The surveillance of the recombinant adenovirus-based COVID-19 vaccine ChAdOx1-S (Oxford, Astra-Zeneca) which contains chimpanzee adenovirus (AdV) encoding the SARS-CoV-2 spike glycoprotein, revealed rare thromboses with concurrent thrombocytopenia, including venous thromboses in unusual sites such as cerebral venous sinus thrombosis (CVST), intestinal venous and arterial thromboses [1]. Soon thereafter, the Pharmacovigilance Risk Assessment Committee (PRAC) of the European Medicines Agency (EMA) reported also a signal concerning thromboembolic events with thrombocytopenia after vaccination with the Ad26. COV2-S COVID-19 vaccine (Johnson & Jonhnson)[2], which contains the cloned SARS-CoV-2 spike glycoprotein in AdV type 26. Both above-mentioned vaccines are abbreviated in the following as AdV-S. The above described thrombotic complications after AdV-S have in the meantime be named TTS (thrombosis thrombocytopenia syndrome), VIPIT (vaccine-induced prothrombotic immune thrombocytopenia) or VITT (vaccine-induced thrombocytopenia)[3–5].

For other SARS-CoV-2 vaccines, which do not use adenovirus vectors, similar observations were only reported until now in two women [6, 7]. Most of the cases reported to date have occurred in persons under 60 years of age within 2 weeks of receiving the first vaccine dose [3]. Since females consented more frequently to be vaccinated there were more female than male vaccinees involved, but this observation is in line with the experience concerning risk factors for cerebral venous sinus thrombosis [8–11]. The occurrence of thrombosis focused on CVST after AdV-S vaccination, and has to be balanced with the frequency of spontaneous cerebral venous sinus thrombosis [12], and CVST associated with SARS-CoV-2 infection [13, 14]. The time period of the occurrence of CVST in COVID-19 patients even under anti-thrombotic treatment is reported to be ca 3–30 days after onset of infection [3, 15]. It was reported that in patients with COVID-19 pneumonia an incidence between 7.7 and 28% of cerebral thrombosis was found in Intensive Care Units, dependent on age, gender, comorbidities, and cytokine release, compared to a general incidence of 2–5–15 cases per million people [9]. The frequency of atypical thrombosis after vaccination has to be weighted with that of thrombosis in the general population [16], and the higher frequency of thrombosis in COVID-19 patients in emergency departments, which will be strongly reduced by vaccination [17, 18]. Factors contributing to the preferential affection of female gender in the observed thromboses after AdV-S vaccination may include increased seasonal adenovirus production supported by beta-estradiol [19] and hormonal activation of thrombocytes [20]. There may be further some contribution to a tendency of hypercoagulation by genetic factors, since the