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 it was now advised for people under 50 not to receive the AstraZeneca vaccine unless the "benefit clearly outweighed the risk".
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 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 but it wouldn't be known how much for some time.
"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.
Health Department Secretary Brendan Murphy said it was a highly precautionary measure made out of an abundance of caution.
"All vaccines have adverse effects, some serious. Flu vaccines do," he said.
"Given that this syndrome seems to occur mainly in younger people for whom the risk of severe COVID is not so great, then there is a basis to have a preferred recommendation for those under 50."
There has been one case of the blood clot syndrome in Australia, in a Victorian man.
Professor Murphy said healthcare workers would be prioritised for the Pfizer vaccine in the first stage of the rollout.
"Those over 70 and 80 will continue to get AstraZeneca at their GPs and be confident in its efficacy and its safety," he said.
"For those healthcare workers under 50, they will now be prioritised to Pfizer, and that might delay that particular phase of 1b. But that's the only phase that might be delayed."
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.
More to come.
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