The European Medicines Agency (EMA) said on Wednesday that cases of blood clotting could not be linked to specific factors such as age, sex or medical history and that a plausible explanation is that thrombosis happens because of the immune response to vaccines.

Based on current evidence, specific risk factors such as age, sex or earlier thrombosis, have not been confirmed. Blood clots have been reported in all groups, EMA Executive Director Emer Cook told a news conference on the conclusions of the EMA’s Pharmacovigilance Risk Assessment Committee (PRAC) on the AstraZeneca vaccine.

A plausible explanation is that this is an immune response, leading to a condition similar to one seen sometimes in patients treated with heparin, she added.

Following its analysis, PRAC concluded that blood clotting should be cited as a vaccine side-effect but it was stressed that the benefits of the AstraZeneca vaccine still far outweigh the risks.

EU health ministers are expected to meet today, following the EMA’s news conference.

The EMA had investigated reports of extremely rare cases of blood clots in the brain, known as cerebral venous sinus thrombosis (CVST). In the European Economic Area, which consists of EU member countries, Iceland, Liechtenstein and Norway, 9.2 million people have been vaccinated with the AstraZeneca vaccine.

In the United Kingdom, 30 cases of blood clots have been reported, including seven fatal, among 18.1 million people vaccinated by 24 March.