• Tue. Jun 15th, 2021

Rare cases of blood clotting linked to the administration of COVID-19 vaccine AstraZeneca has thrown the medical condition under the spotlight


Jun 10, 2021

Rare cases of blood clotting linked to the administration of COVID-19 vaccine AstraZeneca has thrown the medical condition under the spotlight, but what is blood clotting, and what are the symptoms?

Key points:

  • Very rare cases of vaccine-induced blood clotting symptoms can occur four to 20 days after AstraZeneca’s administration
  • The blood vessels involved are smaller than those affected by deep vein thrombosis, which can occur on long plane flights
  • The symptoms have only been reported in about one in 250,000 people vaccinated with AstraZeneca in Europe

Blood clotting is the body’s natural response to injury or trauma.

Cell fragments called platelets activate to stick to a ruptured blood vessel wall, seal it up, and stop it from bleeding.

The platelets work with dissolved clotting factors in the blood that form a strong net called fibrin.

University of Adelaide haematologist and blood researcher Dan Thomas said blood clots could move around the body when the circumstances that produced the clotting overwhelmed the factors that dissolved clots.

For example, when blood is flowing too slowly during prolonged periods of inactivity, like on long plane trips, clotting could form deep within a vein.

“Symptoms of a usual blood clot include leg swelling, calf muscle pain, shortness of breath, pain when breathing in deeply, or coughing up blood,” Dr Thomas said.

Authorities started rolling out AstraZeneca to Australians in March.

What are the symptoms?

Dr Thomas said the symptoms of vaccine-induced clotting, as they were known so far, included:

  • Severe gut pain commencing 4–20 days after receiving the vaccine
  • A persistent headache unresponsive to paracetamol
  • A general feeling of systemic unwellness beyond four days after the vaccination
  • Stroke-like symptoms, seizures or vomiting

He said the stroke-like symptoms, also known as a transient ischaemic attack, could feel like:

  • Dizziness
  • Transient loss of speech or balance
  • Weakness on one side of the body
  • Confusion
  • Blurred vision

Dr Thomas said platelets had been activated by antibodies in a rare phenomenon where the body thought they were bad because they were sitting next to some of the vaccine, which resembled a foreign pathogen.

“Fortunately, it looks as though these antibodies can go away over time and we can accelerate their removal by treatment with immunoglobulin.”


The AstraZeneca vaccine is now only preferred for Australians over 50.

Dr Thomas said people taking the medication heparin to prevent blood clots could be at increased risk of clotting disorders from COVID-19, or the AstraZeneca vaccine.

“Most importantly, if anyone is worried they may be getting a clotting reaction after being vaccinated, this can be immediately diagnosed with a blood test,” he said.

A distinct type of clotting

The Australian Technical Advisory Group on Immunisation (ATAGI) said symptoms were different to those commonly associated with vaccines.

The common side effects of headaches, muscle aches, fevers and chills can begin within 24 hours of being vaccinated and last up to two days, but the onset of clotting symptoms began four to 20 days after vaccination.

ATAGI said the symptoms resulted from a distinct type of “thrombosis associated with thrombocytopenia”.

“Most cases involved a type of clot in the brain called cerebral venous sinus thrombosis (CVST),” it said in a statement.

The ATAGI said CVST had also been noted as a complication of COVID-19 itself.

Dr Thomas said on rare occasions the virus had “caused loss of blood flow to a limb”, which would present as whiteness in a person’s feet accompanied by pain.


Some 1,178,302 vaccine doses were facilitated by the government by April 11.(Photo: Insiders)

Pfizer preferred for under 50s

The Federal Department of Health said about one in every 250,000 people vaccinated with AstraZeneca in Europe had been diagnosed with the rare blood clotting.

So far, there had been just two reported cases likely linked to AstraZeneca in Australia.

The department said there was a potentially greater risk for people below the age of 50 to develop thrombosis with thrombocytopenia after being administered the vaccine.

It had led to a preference towards the Pfizer vaccine for that age group in Australia, while people above 50 would continue to be given the AstraZeneca vaccine.

“It is important to note the AstraZeneca vaccine remains highly effective at preventing death and severe illness among people who have contracted COVID-19,” said health department heads in a joint statement last week.

“And that the incidence of the blood-clotting syndrome is very rare.”

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