New advice has been issued for people under 50 about the AstraZeneca vaccine, over concerns of a link to blood clotting.
Chief medical officer Paul Kelly said people under 50 should not receive that vaccine unless the "benefit clearly outweighed the risk".
The use of Pfizer is preferred over AstraZeneca in people under 50, who haven't already received the first dose of AstraZeneca.
Professor Kelly said the link to blood clots was a very rare, but serious concern.
"At the moment, it seems to be around 4 to 6 per million doses of vaccine," he said.
"The third recommendation is people that have had their first dose of the COVID-19 AstraZeneca without any serious adverse effects can safely be given their second dose. This includes adults under the age of 50.
"People who have had blood clots associated with low platelet levels after their first dose of COVID-19 AstraZeneca should not be given the second dose."
Prime Minister Scott Morrison said this would affect the vaccine rollout.
"We expect that this will require some changes to the arrangements we have as part of the vaccination rollout," he said.
"We discussed that this morning at my media conference earlier today [Thursday], that that could be possible. And this includes when we might expect our first doses, ultimately, to be able to be offered to all Australians."
The Prime Minister said the advice was received from the expert vaccine panel late on Thursday evening.
"The key principle of our management of the COVID-19 pandemic has been always to base our decisions on the expert medical advice. It has not been our practice to jump at shadows. It has not been our practice to take unnecessary precautions," Mr Morrison said.
Australia's expert vaccine panel met on Thursday to discuss the safety of the vaccine amid concerns about links to blood clotting.
The development comes after the UK medical regulator on Wednesday advised against the use of the AstraZeneca vaccine for people aged 18-30, after reports of rare cases of blood clots.
That prompted the United Kingdom to offer people aged under 30 an alternative vaccine due to the risk.
Other countries are considering attaching warning labels.
The European Medicines Agency has not made a specific recommendation, but found women and people under 60 were at a higher risk of developing the rare side effect.
Australia's drug regulators are holding urgent meetings to consider the findings before providing the government with recommendations.
Health Minister Greg Hunt said earlier on Thursday said the vaccine would be restricted if experts recommended it.
More than 996,000 coronavirus vaccine doses have now been administered nationally.
This story AstraZeneca rollout slowed amid rare blood clot concerns first appeared on The Canberra Times.
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