Thrombocytopenia after immunization of Canadian children, 1992 to 2001

Background. Thrombocytopenia occasionally follows immunization of children, especially after administration of measles-containing vaccines. The purpose of this study was to describe the clinical features of postimmunization thrombocytopenia, with emphasis on the rate of complications and outcome. Methods. A prospective survey was conducted by 12 pediatric centers in Canada during 1992 to 2001. At each center a nurse monitor searched for inpatient cases. Cases were defined as having onset of clinical signs or laboratory measures of thrombocytopenia (platelet count, <50 × 10⁹/l) within 30 days after immunization. Cases were described in a standardized manner, including follow-up data as available. Results. Sixty-one cases were detected, an average of 6 per year or ∼ 1 case per 15 000 general hospital admissions. Median age of cases was 13 months. The mean platelet count at diagnosis was 8.6 × 10⁹/l. Most cases (79%) followed measles-containing vaccines. Only 1 child had a serious (fatal) complication. Platelet counts returned to normal within 30 days of onset in 46 of 57 children (80.7%) with information available. Five children (8.2%) had persistent or intermittent thrombocytopenia for 3 months or more. Conclusion. Thrombocytopenia associated with routine immunization of children is rare and usually benign, resolving within 1 month in most children.