Dr Rolf Marschalek, a biochemistry professor at Goethe University in Germany, told the Financial Times clots may be caused by the way vaccine enters the body. The paper says the problem is 'completely absent' in mRNA vaccines like Pfizer's and Moderna's. Dr Marschalek suggests that the vaccine is delivered to the nucleus of the cell – a blob of DNA in the middle – rather than to the fluid around it that acts as a protein factory. Bits of coronavirus proteins that get inside the nucleus can break up and the unusual fragments then get expelled out into the bloodstream, where they can trigger clotting in a tiny number of people, Dr Marschalek said.