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What we know so far about blood clots linked to AstraZeneca vaccine

People struck down by an extremely rare vaccine-related clotting disorder may take weeks to recover and in more serious cases require ongoing treatment.
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People struck down by an extremely rare clotting disorder that has been associated with the AstraZeneca COVID-19 vaccine may take weeks to recover and more serious cases could require ongoing treatment.

With so few cases detected globally, experts say people with the emerging and sometimes fatal syndrome are being treated on a case-by-case basis.

A nurse holds a vial of the AstraZeneca COVID-19 vaccine, manufactured by the Serum Institute of India and provided through the global COVAX initiative, in Machakos, Kenya.

A nurse holds a vial of the AstraZeneca COVID-19 vaccine, manufactured by the Serum Institute of India and provided through the global COVAX initiative, in Machakos, Kenya.Credit:AP

“Every case is essentially being treated on its merits because we just don’t have enough cases for there to be a robust protocol that’s been replicated over and over,” infectious diseases expert Professor Paul Griffin, who sits on Australia’s AstraZeneca advisory board, said.

“It is such a rare syndrome, which is why it’s causing so much alarm at the moment.”

A 44-year-old man who received the AstraZeneca vaccine remains in Melbourne’s Box Hill Hospital with Australia’s first known probable case of the rare syndrome dubbed vaccine-induced prothrombotic immune thrombocytopenia (VIPIT). He was admitted with abdominal clotting about 12 days after receiving the AstraZeneca vaccine.

The case comes as dozens of rare blood clots have been reported amid the tens of millions of AstraZeneca shots that have been administered globally.

The European Medicines Agency confirmed overnight it had found a possible link between the vaccine and blood clotting issues in adults. Young people in Britain will be offered an alternative to the AstraZeneca vaccine over fears about a potential link to blood clots, in a decision that Australian authorities continue to mull over.

So, what exactly is VIPIT? What is the prognosis and how is it treated?

What does treatment involve?

Treatment for the unusual blood clot disorder does not appear at this point to differ greatly from other types of blood clots, Bill Renwick, head of haematology at Western Health, said.

Generally, a patient with a venous thrombosis (blood clots in the veins) requires treatment for a minimum of three months with blood thinners, but this could be extended to six months, or even indefinitely, depending on why the clot occurred in the first place.

“Some patients have no identifiable cause for their blood clots and this may lead to a recommendation for indefinite treatment with anticoagulants,” Dr Renwick said.

Our blood is filled with platelets – small, colourless cell fragments in our blood that form clots and stop or prevent bleeding.

Cases of VIPIT are fuelled by four factors including low levels of platelets, high levels of blood clotting breakdown products, the formation of blood clots, and the presence of a specific antibody that over-activates platelets in the blood.

In the case of the Victorian man, where a clot was discovered in his abdomen, blood thinners such as heparin or warfarin or newer treatments such as rivaroxaban and apixaban are likely to be used as the first mode of treatment.

“Treatment with anticoagulants (blood thinners) is the same whether it’s a simple deep vein thrombosis in the calf or a clot in a vein in the abdomen,” Dr Renwick said.

But these traditional treatments must be administered carefully.

“The trouble with traditional anticoagulant treatment in these cases is that some of the people, because they also have such low platelets, they actually have issues with bleeding as well as clotting, so blood thinning medicine needs to carefully used,” Professor Griffin said.

“Hopefully one of things that will change in time is when we have more of an understanding as to the mechanism, and how to diagnose it earlier, then potentially we’ll have more specific therapy that we’ll be able to use in a more evidence-based way.”

Complications can also arise from having a blood clot in the abdomen because the veins in this part of the body drain the blood coming back from our internal organs, including the bowel.

So, if there is a blood clot in the abdomen, for example, pressure can build and lead to an inability of the arterial blood to supply blood to the tissues and affected tissues may die.

“Sometimes the treatment with the anticoagulants is early enough to prevent infarction, sometimes it’s not,” Dr Renwick said. “Sometimes part of a bowel may infarct and need to be surgically resected. How long a patient stays in hospital depends on how affected they are and whether a surgical operation is required.”

Jose Perdomo a senior research officer in the Haematology Research Unit at the St George and Sutherland Clinical School at UNSW said more complex cases of the blood clotting disorder may need to be treated with intravenous immunoglobulin threapy.

This involves using a pool of immunoglobulins (antibodies) from the plasma of thousands of healthy donors to treat the clot.

What is the prognosis?

In similar blood clotting conditions to VIPIT, mortality rates can hover anywhere between 30 and 40 per cent.

The new disorder appears to be very rare, affecting about one in every 100,000 people who have been been vaccinated.

Dr Perdomo estimated a person who receives prompt hospital treatment could recover within weeks, but their recovery time depended on the level of antibodies circulating their body.

“It can take many weeks for people to recover with treatment,” he said. “The difficulty is there is not much data out yet, so we don’t know with absolute certainty.”

He said VIPIT was strikingly similar to a blood clot condition observed in people who have been treated with the drug heparin, a blood thinner used to prevent the formation of clots.

In some cases, there has been a potentially dangerous immune-driven adverse reaction to the drug, leading to a condition known as heparin-induced thrombocytopenia.

This occurs when patients, who have been given heparin, develop an antibody that interacts with platelets (blood cells involved in blood clots) which can lead to over-activation of platelets and cause blood clots.

“In heparin-induced blood clots it is slightly easier to treat because the heparin is withdrawn and the condition usually resolves as well,” Dr Perdomo said.

“But in the case of VIPIT there is nothing extra that you can take out so it can take many weeks for people to recover.”

Reports of overseas cases indicate symptom onset was between four and 20 days after vaccination and have generally been severe, with patients requiring hospitalisation.

Professor Griffin, who is also director of infectious diseases at Mater Health in Queensland, said no medical intervention is free of risk.

He stressed the risk of these clotting issues linked with the AstraZeneca vaccine remains exceedingly rare at less than 5 per million doses administered.

“For comparison, a very common intervention in the form of the oral contraceptive pill carries a risk of clotting issues in the order of four of every 10 000 people taking this pill,” Professor Griffin said.

“When assessing the risks and benefits we also have to keep in mind the risks of not vaccinating.”

Many European countries suspended use of AstraZeneca’s vaccine earlier this month following initial reports of the symptoms, which have led to at least 15 deaths.

Available evidence so far continues to suggests the syndrome is extremely rare. Norway has reported a rate of 1 in every 25,000 doses, Germany has reported a rate of 1 in 100,000, while Europe’s overall figures are 1 in 210,000. Britain has recorded a much lower rate: 30 reported cases from 18.1 million AstraZeneca injections – about 1 in 500,000.

In comparison, a 65-year-old has between a 1 or 2 in 100 chance of dying if infected with COVID-19, with that risk continuing to rise with age.

Australian hospitals are now scouring patient records for cases linked to the rare clotting disorder.

The European Medicines Agency and Australia’s Therapeutic Goods Administration have maintained that the benefits of the vaccine continue to outweigh any risks.

With Liam Mannix

Melissa Cunningham is The Age's health reporter.

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