Even though the link is not firm yet, they're calling the condition vaccine-induced immune thrombotic thrombocytopenia or VITT. It's characterized by unusual blood clotting combined with a low number of blood-clotting cells called platelets. Patients suffer from dangerous clots and, sometimes, hemorrhaging at the same time.
If vaccines cause it, it's still very rare and unusual, they wrote. It might not even be happening any more often in recently vaccinated people than among the population in general. But if vaccination can cause the condition, it would be important to recognize that and treat it appropriately -- because the usual treatment for blood clots is not recommended for VITT.
While blood clots in the brain have received the most attention, patients have also had clots in other large veins and arteries. ASH published guidance saying normal post-vaccination malaise, headache and fever are not of concern.