Changes in Measurement of Haemophilus influenzae serotype b (Hib) Vaccination Coverage --- National Immunization Survey, United States, 2009

The National Immunization Survey (NIS) has introduced a new method for measuring Haemophilus influenzae serotype b (Hib) vaccination coverage. Since its inception in 1994, NIS has considered a child aged 19--35 months to be fully vaccinated with Hib vaccine if the child had received 3 or more doses of any Hib-containing vaccine (3+Hib), regardless of vaccine product type received. However, for some Hib vaccine product types, 4 doses are needed to be fully vaccinated. Because NIS data have not distinguished between Hib vaccine product types, a child who received 3 doses of a vaccine product that requires 4 doses was misclassified as fully vaccinated. Since January 2009, NIS has requested that vaccination providers report data on Hib vaccine product types. Using this new information, two new measures were created: 1) primary series completed and 2) fully vaccinated (primary series completed plus booster dose). To determine the effects of the new method, CDC used preliminary data from the first half of 2009 NIS to compare the new measures with the previous 3+Hib measure. The findings, which were influenced by a nationwide shortage of certain Hib vaccine types, indicated that 92.9% of children aged 19--35 months in the United States had received the primary Hib series according to interim recommendations of the Advisory Committee on Immunization Practices (ACIP), and 56.9% were fully vaccinated. Using the previous method, 82.9% were fully vaccinated (3+Hib). When interpreting Hib vaccination coverage estimates and analyzing trends with NIS Hib vaccination coverage data in the future, analysts and state immunization programs should be aware of this change in Hib measurement.

Before 2009, two manufacturers produced Hib vaccines licensed for children in the United States: Merck & Co., Inc. (West Point, Pennsylvania) and Sanofi Pasteur (Swiftwater, Pennsylvania). Merck's Hib vaccines require a 2-dose primary series with doses at ages 2 months and 4 months and a booster at age 12--15 months. Sanofi Pasteur's Hib vaccines require a 3-dose primary series at ages 2, 4, and 6 months and a booster at age 12--15 months. Before January 2009, NIS did not request Hib vaccine product type and considered fully vaccinated with Hib vaccine all children who had received 3 or more doses of any Hib-containing vaccine.

In December 2007, Merck recalled several lots of two types of Hib conjugate vaccine and temporarily suspended production of both products, leading to a shortage of Hib conjugate vaccines during December 2007--September 2009. The recalled vaccines were PedvaxHIB, a monovalent Hib vaccine, and Comvax, a Hib-hepatitis B (HepB) combination vaccine. Because of the limited Hib vaccine supply, ACIP recommended temporary suspension of the booster dose for most children in the United States* (1). This revised recommendation underscored an NIS measurement problem; a child who received the 2-dose primary series with a Merck product in accordance with the revised recommendation would not be counted as fully vaccinated according to the 3+Hib measure, whereas a child who received a 3-dose primary series with a Sanofi Pasteur product in accordance with the revised recommendation would be counted as fully vaccinated according to the 3+Hib measure.

NIS is an ongoing, national survey used to estimate vaccination coverage in the United States among children aged 19--35 months in the 50 states and selected local areas and, beginning in 2009, in the U.S. Virgin Islands. NIS is a random-digit-dialed telephone survey of households with children aged 19--35 months at the time of interview. The household telephone survey is followed by the Immunization History Questionnaire,† which is mailed to a child's vaccination providers, if permission is granted by the parent or guardian. Beginning in January 2009, the names of the Hib vaccine product types were added to the Immunization History Questionnaire§ (2).

With the addition of vaccine product type information, two new measures of Hib vaccination coverage were created: 1) vaccinated with the primary series and 2) vaccinated with the primary series and a booster dose (fully vaccinated). According to ACIP vaccination recommendations, children who receive a mixture of vaccine product types require 3 doses to complete the primary series (3). The two new measures are presented in this report and compared with the 3+Hib measure in reports published previously (4).

