LONDON: Scientists in Europe believe they may have found the cause of blood clots in the brains of a small number of people who have received the Oxford-AstraZeneca coronavirus vaccine.
Its use was suspended by over a dozen EU states after the discovery of around 30 cases of cerebral sinus vein thrombosis (CSVT) in recipients across the bloc.
The suspensions came as authorities sought to establish whether there was any evidence to link the clots directly to the vaccine.
AstraZeneca said its data suggested that clots were occurring at a lower rate among vaccine recipients than across Europe’s population as a whole, with clots occurring in fewer than one in every 2 million vaccinated people.
Now two teams, in Germany and Norway, say there is a chance that the vaccine may, in very rare circumstances, cause the immune system to attack the body’s blood platelets, potentially leading to thickening and causing clots.
But they added that there is still no conclusive evidence that the vaccine is dangerous, and that the development may also be occurring naturally.
EU and UK health regulators have said the vaccine is safe to use, and neither team’s findings have yet been peer-reviewed.
The teams, from Oslo University Hospital and Greifswald University in Germany, say antibodies created by the body after receiving the vaccine could be mistaking platelets in the blood for an infection, attacking them and causing the body to then create more platelets, causing clots.
Similar effects have been observed in patients receiving other drugs, including heparin, which causes a clotting condition called heparin-induced thrombocytopenia (HIT).
The Greifswald team examined nine cases of clotting post-vaccination, seven of whom were found to have CSVT. Tests showed four had elevated platelet levels in their blood, similar to HIT sufferers.
A sample of 20 vaccinated people who did not develop clots, meanwhile, showed normal levels of platelets.
The team said it would submit its findings to British medical journal The Lancet shortly, and advised anybody who noticed bruising, swelling, dizziness or headaches after receiving the AstraZeneca vaccine to seek medical advice as all those symptoms were experienced by those who went on to develop clots.
Oslo University Hospital’s Prof. Pal Andre Holme told Norwegian newspaper VG that his team had found a similar pattern, albeit with a smaller sample size than the Greifswald team.
“Our theory is that this is a strong immune response that most likely comes after the vaccine,” he said. “There is no other thing than the vaccine that can explain this immune response.”
France, Italy and Germany have since resumed use of the vaccine, but it remains suspended in Norway, Denmark and Sweden.
Canada suspended its use in people under the age of 55 over fears of blood clots. It has yet to be approved in the US.