that there was a ''possibility of permanent renal dysfunction and possible brain damage,'' but no follow-up information was available. The other four cases occurred in children who had received MMR in addition to the Hib vaccines (two children received HbOC vaccine, one child received PRP-D vaccine, and one child received an unspecified Hib vaccine). Thrombocytopenia developed 5 to 13 days after immunization, and all four patients recovered. Two of the children had only petechiae and purpura in association with the thrombocytopenia, one child had a high fever and maculopapular rash in addition to petechiae, and one child had fever, a maculopapular rash, erythema multiforme, petechiae, and purpura.

Milstien and coworkers (1987) published a summary of the adverse reactions reported to the U.S. Food and Drug Administration (FDA) during the first year after marketing of the PRP vaccine (1985-1986). During that time, 4.5 million doses of vaccine were sold. Included in that summary was one report of petechiae (platelet count not given), one case of idiopathic thrombocytopenic purpura (ITP) (platelet count, 15,000/mm3) diagnosed 18 days after immunization, and one child who developed a low platelet count associated with hemolytic-uremic syndrome that began 2 days after receiving PRP vaccine.

Controlled Observational Studies

A study of the effect of two doses of PRP and PRP-D vaccines on hematologic indices was conducted in 61 adults (Lepow et al., 1984b). No effect of either vaccine on platelet count was observed.

Controlled Clinical Trials

In several large prospective trials of PRP and Hib conjugate vaccine efficacy and safety (Black et al., 1987, 1991b; Eskola et al., 1990a; Peltola et al., 1977; Santosham et al., 1991b; Vadheim et al., 1990), no subjects developed petechiae or purpura. Platelet counts were not measured, so the incidence of asymptomatic thrombocytopenia after immunization with Hib vaccines is unknown.

Causality Argument

The information concerning thrombocytopenia and Hib vaccines is limited. There is no biologic plausibility (data from animal models or experimental studies) to suggest a mechanism for thrombocytopenia following immunization for Hib. One case of thrombocytopenia without clinical illness occurred in a clinical trial study subject who had other possible risk factors that could have been responsible for the decline in platelet count

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