(NEW YORK) — The Food and Drug Administration and the Centers for Disease Control and Prevention have announced that they are calling for an immediate pause on the use of the single-dose Johnson & Johnson vaccine on Tuesday after discovering six cases in the United States of a rare and severe type of blood clot that developed about two weeks after the vaccine was administered in these patients.

“[The] CDC will convene a meeting of the Advisory Committee on Immunization Practices (ACIP) on Wednesday to further review these cases and assess their potential significance,” read a joint CDC and FDA statement that was issued on Tuesday morning. “[The] FDA will review that analysis as it also investigates these cases. Until that process is complete, we are recommending a pause in the use of this vaccine out of an abundance of caution. This is important, in part, to ensure that the health care provider community is aware of the potential for these adverse events and can plan for proper recognition and management due to the unique treatment required with this type of blood clot.”

More than 6.8 million doses of the vaccine has already been administered in the United States.

The CDC and the FDA said that they are currently reviewing data involving these six cases — all of which occurred among women between the ages of 18 and 48 — where symptoms occurred between six to 13 days after they were vaccinated with the Johnson & Johnson 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),” the statement read. “Treatment of this specific type of blood clot is different from the treatment that might typically be administered. Usually, an anticoagulant drug called heparin is used to treat blood clots. In this setting, administration of heparin may be dangerous, and alternative treatments need to be given.”

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