During the last months many countries have started the immunization of millions of people by using vector-based vaccines. Unfortunately, severe side effects became overt during these vaccination campaigns: cerebral venous sinus thromboses (CVST), absolutely rare under normal life conditions, were found as a severe side effect that occured 4-14 days after first vaccinations. Besides CVST, Splanchnic Vein Thrombosis (SVT) was also observed. This type of adverse event has not been observed in the clinical studies of AstraZeneca, and therefore led immediately to a halt in vaccinations in several european countries. These events were mostly associated with thrombocytopenia, and thus, similar to the well-known Heparin-induced thrombocytopenia (HIT). Meanwhile, scientists have proposed a mechanism to explain this vaccine-induced thrombocytopenia. However, they do not provide a satisfactory explanation for the late thromboembolic events. Here, we present data that may explain these severe side effects which have been attributed to adenoviral vaccines. According to our results, transcription of wildtype and codon-optimized Spike open reading frames enables alternative splice events that lead to C-terminal truncated, soluble Spike protein variants. These soluble Spike variants may initiate severe side effects when binding to ACE2-expressing endothelial cells in blood vessels. In analogy to the thromboembolic events caused by Spike protein encoded by the SARS-CoV-2 virus, we termed the underlying disease mechanism the “Vaccine-Induced Covid-19 Mimicry” syndrome (VIC19M syndrome).