The Risk of Immune Thrombocytopenic Purpura After Vaccination In Children and Adolescents

Background: Immune thrombocytopenic purpura (ITP), a rare and often serious disease, has been associated with measles-mumps-rubella vaccination (MMR). The risk of ITP after other childhood vaccines is unknown.

Objectives: To determine the risk of ITP after childhood vaccinations.

Methods: Using data from 5 managed care organizations for 2000-2009, we identified a cohort of 1.8 million children ages 2 months to 17 years. Potential incident cases of ITP were identified by ICD-9 codes and platelet counts <50,000. All cases were verified by chart review. Incidence rate ratios (IRR) were calculated using the self-controlled case series method with a 1-42 day risk window within discrete age categories. IRRs were calculated for routinely used childhood and adolescent vaccines.

Results: A total of 1.8 million children received 15 million vaccine doses. There were 204 chart-confirmed ITP cases. Most cases were acute and mild with no long term sequelae. The IRR for the first dose of MMR vaccine, administered at ages 12-19 months, was significantly elevated (IRR, 6.99, 95% confidence interval (CI) 1.56, 31.22). Elevated IRRs were also found for hepatitis A vaccine (Hep A) administered at ages 7 to 17 years (IRR, 18.33, 95% CI 2.23, 150.51), and for varicella vaccine (VZV) (IRR, 14.07, 95% CI 1.13, 174.57) and tetanus-diphtheria-acellular pertussis vaccine (Tdap) (IRR, 19.1, 95% CI 3.02, 121.49) administered at ages 11-17 years. For Hep A, VZV, and Tdap, elevated IRRs were based on 1-2 vaccine-exposed cases.

Conclusions: We confirmed a significant association between ITP and MMR vaccine. Because of the small number of exposed cases and potential confounding, the possible association of ITP with Hep A, VZV, and Tdap in older children requires further investigation.