Thrombocytopenia is a decrease in the number of platelets, the cells involved in blood clotting. Thrombocytopenia may stem from the failure of platelet production, a shortened platelet life span, or an abnormal distribution of platelets within the body (Lee et al., 1993). Thrombocytopenia occurs in children of all ages, with an incidence of 31.9 cases per 1 million children per year (Cohn, 1976). Approximately 70 percent of cases occur following viral illnesses (Lightsey, 1980). In most cases, thrombocytopenia in children is mild and transient, and it is often discovered only incidentally when a complete blood count is performed. Severe thrombocytopenia associated with spontaneous bleeding, including bleeding into the skin, is called thrombocytopenic purpura.
A possible association of Hib vaccine with thrombocytopenia was noted by Granoff et al. (1984) in a trial of the PRP-D vaccine in adults. Thirty subjects were randomly assigned to receive two doses of either the PRP or the PRP-D vaccine. Because this was one of the first trials of Hib conjugate vaccine in human subjects, multiple hematologic, renal, and hepatic tests were performed after the administration of each dose. One subject developed transient, asymptomatic thrombocytopenia after receiving the second injection of PRP-D vaccine. His platelet count fell from 173,000/mm3 before the second injection to 80,000/mm3 7 days after the injection. There was no associated evidence of bleeding, and the platelet count was 153,000/mm3 12 days later (normal range, 150,000 to 50,000/mm3). The causal relation between PRP-D vaccine and thrombocytopenia in this subject was not clear because he had received tetanus toxoid 4 weeks earlier and also was taking a number of medications for migraine headaches. None of the other subjects in that study experienced a decrease in platelet count, including the remaining 14 subjects who received PRP-D vaccine and the 15 subjects who received PRP vaccine. A second 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.