On Apr. 13, 2021, the FDA and CDC announced that they were are 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 and recommended a pause in the use of this vaccine out of an abundance of caution.

A type of blood clot called cerebral venous sinus thrombosis 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.