During 2020 and 2021, the global pandemic of coronavirus disease 2019 (COVID-19) due to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has resulted in high death rates and acute and chronic morbidity in all countries. The rapid development of new mRNA vaccines to SARS-CoV-2 brings hope that the spread of this virus can be controlled. The ChAdOx1 nCoV-19 vaccine developed by a collaboration between the University of Oxford and AstraZeneca showed efficacy in clinical trials, with a good safety profile. However, there have been recent reports of the rare development of thrombotic events in young women following vaccination with ChAdOx1 nCoV-19, particularly of the rare condition of cavernous sinus thrombosis. Studies have begun to investigate whether antibodies to the SARS-CoV-2 spike cross-react with platelet factor 4 (PF4/CXLC4) and mimic autoimmune heparin-induced thrombocytopenia. This Medical Science Monitor Editorial aims to briefly update the current status of studies on a possible rare complication of using new mRNA vaccines to prevent COVID-19.