Rare blood clots – what you need to know

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Blood clot, colored scanning electron micrograph

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Investigations are taking place into whether cases of a rare type of blood clot are connected to the Oxford-AstraZeneca coronavirus vaccine.

The link between the vaccine and these rare clots – known as CVSTs – is not yet proven, but experts say evidence is “firming up”.

The Joint Committee on Vaccination and Immunisation (JCVI), the group of UK experts who advise how vaccines should be used, is now recommending healthy people under 30 be offered a different vaccine.

What are CVST clots?

CVST stands for cerebral venous sinus thrombosis. Put simply, this is a clot found in a large vein in the brain.

Blood normally travels through veins from the brain back to the heart.

The CVST clot can block the flow of blood in the brain, reducing oxygen supply and potentially causing damage – stopping the central nervous system from working properly.

Rare clots have also been found in other areas, such as the large veins in the abdomen, in people who have had the Oxford-AstraZeneca jab, and, in a handful of cases, in some arteries (the vessels carrying blood from the heart to organs).

Why do the clots happen?

No-one is yet sure, but there are certain factors in common.

In the CVST cases under investigation, the patients were all found to have low numbers of platelets – these are the blood cells that normally help repair bleeding in the body.

Patients were also found to have a particular antibody (an immune protein that often helps fight against infection) in their blood that activates platelets.

The antibody causes the platelets to mistakenly clump together and form clots, and in turn activates other parts of the body’s clotting system.

A combination of clots and low platelets is one of the features doctors are now looking out for.

But there is a lot more that needs to be understood about the process.

Cerebral venous thrombosis - CTA scan

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How can I tell if I have one?

Anyone who has any of the following symptoms four days or more after having the vaccine should seek prompt medical help:

  • A severe, persistent headache
  • Blurred vision
  • Shortness of breath
  • Chest pain
  • Leg swelling
  • Persistent stomach/abdominal pain
  • Unusual bruising or red/purple pinpoint spots beyond the injection site where the vaccine is given
  • Neurological symptoms such as weakness in the legs or seizures

How long after the vaccine do clots appear?

Most cases have been seen between four days and a few weeks of people having the jab.

Medical experts in the UK now suggest doctors should consider this rare condition as a diagnosis in anyone who has matching symptoms up to a month after they have had the vaccine.

How rare are the clots?

According to the UK medical regulator, the MHRA, 79 cases have been identified, and 19 people have died.

This is out of a total of more than 20 million doses of the AstraZeneca vaccine which have been given in the UK.

Data continues to be analysed and there is likely to be more information as time goes on.

From the current information the general risk of these types of clots following a jab is estimated to be one in 250,000.

Though still rare, this risk appears to increase in younger people.

The JCVI has decided that – given that healthy people under 30 are at lower risk of becoming seriously ill from Covid – the balance of risks and benefits means they should be offered a different jab, if one is available.

Experts are clear that for the vast majority of people the benefits of the vaccine vastly outweighs the risks.

AZ vaccine being given in London

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Can the clots be treated?

Generally, the quicker that cases are identified, the better for the patient.

Treatment can involve a number of medicines and includes immunoglobulins – immune proteins – given through a drip.

Some patients will be sent home with new generation blood thinners and have regular monitoring.

According to blood specialist, Prof Adrian Newland, current evidence suggests 75% of people survive.

An expert panel of blood specialists in the UK have put together guidance for doctors, which is being constantly updated.

Are other Covid vaccines linked to clots?

Prof Anthony Harnden, deputy chair of the JCVI, says there have been a couple of cases with Pfizer and a case within the clinical trial of the Janssen vaccine, but says: “They are extremely rare and so it is hard to decide if they are caused by vaccine or not.”

He says that if a link is proven to the AstraZeneca vaccine, scientists would need to look closely at other jabs – such as Janssen – which use similar methods to deliver the vaccine.

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