1. What are the risks associated with the Oxford/AstraZeneca vaccine?

This is clearly a fast-moving subject and I can only comment from a general practice point of view not a consultant haematologist or researcher.

The risks, as with any vaccine, include allergy, pain tenderness at site, fatigue, chills, temperature, nausea, joint pains, muscle aches and possibly this very rare new risk of clotting.

2. When exactly do blood clots occur after receiving the AstraZeneca vaccine?

  • As of 31st March, there were 79 cases of thromboembolic events with concurrent thrombocytopenia, low platelets following vaccination.
  • 44 cases of cerebral venous sinus thrombosis with thrombocytopenia and
  • 35 cases of other major thrombotic events including arterial, with thrombocytopenia
  • 51 women and 28 men aged 18-79 years – estimated doses of vaccine given 20.2 million so risk of 4.4 cases per million doses
  • 19 cases had a fatal outcome 

Some had positive platelet factor antibodies and elevated D-dimer and in these occurred after the first dose of the vaccine. More research and clinical trials are required. Background rate of cerebral venous sinus thrombosis (cvst) not known during pandemic. 

The commission on human medicines has not yet established a causal relationship between the vaccine and these events but investigations are continuing. Overall benefits of the vaccine need to be weighed up with risk. The MHRA is still not recommending age restrictions in COVID-19 vaccine AstraZeneca use. However, there may be a trend for increasing incidence with decreasing age with a slightly higher incidence in the younger adult age group. The risk of severe disease associated with COVID-19 increases with age, with youngest adults at lowest risk.

The rare reaction of low platelet count and clots appears to be an idiosyncratic reaction on first exposure to the AstraZeneca COVID-19 vaccine.

The Joint Committee on Vaccination and Immunisation (JCVI) has weighed the relative balance of benefits and risks and advise that the benefits of prompt vaccination with the AstraZeneca COVID-19 vaccine far outweigh the risk of adverse events for individuals 30 years of age and over and those who have underlying health conditions which put them at higher risk of severe COVID-19 disease. JCVI currently advises that it is preferable for adults aged under 30 years without underlying health conditions that put them at higher risk of severe COVID-19 disease, to be offered an alternative COVID-19 vaccine, if available. People may make an informed choice to receive the AstraZeneca COVID-19 vaccine to receive earlier protection.

There are no reports of this side effect following the 2nd dose as yet to date and the JCVI is still recommending those who had the first dose with no side effects to have the AstraZeneca 2nd dose irrespective of age.

In terms of when clots occur, the MHRA is suggesting to look out for symptoms four days post vaccine, the cases occurred within the first 14 days post vaccine.

3. Who is most at risk of blood clots?

Any patient with a history of major venous and arterial thrombosis with low platelets is now recommended not to have the AstraZeneca vaccine. Risk benefit analysis in patients with a history of cerebral venous sinus thrombosis, acquired or hereditary thrombophilia, heparin-induced thrombocytopenia or antiphospholipid syndrome should only be considered when the potential benefit outweighs any potential risks. Patients who have experienced major venous and arterial thrombosis occurring with thrombocytopenia following vaccination with any COVID-19 vaccine should not receive a second dose of COVID-19 Vaccine AstraZeneca.

Healthcare professionals should be alert to the signs and symptoms of thromboembolism and/or thrombocytopenia. Vaccinated individuals should be instructed to seek immediate medical attention if they develop new symptoms such as shortness of breath, chest pain, leg swelling, persistent abdominal pain, any neurological symptoms or signs (such as blurred vision, confusion or seizures) or unusual skin bruising and/or petechiae after vaccination. Vaccinated individuals should also seek immediate medical attention if they develop new onset or worsening persistent headaches which do not respond to simple painkillers four or more days after vaccination. Patients with thromboembolic events and concurrent thrombocytopenia should be urgently referred to a secondary healthcare centre and to a specialist in haematology for advice on further management.

Pregnancy predisposes to thrombosis.

4. How/Why exactly is the AstraZeneca vaccine causing blood clots?

A study by Greinacher and other dated 28th March as a research article suggested that the vaccination induces platelet activating antibodies clinically resembling heparin-induced thrombocytopenia (HIT) by direct binding of the virus to platelets with the adenovirus causing platelet pre activation.

5. Can anything prevent the risk of blood clots? Does aspirin prevent blood clots?

The study suggested the NOACs (novel oral anticoagulants) Rivaroxaban and Apixaban can be considered for treatment along with immunoglobulin.

6. A Canadian panel is recommending the Oxford/AstraZeneca Covid vaccine is paused for under-55s? But what do you think about this move specifically? Do you believe this is a good step? Why/why not?

Follow Joint Committee on Vaccination and Immunisation guidelines under 30, preferable to offer a different vaccine.

7. Do you have any more comments to make about the risk of blood clots associated with the Oxford/AstraZeneca vaccine?

The risks are extremely small and are to be weighed up with the risks of COVID-19 disease.

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