The small risk of patients developing a rare blood-clotting condition after receiving a dose of the AstraZeneca vaccine has led to a major roadblock and delay for Australia's rollout of the COVID jab.
While having blood clotting after getting the vaccine remains a rare event, how can the condition come about?
What happened with blood clots and the vaccines in Australia?
Those under 50 have been urged to get the Pfizer vaccine instead of the AstraZeneca, after advice was handed down by Australia's vaccine advisory group.
The advice was given after a man in his 40s in Melbourne developed the rare blood-clotting side effect on Good Friday after receiving the AstraZeneca vaccine.
Since then, a second case was picked up in a woman from Western Australia who received the vaccine in March.
While there have been two cases of the rare clotting disorder, the rate of it developing is still one per 350,000 doses.
The head of the Therapeutic Goods Administration has urged for people to get their vaccine when it's their time, saying the chances of winning the lottery were higher than developing the clotting condition.
Is this just in Australia where this has happened?
There have been several cases of blood clotting linked to the AstraZeneca vaccine in Europe, where some countries have paused that vaccine's rollout or limited it to certain age groups.
In Europe, there have been 222 cases of blood clots that have been confirmed.
However, more than 34 million people across the continent had received that particular vaccine.
Over in the US, health agencies have called for a pause of the single-dose Johnson and Johnson vaccine after similar cases of blood clotting.
There had been six cases of clotting from more than 6 million doses of the vaccine.
Australia has opted not to purchase the Johnson and Johnson vaccine doses due to clotting concerns.
How can the blood clots develop after people get the vaccine?
In most cases, blood clots develop after long periods of inactivity, such as long-haul flights, coming out of surgery, or being immobile for a long period of time.
However, the types of blood clots more commonly associated with the vaccine have to do with the body's immune response.
Health experts have even coined a new term for the type of blood clots being seen: vaccine-induced immune thrombotic thrombocytopenia.
Dr Jose Perdomo, a senior research officer at the University of New South Wales' Hematology Research Unit, said the clotting linked to vaccines was due to antibodies forming in the immune system.
"These are antibodies with a particular coating and they form this particular complex that then invades the blood cells," he said.
"This type of clotting is different to the more common deep-vein thrombosis because it doesn't respond well to anti-coagulants."
While more common blood clots are often found in areas such as the leg, Dr Perdomo said the vaccine-induced blood-clots had the potential to be formed anywhere in the body.
"It could happen in any place and could block vital organs or lead to a stroke or respiratory failure," he said.
Associate professor at the Thrombosis and Haemostasis Society of Australia and New Zealand Vivien Chen said what was driving the blood clots in the vaccine-related cases were different to standard blood clots.
She said the antibodies activated platelets in the bloodstream, which led to a lower platelet count.
Why are these blood clots so serious?
Part of the reason why there has been new advice given for the AstraZeneca vaccine has been the relatively high death rate for the vaccine-induced thrombosis, should the condition be developed.
While the chance of getting the rare condition following the vaccine was one in 350,000, should it be developed, health experts have said there was a death rate of about 25 per cent.
"That type of reaction is very serious," Dr Perdomo said.
"Even though the condition is rare, the chances of dying from it are high, so that's the reason for concern."
The prospect of the blood clot forming in potentially any part of the body as opposed to just the one location also represented a concern.
How come the risk is lesser for older people?
Following the new vaccine advice, those under 50 in Australia have been advised to have the Pfizer vaccine instead of the AstraZeneca.
Australian health authorities said that age limit was put in place due to the benefit of them getting the vaccine outweighed the risk of potential blood clots for those over the age of 50.
Most cases of blood clotting globally that have publicised have been reported in women under the age of 60.
Dr Permodo said more research was being conducted into why older people were less likely to develop the blood clotting after getting the vaccine.
However, one potential reason could be to do with the immune system, he says.
"One of the possible ideas is that the older population have a less reactive immune system, but that's a theory that is still being studied," he said. "The condition, it seems, is more prevalent in women than men."
Can other medication cause blood clots?
Other medication, such as the contraceptive pill, has been known to carry a risk of blood clotting.
While the risk of getting a blood clot from the vaccine is about one in every 350,000 doses, the risk of a clot from the pill is roughly about one in 2500.
However, Dr Chen said the risk of clotting presented for other medication was not for the type of clotting associated with the vaccine. "In that case [of medication such as the pill], we're talking about the risk of standard clotting such as deep-vein thrombosis, which is different to the clotting that's associated with the vaccine," she said.
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