BLOOD clots after the AstraZeneca Covid jab are "extremely rare", but health authorities continue to monitor the link - here's everything you need to know. 

People under 40 in the UK are being offered an alternative to the AstraZeneca vaccine following reports of extremely rare blood clots on the brain coupled with low blood platelet count.

The Medicines and Healthcare products Regulatory Agency (MHRA) had said the benefits of the Oxford/AstraZeneca vaccine continue to outweigh risks for most people.

It has not proven the vaccine causes the clots but has said the link is getting firmer.

However, current guidance is that most under-40s will be offered the Pfizer or Moderna vaccines.

What are CVST clots and why do they happen?

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

A blood clot can develop when an infection in the face or skull spreads to the cavernous sinuses, which are hollow spaces under the brain, behind each eye socket.

The blood clot develops to prevent the infection from spreading further, but it can restrict the blood flow from the brain, which can damage the brain, eyes and nerves running between them.

Sometimes, clots can develop without infection.

No one is sure why clots happen with the AstraZeneca vaccine however in cases that have been investigated, patients were found with low levels of platelets.

Platelets are blood cells that help repair bleeding in the body.

A specific antibody that activates the platelets, telling them to mistakingly clump together and form clots, was also found in patients. 

This combination provides some clues as to why clots are formed and is what doctors are now looking for.

How rare are they?

Up to May 19, the health regulator MHRA had received reports of 332 cases of major blood clots with low platelet counts across the UK following vaccination with the AstraZeneca jab.

Cerebral venous sinus thrombosis was reported in 120 of these cases. 

There have been 58 deaths.

The JCVI, which advises UK health departments on immunisation, said there is an “extremely small risk” of people suffering blood clots after having the AstraZeneca jab.

But they added that the risk of serious illness with Covid-19 also drops for younger people as infection rates fall across the country.

Professor Wei Shen Lim, Covid-19 chairman for JCVI, said: “We have continued to assess the benefit/risk balance of Covid-19 vaccines in light of UK infection rates and the latest information from the MHRA on the extremely rare event of blood clots and low platelet counts following vaccination.

“As Covid-19 rates continue to come under control, we are advising that adults aged 18 to 39 years with no underlying health conditions are offered an alternative to the Oxford/AstraZeneca vaccine, if available and if it does not cause delays in having the vaccine.”

How can I tell if I have one and how long after the vaccine do clots appear

In most cases of cavernous sinus thrombosis, the eyes are affected, while the most common initial symptom of cavernous sinus thrombosis is a headache.

Other symptoms include a high temperature, vomiting, seizures and changes in mental state like feeling confused. Read more about the symptoms here.

Although serious side effects are very rare, the Government advises that if you experience any of the following from around four days to four weeks after vaccination you should seek medical advice urgently:

  • a new, severe headache which is not helped by usual painkillers or is getting worse
  • a headache which seems worse when lying down or bending over
  • an unusual headache that may be accompanied by:
  • blurred vision, nausea and vomiting
  • difficulty with your speech
  • weakness, drowsiness or seizures
  • new, unexplained pinprick bruising or bleeding
  • shortness of breath, chest pain, leg swelling or persistent abdominal pain

The MHRA also said that, as a precautionary measure, anyone who has a severe headache that is not relieved with painkillers or is getting worse, should seek prompt medical attention at any point from around four days to four weeks after vaccination.

What about under-40s?

Under-40s are to be offered an alternative to the Oxford/AstraZeneca Covid-19 vaccine as a precautionary move.

The Joint Committee on Vaccination and Immunisation (JCVI) has raised the age threshold after the UK’s medicines regulator reported new figures earlier this month.

The policy of offering an alternative vaccine previously only applied to the under-30s.

The change comes despite the JCVI and the Medicines and Healthcare products Regulatory Agency (MHRA) saying there are no fresh safety concerns.

The MRHA has said the benefits of the vaccine continue to outweigh any risks for the vast majority of people.

Can the clots be treated?

The sooner clots are identified is generally better for the patient. 

Treatment will often involve a mixture of medicine and antibodies given through a drip, with patients then given blood thinners to take home. 

Patients will also be regularly monitored.