(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.”

The FDA and CDC said that these cases are extremely rare but that an abundance of caution was required.

“COVID-19 vaccine safety is a top priority for the federal government, and we take all reports of health problems following COVID-19 vaccination very seriously,” the statement continued. “People who have received the J&J vaccine who develop severe headache, abdominal pain, leg pain, or shortness of breath within three weeks after vaccination should contact their health care provider. Health care providers are asked to report adverse events to the Vaccine Adverse Event Reporting System.”

Johnson & Johnson issued a statement immediately following the announcement by the FDA and the CDC.

“The safety and well-being of the people who use our products is our number one priority. We share all adverse event reports about individuals receiving our COVID-19 vaccine, along with our assessment of these reports, with health authorities in compliance with regulatory standards,” said Johnson & Johnson. “We are aware that thromboembolic events including those with thrombocytopenia have been reported with COVID-19 vaccines. At present, no clear causal relationship has been established between these rare events and the Janssen COVID-19 vaccine. We continue to work closely with experts and regulators to assess the data and support the open communication of this information to healthcare professionals and the public.”

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