Scientists Say They Found Cause of Rare Blood Clotting Linked to AstraZeneca Vaccine

Scientists in Europe said they had identified a mechanism that could lead the AstraZeneca PLC vaccine to cause potentially deadly blood clots in rare instances as well as a possible treatment for it.

Two teams of medical researchers in Norway and Germany have independently found that the vaccine could trigger an autoimmune reaction causing blood to clot in the brain, which would offer an explanation for isolated incidents across Europe in recent weeks.

Several European countries briefly halted their rollouts of the vaccine this week after more than 30 recipients were diagnosed with the condition known as cerebral venous sinus thrombosis, or CVST. Most of the people affected were women under the age of 55.

The issue affected a tiny portion of those who had received the shot, however, and after investigating, the European drugs regulator ruled that the benefits outweighed the potential risks of the vaccine, and recommended vaccinations resume.

Some countries, such as Germany, France and Italy, resumed vaccinations with AstraZeneca’s shot on Friday, with an added warning that it could be linked to blood clotting. The French healthcare authority, which recorded three cases of CVST connected to the vaccine, advised the government on Friday to only administer the shot to people older than 55.