Last Friday, the Centers for Disease Control and Prevention (CDC) and the U.S. Food & Drug Administration (FDA) lifted the pause in administration of the Johnson & Johnson (Janssen) COVID-19 vaccine in the U.S. The temporary pause was due to reports of a serious condition called cerebral venous sinus thrombosis (CVST), which refers to blood clots in the brain’s veins—not in the arteries, as is the case for most strokes—in combination with thrombocytopenia (low blood platelet count). CVST and thrombocytopenia together is called thrombosis-thrombocytopenia syndrome (TTS). When TTS is linked to receiving a COVID-19 vaccine, it is called vaccine-induced immune thrombotic thrombocytopenia (VITT). CVST has also been associated with cases of TTS in adults who received the AstraZeneca COVID-19 vaccine available in Europe, according to the European Medicines Agency, the agency responsible for the scientific evaluation, supervision and safety of medicines in the European Union.