Health officials have paused the rollout of the Johnson & Johnson vaccine due to concerns that the vaccine may have caused blood clots in several women. As of now the J&J vaccine is the only one which does not require a booster shot. 
Almost seven million Americans have received the J&J vaccine, and there are six reported cases of such blood clots. 
“We are recommending a pause in the use of this vaccine out of an abundance of caution,” said the joint statement from the CDC and FDA released on Tuesday.
A severe type of blood clot, called cerebral venous sinus thrombosis (CVST), was reported alongside a low platelet count. This type of clot requires a different treatment than most common blood clots.
While such cases are incredibly rare, there has been much discussion and fear regarding those who have already received the vaccine. According to the statement all six women experienced symptoms within six to 13 days after vaccination. 
This is only a concern with the Johnson & Johnson vaccine. The Pfizer and Moderna vaccines are still considered safe.