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

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 vaccination 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.

Others, including Norway, Sweden and Denmark, said they needed more research before restarting their rollouts. Norway registered three cases of CVST, one of them fatal. The country vaccinated around 120,000 people with the shot. Finland suspended the use of AstraZeneca on Friday, after recording two cases of what the authorities called unusual blood clotting.

Pål André Holme, a professor of hematology and chief physician of the Oslo University Hospital who headed an investigation into the Norwegian cases, said his team had identified an antibody created by the vaccine that was triggering the adverse reaction.

“Nothing but the vaccine can explain why these individuals had this immune response,” Prof. Holme said.

Norway’s health authority cited the findings when announcing that it would not resume the vaccination.

A team of German researchers around Andreas Greinacher, professor of transfusion medicine at the Greifswald University Clinic, said they had independently came to the same conclusion as Prof. Holme in a statement and a press conference on Friday.

In Germany, 13 cases of CVST were detected among around 1.6 million people who received the AstraZeneca vaccine. Twelve patients were women and three died.

The German researchers, who coordinated with colleagues in Austria, Ireland and Britain, said in a statement that patients who show symptoms four days after vaccination, such as headaches, dizziness or impaired vision, could be quickly diagnosed with a blood test. Prof. Greinacher said the news meant that people should not fear the vaccine.

“Very, very few people will develop this complication,” Prof. Greinacher said in a press conference Friday. “But if it happens we now know how to treat the patients.”

The German government said it was examining the findings, but stuck to its decision to resume use of the AstraZeneca vaccine.

Neither the German nor the Norwegian findings were published or peer reviewed. Prof. Greinacher said he had submitted his findings for publication to the British medical journal The Lancet.

The German Society for Thrombosis and Hemostasis Research reviewed Prof. Greinacher’s work and issued a statement Friday advising physicians how to diagnose and treat the condition should it arise in vaccine recipients.

Dr. Robert Klamroth, deputy-chairman of the Society for Thrombosis and Hemostasis Research, said the rare autoimmune reaction occurred more frequently in Germany because the country initially only authorized the vaccine for people younger than 64. Britain, which had fewer incidents but vaccinated many more people, was predominantly giving the shot to older recipients.

Once diagnosed, the condition should be treated with blood thinning medication and immunoglobulin, which targets the antibody that causes the problem.

“We believe the most likely hypothesis is that this particular vaccine is causing a rare autoimmune reaction that triggers antibodies, which then interact with the platelets, but we don’t know why this is happening,” Dr. Klamroth said.

Write to Bojan Pancevski at bojan.pancevski@wsj.com