Abstract
In response to the COVID-19 pandemic, several vaccines were developed and rolled out at unprecedented speed, and notwithstanding this rapid pace of development, the results from initial clinical trials involving tens of thousands of adult subjects generally indicated that most vaccines were remarkably effective and safe, with no major safety warnings noted. However, with more than 2 billion vaccination doses administered to date, reports of rare adverse events following immunization (AEFI) are beginning to emerge. In late February 2021, atypical thrombotic events following immunization with the adenoviral vector-based ChAdOx1 nCov-19 vaccine were first reported, and similar events have also been observed in recipients of the adenoviral vector-based Ad26.COV2.S vaccine and the mRNA-based BNT162b2 and mRNA-1273 vaccines. These manifestations of atypical thrombosis and thrombocytopenia following COVID-19 vaccine immunization are now collectively referred to as vaccine-induced immune thrombotic thrombocytopenia (VITT). Although the reported incidence remains very low and does not affect the overall benefit of immunization, it is also true that if left untreated, VITT can be debilitating or even fatal. Therefore, this review seeks to provide a comprehensive overview regarding the incidence, pathogenesis, presentation, diagnosis, and treatment of VITT, as well as considerations for special populations, based on the currently available evidence in the literature. It is hoped that this will enhance awareness of this vaccine side effect, so that cases of VITT may be identified and treated in a timely and appropriate manner.