In response to injury or other signals, components of the blood known as platelets and fibrin bind blood cells together into sticky masses called clots. If the process continues unchecked, the clots can prevent blood from flowing. Some scientists speculate the vaccine may have triggered an immune reaction in patients that activated platelets to form clots.

The affected patients developed blood clots in the dural venous sinuses, which help drain blood from the brain. The condition, known as cerebral venous sinus thrombosis, can increase pressure within the skull, leading to hemorrhaging and stroke.