Incidence, Risk Factors, and Association with Vaccination of Thrombocytopenia and Immune Thrombocytopenia (ITP): A Scoping Review.

BACKGROUND:The Brighton Collaboration published its standard harmonized case definition for thrombocytopenia in 2007 to facilitate collection of valid and reproducible vaccine safety evidence and enable valid comparisons across studies. A companion guide to this case definition (including background incidence, risk factors, evidence for or against an association with vaccination, medical codes, data collection forms, and algorithms to support level of diagnostic certainty assessment) was initially published in 2021 and updated in 2025 based on a formal scoping review. This article reports the results of this scoping review, summarizing current evidence regarding the background incidence of and risk factors for thrombocytopenia, including immune thrombocytopenia (ITP) and its potential association with vaccination. METHODS:Two PubMed searches were conducted in July 2024: one for background incidence and non-vaccine risk factors, and one for vaccine associations. Search results were screened and relevant data were extracted and added to the corresponding companion guide tables. FINDINGS:Twenty-two articles contributed to the updated summary of background incidence, 36 articles contributed to the updated summary of non-vaccine risk factors, and 93 articles contributed to the updated summary of vaccine associations. Estimates of the background incidence of thrombocytopenia range between 1.6 and 92.1 per 100,000 person-years, depending on the study, region, and age range. Non-vaccine risk factors for thrombocytopenia include age, sex, genetics, season, geography, comorbidities, iatrogenesis, infection, medication, and food. While studies have consistently found an increased risk of ITP with measles-containing vaccines, and multiple studies found an increased risk following adenoviral COVID-19 vaccination, studies of other vaccines (including mRNA and inactivated COVID-19 vaccines) have largely found no association. CONCLUSION:Though rare, thrombocytopenia has many risk factors, including vaccination against measles. This review should be useful to stakeholders attempting to assess the occurrence of thrombocytopenia across a variety of settings, including as an adverse event following immunization.