EXPLAINER: What’s known about J&J’s vaccine and rare clots

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FILE In this March 31, 2021, file photo, a nurse fills a syringe with Johnson & Johnson's one-dose COVID-19 vaccine at the Vaxmobile, at the Uniondale Hempstead Senior Center, in Uniondale, N.Y. U.S. health regulators on Tuesday, April 13, is recommending a pause in using the vaccine to investigate reports of potentially dangerous blood clots. (AP Photo/Mary Altaffer, File)

A rare, rogue immune response is the main suspect as authorities investigate highly unusual blood clots following use of two similar COVID-19 vaccines from Johnson & Johnson and AstraZeneca.

The U.S. recommended that states pause giving the J&J vaccine on Tuesday while authorities examine six reports of the unusual clots, including a death, out of more than 6.8 million Americans given the one-dose vaccination so far.

But the small number of cases sparked concern because just last week, European authorities said similar clots were possibly linked to the AstraZeneca vaccine, which is not yet OK'd in the U.S. That led some countries to limit its use to certain age groups.

Also Tuesday, J&J delayed its imminent European rollout.

WHAT MAKES THESE CLOTS DIFFERENT?

These are not typical blood clots. They’re weird in two ways.

First, they’re occurring in unusual parts of the body, such as veins that drain blood from the brain. Second, those patients also have abnormally low levels of platelets -- cells that help form clots -- a condition normally linked to bleeding, not clotting.

Scientists in Norway and Germany first raised the possibility that some people are experiencing an abnormal immune system response to the AstraZeneca vaccine, forming antibodies that attack their own platelets. That’s the theory as the U.S. now investigates clots in J&J vaccine recipients, Dr. Peter Marks, the Food and Drug Administration’s vaccine chief, said Tuesday.