Scientists say they found a rare cause of blood clotting linked to the AstraZeneca vaccine

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

The problem affected a small part of those who had received the vaccine, however, and after investigating, the European drug regulator determined that the benefits outweighed the potential risks of the vaccine and recommended restarting the vaccination.

Some countries, such as Germany, France and Italy, resumed vaccination with the injection of AstraZeneca on Friday, with an additional warning that it could be related to blood clotting. The French health authority, which registered three cases of CVST linked to the vaccine, advised the government on Friday to administer the vaccine only to people over 55.

Others, including Norway, Sweden and Denmark, said they needed more research before restarting their launches. Norway has recorded three cases of CVST, one of which is fatal. The country vaccinated about 120,000 people with the vaccine. Finland suspended the use of AstraZeneca on Friday, after recording two cases of what the authorities called unusual blood clotting.

Pål André Holme, professor of hematology and chief physician at Oslo University Hospital who led an investigation into Norwegian cases, said his team 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,” said Prof. Holme.

The Norwegian health authority cited the findings when announcing that it would not resume vaccination.

A team of German researchers around Andreas Greinacher, a professor of transfusion medicine at Greifswald University Clinic, said that he independently reached the same conclusion as Prof. Holme in a statement and a news conference on Friday.

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

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

“Very, very few people will develop this complication,” said Prof. Greinacher at a news conference on Friday. “But if it does, we now know how to treat patients.”

He said that after a quick diagnosis, the condition could be treated at any mid-sized hospital.

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

AstraZeneca did not immediately respond to a request for comment. Nor have drug regulators in Britain, Germany, Austria and the Netherlands, where vaccinations with the AstraZeneca vaccine have resumed or have not been suspended this week.

The European Medicines Agency, or EMA, which regulates medicines for most European countries, said it had assessed the cases in Germany and Norway and discussed them with the competent national authorities.

An EMA spokeswoman said the vaccine may be associated with very rare cases of blood clots, including CVST, but that the vaccine’s benefits outweigh that risk.

“A causal relationship with the vaccine has not been proven, but it is possible and deserves further analysis,” said the spokeswoman in a statement.

Neither the German nor the Norwegian results have been published or peer-reviewed. Prof. Greinacher said he submitted his findings for publication in the British medical journal The Lancet.

The German Society for Research on Thrombosis and Hemostasis reviewed the work of Prof. Greinacher and issued a statement on Friday advising doctors on how to diagnose and treat the condition if it appears in vaccine containers.

Dr. Robert Klamroth, vice president of the Society for Thrombosis and Hemostasis Research, said the rare autoimmune reaction occurred more often in Germany because the country initially authorized the vaccine only for people under 64. Britain, which had fewer incidents but vaccinated many more people, was giving the injection predominantly to older recipients.

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

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

Write to Bojan Pancevski at [email protected]