(CNN) -
The US Centers for Disease Control and Prevention and the US Food and Drug Administration recommend that the country stop using Johnson & Johnson's covid-19 vaccine after " Six Rare Cases of a Severe Type of Clot Reported in the US »
These US health agencies were studying concerns about blood clots and the Johnson & Johnson COVID-19 vaccine and are working to assess whether the injection is associated with a very small increased risk of rare blood clots, a federal official told CNN.
"The CDC and FDA are taking these concerns about blood clots and the J&J vaccine seriously and are diligently gathering data," the official said.
"The CDC is very concerned and is working hard on this and monitoring this closely," said the expert, who spoke on condition of anonymity due to the sensitive nature of the issue.