Updated project metadata. Vector-based SARS-CoV-2 vaccines such as AstraZeneca's ChAdOx1 nCoV-19 have been associated with vaccine-induced thrombosis (TTS/VITT). We studied ChAdOx1 nCov-19 (AstraZeneca) and Ad26.COV2.S (Johnson & Johnson) and compared the vaccines using proteomics with mass spectrometric measurements. We could demonstrate that ChAdOx1 nCoV-19 contained significant amounts of human proteins in addition to adenoviral proteins. This was not the case for Ad26.COV2.S. Among the detected human proteins, proteasome subunits beta 1/delta and beta 5/MB1 were identified. In summary, both studied SARS-CoV-2 vaccines are markedly different concerning their content of contaminating proteins, which is most likely a determining factor for different frequencies of TTS/VITT after their application.