Figure 1.

Schematic depicting the mechanism of action of adenoviral vector-based COVID-19 vaccines (with the ChAdOx1 nCov-19 vaccine as an example) and a hypothetical model of how this could induce vaccine-induced immune thrombotic thrombocytopenia. Adenoviruses in the vaccines infect coxsackievirus and adenovirus receptor-positive (CAR+) cells after intramuscular injection, and these CAR+ cells subsequently produce SARS-CoV-2 spike protein, which is taken up by antigen-presenting cells (APC) for stimulation of T cells, resulting in the activation of adaptive immune responses. However, the SARS-CoV-2 spike proteins may also activate platelets, leading to the release of platelet factor 4 (PF4) from platelet α-granules, and ultimately resulting in stimulation of the coagulation system and clinically significant thromboembolic complications. Further research is needed to confirm this hypothetical model. T_FH cells, T follicular helper cells.