Researchers say they may have found a reason for a rare blood clotting condition that has occurred in some people who received the AstraZeneca COVID-19 vaccine.

A Ukrainian army medic prepares a dose of the Oxford/AstraZeneca vaccine against the coronavirus disease, marketed under the name Covishield and produced in India, before vaccinating a serviceman at a military hospital in Severodonetsk, a government-controlled town located roughly 120 kilometres (75 miles) to the north of the separatist stronghold of Donetsk in eastern Ukraine, on March 9, 2021. (Photo by Anatolii STEPANOV / AFP) (Photo by ANATOLII STEPANOV/AFP via Getty Images)