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A Melbourne man has developed a blood clot after taking the AstraZeneca vaccine.Credit:AP

“We cannot say there is a causal link. That’s the bottom line,” said Associate Professor Vivien Chen,lead author of the Thrombosis and Haemostasis Society’s just-released guidance on VIPIT. “This is a new,emerging syndrome. The scientific investigations to show causality have not been done.”

However,Professor Jim Buttery,head of epidemiology and signal detection at the Victorian immunisation safety service,said he now believed there was evidence of a link.

“Although not conclusively proven yet,it is likely there is a causal link between the AstraZeneca vaccine and this rare subset of thrombosis with thrombocytopenia,” he toldThe Agevia email.

“Based on the data available to date at the moment it appears the UK detected 30 cases from 18 million doses of vaccine,approximately 1 in 500,000 doses.”

Available evidence suggests the syndrome is extremely rare. Norway has reported arate of 1 in every 25,000 doses,Germany has reported a rate of 1 in 100,000,while Europe’s overall figures are1 in 210,000. Britain hasrecorded a much lower rate:30 reported cases from 18.1 million AstraZeneca injections – around 1 in 500,000.

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

TheEuropean Medicines Agency and Australia’sTherapeutic Goods Administration say the benefits of the vaccine continue to outweigh any risks.

Initially,much of the focus was whether AstraZeneca’s vaccine may raise the overall risk of blood clots. There is nowevidence to show this is not the case. However,that concern turned out to be something of a red herring.

Our blood is filled with platelet cells,which are able to clot together when we are bleeding and clog the wound. This is why you stop bleeding soon after cutting yourself.

Cases of VIPIT are characterised by four things: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.

“It’s predominantly blood clots in what we would call unusual places,” said Dr Chen.

The unusual antibodies are a hallmark of the syndrome. They activate the platelets,which has the dual effect of causing blood clots to form while also removing other platelets from the bloodstream.

“There are several mechanisms by which the antibodies could develop,and as yet we don’t know the exact mechanism,” said Professor Paul Monagle,a paediatric haematologist at the University of Melbourne who is studying the syndrome.

“One possibility is it is creating antibodies,and those antibodies are recognising some sequence on the platelets,instead of the vaccine.”

Potential risk factors also remain unclear. Most of the cases reported so far were in women younger than 55.

That has led several European countries to stop giving AstraZeneca to people aged under 60. However,it could be that age and gender are not risk factors. Many European countries gave AstraZeneca to younger groups first,as well as healthcare workers –of which a large proportion are women.

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