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

Several European countries briefly stopped introducing the vaccine this week after more than 30 recipients were diagnosed with cerebral venous sinus thrombosis or CVST. Most affected people were women under the age of 55.

However, the issue affected a small proportion of those who received the injection, and after investigation, the European medicines regulator ruled that the benefits outweigh the potential risks of the vaccine, and recommended vaccinations are resumed.

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

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

Pål André Holme, professor of hematology and chief physician at Oslo University Hospital who led an investigation into the Norwegian cases, said his team had identified an antibody produced by the vaccine that caused the side effect.

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

The Norwegian health authority cited the findings when it announced not to resume vaccination.

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

In Germany, 13 cases of CVST were detected in approximately 1.6 million people who received the AstraZeneca vaccine. Twelve patients were female and three died.

The German researchers, who worked with colleagues in Austria, Ireland and Great Britain, said in a statement that patients who show symptoms such as headache, dizziness or impaired vision four days after vaccination can be quickly diagnosed with a blood test. Prof. Greinacher said the news meant people shouldn’t have to fear the vaccine.

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

He said the condition could be treated in any medium-sized hospital after a quick diagnosis.

The German government said it was investigating the findings, but stuck to 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 reviewed the cases from Germany and Norway and discussed them with the relevant national authorities.

An EMA spokeswoman said the vaccine could be linked to very rare cases of blood clots, including CVST, but the benefits of the vaccine outweigh that risk.

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

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

The German Association for Thrombosis and Hemostasis Research assessed the work of Prof. Greinacher and released a statement on Friday advising doctors on how to diagnose and treat the condition should it occur in vaccine recipients.

Dr. Robert Klamroth, deputy chair of the Society for Thrombosis and Hemostasis Research, said the rare autoimmune reaction was more common in Germany because the country initially allowed the vaccine only for people under the age of 64. Great Britain, which had fewer incidents, but many vaccinated more people, mainly gave the shot to older recipients.

Once diagnosed, the condition must be treated with blood-thinning medications and immunoglobulin, which target the antibody causing the problem.

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

Write to Bojan Pancevski at [email protected]