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Victoria’s Chief Health Officer Brett Sutton was among those showing up to get an AstraZeneca shot as the state’s three mass vaccination centres threw open their doors to an extended cohort on Wednesday. 

As he did, he declared: “You are more likely to get a clot, whatever age you are, on a long-haul flight to Europe or North America.”

So how does the rare clotting condition associated with the AstraZeneca vaccine compare with other types of blood clots we’re more familiar with?

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Haematologist says long-haul flights are more risky

University of Adelaide haematologist and blood researcher Daniel Thomas said Professor Sutton’s comments were right and deep vein thrombosis (DVT) was “common”.

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“There’s low pressure on planes, people’s knees are often bent at an angle that makes it difficult for blood to return to the heart so the bloodflow is slower, predisposing you to large vein clots below the knee,” he said.

“Often people are a little dehydrated on planes and they may not think of moving around because they’re asleep.”

He said hundreds of people in Australia developed DVT each year, but the risk of death was “much lower” than the 25 per cent fatality observed so far in the clotting syndrome linked to the AstraZeneca vaccine.

“That’s the problem we’re facing — is that if you develop a post-vaccine clotting syndrome, from the few cases we’ve seen so far, the risk of serious complications or death seems to be higher than if you developed a clot from a plane flight,” he said.

Australia’s Therapuetic Goods Administration announced on Friday night it was investigating three fresh cases of clots developing in people who had received the AstraZeneca jab.


Sitting still for long periods of time during activities such as long-haul flights increases your risk of blood clots forming.(iStockPhoto/JennaWagner)

Dr Thomas said it was critically important not to lose sight of the benefits the vaccines had delivered.

What we know so far about clotting and AstraZeneca

The identification of more than 100 cases of very rare blood clots linked to the vaccine across the world caused Australian authorities to re-examine the rollout of AstraZeneca, the vaccine we have the most abundant supply of.

It’s estimated that for every million people given the vaccine, five will develop the rare clots and a quarter of those who do will later die.

The Australian Technical Advisory Group on Immunisation (ATAGI) describes the clotting as thrombosis with thrombocytopenia syndrome (TTS), which is a combination of clotting and a low blood platelet count.

The authority said the condition was most likely to occur between four and 20 days after the first dose of the vaccine.

University of Western Australia haematologist Matthew Linden said experts didn’t completely understand what was causing the clotting, which has mostly been occurring in the brain but can also affect other parts of the body.

He said there was good data suggesting the the vaccine may be creating an inappropriate immune reaction against a protein found in blood platelets.

It’s also known that the conditon has been more prevalent among women under 55.

What’s the official advice in Australia?

The federal government has announced the Pfizer vaccine is now the preferred choice for people under 50, but in some circumstances those under 50 can still take AstraZeneca if they’re making an informed decision.

The rollout of AstraZeneca has been continuing in those aged above 50.

Burnet Institute disease elimination deputy program director Joseph Doyle, who is also an infectious diseases specialist at The Alfred in Melbourne, said authorities had to choose a specific age to manage risk.

“You’ve got to think about the risk of COVID-19 when you’re over 50, 60, 70 and 80.

“We know that for every decade of age, the risk of severe COVID — getting into hospital, intensive care or even dying — increases by about three times.”

joseph Doyle says a ‘risk-balancing decision’ has to be made weighing up the benefits of AstraZeneca against its serious, but rare, side effects.(Supplied: Joseph Doyle)

Dr Doyle said a delicate balance was required to weigh the risks posed by the virus and concerns about rare vaccine side effects.

“There’s been a really, really rigorous appraisal in Australia about that and to be conservative, they’ve decided that 50 is the age,” he said.

“There has to be a risk-balancing decision about when you should think of these serious but small side effects and work out when it’s safe to give the vaccine.”

How are other countries dealing with AstraZeneca?

Dr Doyle said it was also important not to lose sight of the comparitively lucky position Australia was in.

“If we were in a pandemic situation like almost any other country in the world, then we would be vaccinating people well below 50 because there’s more and more benefit,” he said.

The Oxford University/AstraZeneca COVID-19 vaccine.(AP: Steve Parsons/Pool Photo)

In the UK, where more than 20 million doses of the vaccine have been administered, there are warnings about the risk of the clotting condition and its use is being limited to those over 30.

Some other countries using AstraZeneca as part of their immunisation programs, including Canada, France, Germany and South Korea, have limited its use to older age groups of at least 50.

In its official advice, ATAGI said the way Australia assessed AstraZeneca’s suitability would be different to other countries, particular those where COVID-19 was widespread.

“This includes countries in our region such as those currently experiencing very serious outbreaks of COVID-19, such as Timor Leste and Papua New Guinea,” it said.

Another key piece of information that experts have been highlighting is that the virus itself is more likely to cause blood clots than the AstraZeneca vaccine.

Vivien Chen, a haematologist with a focus in coagulation disorders at the University of Sydney, said blood clots were a symptom of coronavirus.

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