According to a high-ranking doctor on the German TV today, the observed effect isn't normal thrombosis but a brain related thromboses, where prophylactic treatment is not possible.
I am a bit out of my depth here, but from what I understood, it is not a normal kind of thrombosis, but more an interaction of the immune system with the thrombocytes in the brain which actually causes bleedings, so aspirine or any other blood thinner would not be helpful.
