These people shared specific features that gave doctors clues that something unusual was happening:

1) They had a low platelet count - Platelets are the blood cells that plug holes in our blood vessels to stop us from bleeding. Platelets also activate the clotting system, but they are not typically low in people who develop blood clots.

2) They developed antibodies to a protein released by platelets (PF4) – People who had VITT developed antibodies against a protein released by platelets called PF4. This reaction caused the platelets to become very active and to increase the chance of developing abnormal blood clots. Forming antibodies against PF4 is a reaction doctors have seen before. A similar reaction happens in people who develop a rare reaction to the anticoagulant heparin (heparin-induced thrombocytopenia; HIT).

3) Their blood clots occurred in unusual places – Most blood clots not related to COVID are found within the blood vessels in legs or arteries of lungs. It is less common for people to have blood clots in large veins in their brain (cerebral venous sinus thrombosis (CVST)) or in blood vessels in their abdomen (splanchnic vein thrombosis) which is what was seen with the people in this study. There are treatments available, but blood clots found in these places do carry a high risk of bad outcomes, including death.