(Granoff et al., 1984). Four cases of thrombocytopenia have been reported through VAERS, and all subjects received other vaccines including a live viral vaccine, MMR. MMR is associated with thrombocytopenia (see Chapter 6). An additional report in VAERS described an event resembling hemolytic-uremic syndrome following immunization for Hib, diphtheria, tetanus, and pertussis (DPT), and polio (OPV). One case of ITP and one case of hemolytic-uremic syndrome were reported to the FDA following immunization with PRP vaccine (Milstien et al., 1987). No information regarding other medications or illnesses in the child with ITP was provided in the summary report. Most cases of hemolytic-uremic syndrome are now known to follow enteritis caused by intestinal pathogens that produce verotoxins (shigellalike toxins) (Centers for Disease Control, 1991b; Rowe et al., 1991). Both patients with hemolytic-uremic syndrome associated with Hib vaccine administration (noted above) presented with diarrhea. Thus, it is most likely that these were related to enteritis caused by verotoxin-producing Escherichia coli or Shigella dysenteriae rather than the Hib vaccines, which contain no toxins.
The evidence is inadequate to accept or reject a causal relation between Hib vaccines and thrombocytopenia.
Early-onset Hib disease following immunization is defined as a case of serious systemic infection caused by Hib that occurs within the 7-day interval following immunization for Hib. The annual incidence of Hib has decreased dramatically with the introduction of Hib vaccines.
In conducting vaccine efficacy trials for Hib vaccines, investigators expected to observe a number of vaccine "failures" (i.e., none of the Hib vaccines was likely to be 100 percent effective in the prevention of disease, particularly after administration of a single dose to young infants). For calculation of efficacy, "immunization" was usually defined as receipt of vaccine more than 14 days prior to the onset of disease; 14 days was considered a reasonable time period for development of a protective antibody response. In studies of vaccine efficacy for the unconjugated PRP vaccine, a wide range of estimates of vaccine efficacy was observed in various locations in the United States (Black et al., 1988; Harrison et al., 1988; Osterholm