Dr Sabine Straus, who chairs the body’s pharmacovigilance risk assessment committee, said the figures might be skewed towards women because currently more recipients of the AstraZeneca vaccine in Europe are women.

“It might be due to the fact that more women have been vaccinated,” she said. “We know that in the different member states, approximately 60 per cent of all vaccines [have been given to] females, but it might also be due to certain other factors that we have not been able to identify yet.”

Pregnant women, and those with clotting disorders, have been advised to discuss with their doctor whether the risks outweigh the benefits

The cause

The MHRA said there was a “strong possibility” that the AstraZeneca vaccine is driving the dangerous clotting although more work was needed to establish “beyond all doubt” that the vaccine causes clotting.

However, on Thursday the EMA went slightly further, adding clotting to the list of side-effects of the AstraZeneca vaccine and saying that a “plausible explanation” was that the jab was causing a rare auto-immune response.

Dr Straus said there was a distinct clinical pattern to these cases, and more cases than would be expected than the background rate.

The EMA said one “plausible explanation” for the condition was an auto-immune response similar to one seen sometimes in patients treated with heparin (heparin-induced thrombocytopenia, HIT).

Heparin is an anti-clotting medication that prevents blood clots from forming, and thrombocytopenia refers to decreased number of thrombocytes, or platelets, in the blood. However, the body sometimes does not take kindly to attempts to thin the blood and the immune system will kick in, creating antibodies which actually increase deadly clots which can travel to veins or the brain.

In March, German and Norwegian scientists also said they believed the jab was causing a rare autoimmune reaction and suggested it could be treated with blood thinning medication.

Sir Munir Pirmohamed, the chairman of the Commission on Human Medicines, said: “It’s really important to understand the mechanisms, because it might provide us with ways of being able to prevent this in future. You would be able to refine the advice that’s given so you could identify risk factors in those individuals.

“We might think about modifying the vaccines so they do not produce this in the future.”

The trials

The clotting events were not picked up in trials because they are so rare. The most common side-effects of the vaccines have included tenderness, swelling and redness at the injection site, headache or muscle ache, joint pain, chills, nausea, vomiting, feeling tired and fever.

Trials have consisted of tens of thousands of people – but the chance of a clotting event is so low that there would need to be hundreds of thousands of people enrolled to catch just one dangerous clot.

Prof Jonathan Van-Tam, the deputy chief medical officer, warned that other vaccines could show similar side effects once rolled out. 

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