 vaccine relative to onset of the pandemic. Influenza activity due to 2009 pandemic influenza A (H1N1) virus peaked in October 2009, before sufficient 2009 H1N1 vaccine was available for widespread vaccination. The 2009 pandemic influenza A (H1N1) was associated with a threefold higher level of hospitalizations and deaths among persons aged <65 years compared with previous influenza seasons [6]. Among children aged 6 months—17 years, national 2009 H1N1 vaccination levels were substantially higher than previous pediatric seasonal influenza vaccination levels [7] and the 30.2% (95% CI 28.4–32.0%) attained for the 2008—09 season (CDC, unpublished NHIS data, 2009). The relatively high 2009 H1N1 vaccination coverage among children (compared to other age groups in the 2009–10 season and compared to previous seasons) is likely a reflection of the focus in many states on childhood vaccination, the use of school–located vaccination clinics in some states, recognition by providers and parents of the risk for severe outcomes among children, and other factors.

Considering both 2009 H1N1 and seasonal influenza vaccines, a record number of influenza vaccinations were administered in the 2009–10 influenza season. The Advisory Committee on Immunization Practices now recommends universal influenza vaccination for all persons aged ≥6 months in the U.S [8]. Yearly monitoring of influenza vaccination coverage is essential to targeting outreach efforts and attaining optimal coverage across all age groups and states in the United States.

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Data Source and Methods

CDC used combined data from the 2009—10 Behavioral Risk Factor Surveillance System (BRFSS) and the National 2009 H1N1 Flu Survey (NHFS) for these analyses [9]. The 2009—10 BRFSS included respondents from 50 states, the District of Columbia, and two territories. For national estimates presented in this report, data from the two territories are excluded. The NHFS included respondents from all 50 states and the District of Columbia (DC). BRFSS respondents were asked if they (or their children, in 43 states and DC) had been vaccinated for the “seasonal flu” in the past 12 months or if they (or their children, in 46 states and DC) had been vaccinated for “H1N1 flu” since September, and if so, in which month. NHFS respondents aged ≥18 years were asked whether they (or their children) had received “seasonal flu vaccination” since August 2009 or “H1N1 flu” vaccination since September 2009 and if so, in which month. Detailed data collection methods for BRFSS and NHFS have been reported previously [3, 4]. The Council of American Survey and Research Organizations (CASRO) NHFS response rates were 34.0% for landline and 25.5% for cellular telephones; cooperation rates were 45% and 57%, respectively. The median state CASRO BRFSS response and cooperation rates were 54% and 76%, respectively.

The cumulative proportion of persons vaccinated with at least 1 dose of the seasonal influenza vaccine and at least 1 dose of the 2009 H1N1 vaccine was determined using the Kaplan–Meier survival analysis procedure on monthly data collected from NHFS and BRFSS during October 2009—June 2010 for the seasonal vaccine and November–June 2010 for the 2009 H1N1 vaccine. To improve precision for subgroups within states, particularly for children, the estimates from BRFSS and NHFS were combined. For participants who indicated they had been vaccinated but had a missing month and year of vaccination, the month and year of vaccination was imputed based on donor pools matched for week of interview, age group, state of residence and race/ethnicity. National and state level vaccination coverage levels were examined by selected age and priority subgroups. Final estimates were compared to estimates from the 2008–09 season using student t–tests. Results from both surveys (NHFS and BRFSS) were weighted and analyzed with SAS and SUDAAN statistical software to account for the complex survey design.

Overall, final estimates in this report, based on an additional four months of interviews, are consistent with previously published interim 2009–10 season estimates [3, 4]. Final estimates tended to be several percentage points higher, consistent with inclusion of a broader vaccination period extending through May 2010.

The findings in this report are subject to certain limitations [3, 4]. Self–reported vaccination status was not validated with medical records and is subject to recall bias. As reported previously, the estimated number of persons receiving seasonal influenza vaccination is an overestimate since the estimated number of persons receiving seasonal influenza vaccination exceeds the number of doses distributed (approximately 114 million) in the United States [3]. Misclassification of 2009 H1N1 vaccine for seasonal influenza vaccine, unique to this season, might have contributed to some over–reporting. Response rates for both surveys were low, which increases the risk of non–response bias. In addition, both BRFSS and NHFS are telephone–based surveys and thus do not include persons without telephone service. BRFSS estimates did not include households with only cellular telephone service. Higher coverage observed for seasonal vaccination in 2009–10 based on combined BRFSS and NHFS data compared to 2008–09 season estimates based on 2009 NHIS may reflect, to some extent, differences in survey methodology (e.g., face–to–face vs. telephone interview mode, inclusion of households in NHIS regardless of telephone status, and higher response rates in NHIS).

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References

1. Use of influenza A (H1N1) 2009 monovalent vaccine: recommendations of the Advisory Committee on Immunization Practices (ACIP), 2009. MMWR Recomm Rep, 2009. 58(RR–10): p. 1–8.
2. Fiore, A.E., et al., Prevention and control of seasonal influenza with vaccines: recommendations of the Advisory Committee on Immunization Practices (ACIP), 2009. MMWR Recomm Rep, 2009. 58(RR–8): p. 1–52.
3. Interim results: state–specific seasonal influenza vaccination coverage – United States, August 2009–January 2010. MMWR Morb Mortal Wkly Rep, 2010. 59(16): p. 477–84.
4. Interim results: state–specific influenza A (H1N1) 2009 monovalent vaccination coverage – United States, October 2009–January 2010. MMWR Morb Mortal Wkly Rep, 2010. 59(12): p. 363–8.
5. Interim results: influenza A (H1N1) 2009 monovalent vaccination coverage — United States, October–December 2009. MMWR Morb Mortal Wkly Rep, 2010. 59(2): p. 44–8.
6. Update: influenza activity – United States, August 30, 2009–March 27, 2010, and composition of the 2010–11 influenza vaccine. MMWR Morb Mortal Wkly Rep, 2010. 59(14): p. 423–30.
7. Influenza vaccination coverage among children and adults – United States, 2008–09 influenza season. MMWR Morb Mortal Wkly Rep, 2009. 58(39): p. 1091–5.
8. Fiore, A.E., et al., Prevention and control of influenza with vaccines: recommendations of the Advisory Committee on Immunization Practices (ACIP), 2010. MMWR Recomm Rep, 2010. 59(RR–8): p. 1–62.
9. Schenker, N. and T.E. Raghunathan, Combining information from multiple surveys to enhance estimation of measures of health. Stat Med, 2007. 26(8): p. 1802–11.

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