US federal health agencies on Tuesday called for an immediate pause for the single-dose Johnson & Johnson COVID-19 vaccine amid reports of clotting issues.

The announcement came after the discovery of six cases of a rare and severe type of blood clot that developed about two weeks after the vaccine was administered.

The Centers for Disease Control and Prevention (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,” the CDC and Food and Drug Administration (FDA) said in a joint statement, ABC News reported.

The FDA will also review that analysis as it also investigates the cases.

“Until that process is complete, we are recommending a pause in the use of this vaccine out of an abundance of caution,” the statement continued.

“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.”

The news comes as the US is trying to outrace new strains of the virus to vaccinate as much of the country’s population as possible.

The US has so far registered more than 31M coronavirus infection cases and nearly 563,000 related deaths, according to a running tally of Maryland’s Johns Hopkins University.

In Europe, the AstraZenaca vaccine has also faced concerns over reported blood clots.

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