LAUSR

During the coronavirus disease 2019 (COVID-19) pandemic, the safety and efficacy of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) vaccines have been proven, and vaccines have been actively administered worldwide. However, thrombosis with thrombocytopenia (TTS) is a rare side effect of adenoviral vector vaccine administration. The mechanism of TTS is that adenovirus vaccines stimulate the production of autoantibodies to platelet factor 4 (PF4) that activate platelets and induce thrombosis. Additionally, cases of cerebral venous thrombosis after BNT162b2 (Pfizer-BioNTech) mRNA vaccination have been reported. However, the mechanism of thrombosis after mRNA vaccination is still unknown. We present a case of venous thromboembolism (VTE) that developed after the second dose of mRNA-1273 (Moderna) immunization in a young male patient with no risk factors for thrombosis. In addition, we confirmed that anti-heparin PF4 IgG was negative in this case. This is the first report suggesting that thrombosis after mRNA vaccination is caused by different mechanism from TTS.