The Influence of Inaccurate Vaccine Brand Identification On the Analysis of Hib Vaccination Coverage Rates From Immunization Information System Data

Background: During 2008, five children ages 5 months to 3 years were reported to the Minnesota Health Department (MDH) with invasive Hib disease; one died. Only one of these children had completed their primary Hib series. In December 2007, Merck & Co., Inc. recalled certain lots of their two Hib vaccines and ceased production of both vaccines.  As a result, only one hib vaccine manufacturer remained to maintain vaccine supply.  It was hypothesized that a shortage of vaccine resulted in lowered Hib vaccination rates leading to an increase in disease.   

Objectives: To provide a more accurate evaluation of Hib vaccine UTD status from immunization information system (IIS) data

Methods: Hib UTD status was evaluated at 9 months of age in two birth cohorts:   11/1/2006 – 3/31/2007 (pre-shortage) and  11/01/2007 – 3/31/2008 (post -shortage). Children having two Merck vaccines at 2 and 4 months were considered up-to-date whereas children having other brands of Hib vaccine needed 3 doses to be considered up-to-date. A second analysis re-assigned the proportion of children with missing brand data to UTD status based on the proportions of Merck vs. non-Merck vaccines distributed by the Vaccines for Children (VFC) program during the respective time periods.

Results: Post-shortage UTD status at 9 months was 62% for PCV, 63% for DTaP, and 53% for Hib.  However, pre-shortage UTD status at 9 months was similar at 66% for PCV, 67% for DTaP, and 56% for Hib. After reassignment of children with missing brand information as UTD based on VFC Merck distributions, the post-shortage Hib UTD rate was  53% the pre-shortage Hib UTD rate was 67%.

Conclusions: Reassignment of UTD status after applying VFC distribution data to the doses missing brand information showed that it was likely that the Hib UTD rates dropped 14% during the year after the shortage.