Hib vaccination coverage previously overestimated

CDC. MMWR. 2010;59:1069-1072.

Before 2009, the National Immunization Survey deemed children who received at least three doses of any Haemophilus influenzae type b-containing vaccine as fully immunized. Certain vaccine product types, however, require four doses to be effective, so clinicians must be aware of the product type before considering a patient fully vaccinated, CDC researchers warned.

In January 2009, the National Immunization Survey (NIS) began requesting data from providers on product type and created two coverage measures: completion of a primary Hib vaccine series and full vaccination — completion of the primary series in addition to a booster dose.

To examine the effects of the new coverage measures on Hib vaccination reporting, CDC researchers compared NIS data using the new measure from the first half of 2009 with the same data using the previous measure for full vaccination.

The researchers included 8,122 children aged 19 to 35 months with survey responses and health care provider records from the 2009 NIS in their analysis. All were born between Jan. 9, 2006, and Dec. 19, 2007.

According to the new measures, 92.9% of children in the sample completed the primary Hib series, and 56.9% were fully vaccinated. Using the previous measure, a substantially higher number of children — 82.9% — were considered fully vaccinated, the researchers said.

The proportion of fully vaccinated children fluctuated between states, ranging from 32.6% in West Virginia to 82.7% in Alaska, when data were interpreted using the new measure. The amount of children completing the primary Hib series also varied and ranged from 82.3% in Montana to 98% in Alabama.

According to the old measures, the number of fully vaccinated children ranged from 50.9% in Rhode Island to 97.7% in New Hampshire.

“These data show that the previous method of measuring Hib vaccination coverage (three or more doses of a Hib-containing vaccine) underestimated coverage with the primary series but overestimated coverage with the primary series and booster,” the researchers wrote. “Use of vaccine product type information indicated a wide range of coverage with the primary series and booster among the states.”

The researchers noted, however, that a December 2007 recall of Merck’s Hib-containing vaccines — PedvaxHIB and Comvax — created a vaccine shortage that lasted through 2009. Subsequent interim recommendations to delay series completion if a child had received one of the Merck vaccines created dissonance among the data, as children who received Sanofi-Pasteur’s ActHIB could complete vaccination as scheduled.

Fifty-two percent of children in the study sample were younger than 12 months at the start of the shortage and were therefore less likely to be fully vaccinated.