For this report, a subset of 2008--09 between-year NIS data was analyzed, consisting of children with interviews in 2009. Interviews for the entire between-year NIS were conducted from July 2, 2008 to August 11, 2009, based on the NIS samples drawn for the third quarter of 2008 through the second quarter of 2009. The household Council of American Research Organizations (CASRO) response rate for the between-year data was 63.3%; provider vaccination records were obtained for 70.4% (n = 18,032 children). Of these, 8,122 children with interviews conducted in the 50 states and District of Columbia during January--June 2009 and with adequate provider data reported by the end of October 2009 were selected for this analysis. This subset consisted of children born during January 9, 2006 through December 19, 2007; 52% of the children were younger than 12 months at the beginning of the Hib shortage and thus were more likely to have had their booster doses deferred. Data were weighted to adjust for households with multiple telephone lines, household nonresponse, and exclusion of households without landline telephones (4).

For some children and certain Hib vaccine doses, product type was not reported. For the estimate of the percentage completing the primary series, 0.9% of children were missing vaccine type information for their first or second Hib dose. For the estimate of the percentage completing the primary series and booster dose, 4.9% of children had missing vaccine type information. The two measures were estimated assuming the Hib vaccines with missing type information were a type requiring 3 doses for the primary series, thereby producing a conservative estimate of coverage.

Using the measures, among children aged 19--35 months overall in the United States, 92.9% received the primary Hib series according to interim ACIP recommendations, and 56.9% were fully vaccinated. Using the previous method, 82.9% were fully vaccinated (3+Hib) (Table). Among states, the percentage completing the primary Hib series ranged from 82.3% in Montana to 98.0% in Alabama, and the percentage fully vaccinated with Hib ranged from 32.6% in West Virginia to 82.7% in Alaska. Using the previous measure, the percentage fully vaccinated (3+Hib) ranged from 50.9% in Rhode Island to 97.7% in New Hampshire.

The Hib vaccine shortage of December 2007--September 2009 made evident the importance of accounting for product type in measuring Hib vaccination coverage. These data show that the previous method of measuring Hib vaccination coverage (3+Hib) underestimated coverage with the primary series but overestimated coverage with the primary series and booster. Use of vaccine product type information indicated a wide range of coverage with the primary series and booster among the states.

Ascertainment of product type for Hib in NIS was successful. Less than 5% of children had missing vaccine product types for determining Hib vaccination coverage for the primary series and booster. NIS also now requests that providers report vaccine product type for rotavirus vaccine (2 or 3 dose schedule, depending on type); rotavirus coverage will be routinely reported by NIS beginning in fall 2010.

In the future, routine reporting of national and state vaccination coverage levels will include the more valid measures of Hib status, based on product types. CDC also plans to use the full year of 2009 NIS data to evaluate 1) compliance with interim recommendations (deferring the booster dose) during the December 2007--September 2009 Hib vaccine shortage, 2) reasons for the wide range in coverage levels across states during the shortage period, and 3) the number of children receiving catch-up vaccinations with the deferred booster dose once the shortage resolved (5).

Reported by

TA Santibanez, PhD, JA Singleton, MS, A Shefer, MD, A Cohn, MD, National Center for Immunization and Respiratory Diseases, Office of Infectious Diseases, CDC.

References

1. CDC. Interim recommendations for the use of Haemophilus influenzae type b (Hib) conjugate vaccines related to the recall of certain lots of Hib-containing vaccines (PedvaxHIB and Comvax). MMWR 2007;56:1318--20.
2. CDC. Licensure of a Haemophilus influenzae type b (Hib) vaccine (Hiberix) and updated recommendations for use of Hib vaccine. MMWR 2009;58:1008--9.
3. CDC. Haemophilus b conjugate vaccines for prevention of Haemophilus influenzae type b disease among infants and children two months of age and older: recommendations of the ACIP. MMWR 1991;40(No. RR-1).
4. CDC. National, state, and local area vaccination coverage among children aged 19--35 months---United States, 2008. MMWR 2009;58:921--6.
5. CDC. Updated recommendations for use of Haemophilus influenzae type b (Hib) vaccine: reinstatement of the booster dose at ages 12--15 months. MMWR 2009;58:673--4.

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