What's causing the blood clots? FDA's Peter Marks says this is just a theory, but that it could be an immune response that occurs very rarely after vaccination, causing activation of blood platelets.

Presser available live here:

Woodcock says FDA/CDC have done an extensive review of the reported events and looked at the background rate of these clots.

Marks says these types of clots have a background rate of 2-14 per million people. BUT says there's a pattern of events similar to Europe events.

Marks: "We have to take the time to make sure we understand this complication."

FDA's Marks says one reason they're raising this is because the standard treatment for blood clots -- heparin -- can actually make THIS type of clot worse. So you need to pause + get the word out. Worst case would be people harmed b/c of erroneous treatment.

Keep in mind that w/ the mRNA shots, we saw cases of anaphylaxis. That's one reason why there's 15 mins of post-injection monitoring, and health workers are on the look out for those reactions.

Nothing's free from risk, it's how you manage it.