Growing concerns over Australia's main Covid-19 vaccine has prompted restrictions on its rollout overseas.
The European Medicines Agency (EMA) overnight revealed the AstraZeneca vaccine is "possibly linked" with blood clotting in the brain, which has killed 19 people in the UK.
Three of those are people under 30.
The finding has led to UK medical regulators advising the vaccine should not be given to people in that particular age bracket.
The EMA said it has so far received 169 cases of the rare blood clot.
It said most of the cases occurred in women under the age of 60 and while they have advised against placing restrictions on the vaccine's rollout, it will provide further food for thought for the Morrison government which has already faced heavy scrutiny over a botched rollout so far.
Belgium pauses vaccine for those under 56
Belgium ignored the EMA's advice by imposing an immediate four-week ban on administering the AstraZeneca vaccine to people under the age of 56 in order to reassess the risks.
"We will keep it this way for the next four weeks and reassess it then," Belgian Health Minister Frank Vandenbroucke said.
Christian Bogdan, a member of Germany's vaccine committee, said the side-effect's prevalence in women aged between 20 and 59, which he said was 20 times higher than other groups, was a "very clear risk signal".
The federal government has acquired more than 50 million doses of AstraZeneca vaccine, most of which will be produced on Australian soil by CSL.
Frontline health care workers, quarantine staff and the elderly are offered the Pfizer vaccine, which the government has acquired 20 million doses of.
Experts are warning that, even if a causal link is proved, the risks to the general population of getting a serious clot are significantly smaller than the risks from possible Covid-19 infection, which can also cause similar clots, or from many other widely used drugs such as the birth control pill.
"The risk of mortality from Covid is much greater than the risk of mortality from these rare side-effects," EMA's executive director Emer Cooke said.
Australia to press ahead with AstraZeneca vaccine
Acting Chief Medical Officer Michael Kidd has repeatedly downplayed the significance of the side effect.
"It is important to note that our colleagues overseas appear to be seeing one to two cases of thrombosis and thrombocytopenia reported in every 1 million people who receive the Covid-19 AstraZeneca vaccine," he told reporters earlier this week.
"By contrast, we know that the risk of death from Covid-19 remains at one to two deaths per 100 people infected."
The federal government has however asked the Therapeutic Goods Administration to urgently analyse the latest findings from Europe, the ABC reported.
The advice will then be discussed at a National Cabinet meeting on Friday.
The federal government has clashed with state counterparts over the vaccine's delayed rollout, particularly after a spike in community cases in Brisbane in recent weeks.
Health Minister Greg Hunt says one million people will have been vaccinated "very soon" however the figure is well below the four million Prime Minister Scott Morrison had touted for the end of March.
NSW Premier Gladys Berejiklian revealed a mass vaccination hub will be set up at Homebush in Sydney's inner west that can immunise up to 30,000 people a week – if the state can secure sufficient supplies from the federal government.
On Wednesday, Mr Morrison denied criticising the European Union despite blaming it for not providing vaccine exports for Australia's slow immunisation rollout.
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