As of Apr. 12, 2021, more than 6.8 million doses of the Johnson & Johnson (Janssen) vaccine were administered in the U.S. The CDC and FDA announced that they had begun reviewing data involving six reported U.S. cases of a rare and severe type of blood clot in individuals after receiving the J&J vaccine.

In these cases, a type of blood clot called cerebral venous sinus thrombosis (CVST) was seen in combination with low levels of blood platelets (thrombocytopenia). All six cases occurred among women between the ages of 18 and 48, and symptoms occurred 6 to 13 days after vaccination.