The COVID-19 vaccine made by AstraZeneca and the University of Oxford can sometimes cause unusual blood clots, accompanied by abnormally low platelets — the small blood cells involved in clotting — according to an investigation by the European Union’s European Medicines Agency (EMA).

These rare blood clots and low platelet counts should be listed as possible side effects of the vaccine, Emer Cooke, the EMA’s executive director, said at a news conference April 7, according to Science Magazine correspondent Kai Kupferschmidt. People receiving the shots should remain aware of these potential side effects, particularly within the first two weeks of their vaccination, she said, according to CNN.