A joint statement by two US  agencies released on Tuesday recommended that there be a temporary pause on the administration of the single-dose Johnson & Johnson COVID-19 vaccine.

Both the U.S. Centers for Disease Control and Prevention and the Food and Drug Administration recommended that inoculation using the Johnson & Johnson vaccine be paused as they investigate  potentially dangerous blood clots.

The announcement means mass vaccination sites in the US that were previously administering the Johnson & Johnson vaccine will immediately pause the administration of the jab.

According to the two agencies, they’re “reviewing data involving six reported U.S. cases of a rare and severe type of blood clot in individuals after receiving the J&J 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,” added the statement.

Some researchers  believe that the blood clotting may be as a result of the  Johnson & Johnson vaccine being an adenovirus vector vaccine — the same type as AstraZeneca’s coronavirus vaccine.

While the AstraZeneca vaccine isn’t in use in the United States,  it has been  authorized in more than 70 countries.

More than 6.8 million doses of the Johnson & Johnson vaccine have already been administered in the U.S., alongside vaccines produced by Pfizer-BioNTech and Moderna.