https://www.scientificamerican.com/article/how-could-a-covid-vaccine-cause-blood-clots/

The very rare occurrence of a mysterious blood-clotting disorder among some recipients of the Oxford–AstraZeneca COVID-19 vaccine has researchers scrambling to uncover whether and how the inoculation could trigger such an unusual reaction.

After weeks of investigation, the European Medicines Agency (EMA) announced on 7 April that there is a possible link between the clots and the vaccine. Even so, the clotting disorder—described today in two reports in The New England Journal of Medicine— is so uncommon that the benefits of the vaccine still outweigh its risks, EMA executive director Emer Cooke told reporters. “These are very rare side effects,” she said. “The risk of mortality from COVID is much greater than the risk of mortality from these side effects.”