The Food and Drug Administration (FDA) and the Centers for Disease Control and Prevention (CDC) are recommending a halt in the rollout of the Johnson & Johnson COVID-19 vaccine (J&J vaccine) after several cases of severe blood clots in recipients.

According to the FDA, there have been six reported cases of the rare and severe type of blood clot in over 6.8 million Johnson & Johnson COVID-19 vaccine recipients.

“Right now, these adverse events appear to be extremely rare,” the agency said on Twitter. “Treatment of this specific type of blood clot is different from the treatment that might typically be administered. 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. FDA will review that analysis as it also investigates these cases.”

However, until that review is completed, the FDA is “recommending this pause.”

“This is important to ensure that the health care provider community is aware of the potential for these adverse events and can plan due to the unique treatment required with this type of blood clot,” the FDA said on Twitter.

In a statement posted to the agency’s website, the FDA said the type of blood clot is called cerebral venous sinus thrombosis, and that it was seen in combination with low levels of blood platelets. All six cases involved women between the ages of 18 and 48, and symptoms occurred between six and 13 days post-vaccination. 

The reports appear similar to a rare, unusual type of clotting disorder that European authorities say is possibly linked to another COVID-19 vaccine not yet cleared in the U.S., from AstraZeneca.

AP reported that more than 6.8 million doses of the J&J vaccine have been administered in the U.S. About 9 million of the company’s doses have been delivered to states and are awaiting administration, according to CDC data.

U.S. federal distribution channels, including mass vaccination sites, will pause the use of the J&J shot, and states and other providers are expected to follow. The other two authorized vaccines, from Moderna and Pfizer, make up the vast share of COVID-19 shots administered in the U.S. and are not affected by the pause.

The announcement hit U.S. stock markets immediately, with Dow futures falling almost 200 points just over two hours before the opening bell. Shares of Johnson & Johnson dropped almost 3%

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