The Federal Government's expert medical taskforce met on Wednesday to consider use of the AstraZeneca vaccine, following advice from the European Union's medical regulator that "very rare cases of blood clots" were a side-effect in the weeks after the vaccine was administered.

The United Kingdom decided to offer other vaccines, such as the one produced by Pfizer, to people aged under 30.

Chief Medical Officer Paul Kelly made the announcement, flanked by the Prime Minister Scott Morrison, in an unexpected press conference on Thursday night.

"The use of the Pfizer vaccine is preferred over the AstraZeneca vaccine in adults aged less than 50 years who have not already received a first dose of AstraZeneca vaccine," Professor Kelly said.

"This is based both on the increased risk of complications from COVID-19 with increasing age, and thus increased benefit of the vaccination, and the potentially lower, but not zero risk, of this rare event with increasing age.

"Immunisation providers should only give a first dose of AstraZeneca COVID-19 vaccine to adults under 50 years of age where benefit clearly outweighs the risk for that individual's circumstances."

Professor Kelly said people who have had their first dose of the AstraZeneca vaccine, and had not experienced any "adverse side effects", can be given their second dose.

"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," he said.

"That's the all but one person that we've had so far in Australia are in that category."

A Melbourne man was hospitalised last week, developing blood clots almost a fortnight after receiving the AstraZeneca vaccine.

Announcement brings changes to national vaccine rollout

Prime Minister Scott Morrison said planning would continue through the night to consider how the advice from the Australian Technical Advisory Group on Immunisation (ATAGI) would change the national vaccine rollout.

"There are, of course, Pfizer vaccines that are in Australia, and we are getting a regular supply of those," Mr Morrison said.

"They can be prioritised against the individuals for whom that will be the more appropriate vaccine for them.

"But we will just have to work through the logistics of that and the calibration of how that is done."

The matter will also be discussed by state and territory leaders at Friday's national cabinet meeting.

The Chief Medical Officer said the decision was made to ensure confidence in the coronavirus vaccine program.

"We've taken very rapid decisions to look at that data carefully and to make the decision that has been made today, and the advice has come from medical experts on that basis," Professor Kelly said.

"We're sharing that with the Australian public so that they can be aware and know that, if we ever get that information, we will immediately and fully be transparent about it."

Australia's vaccination program is currently only in phase 1a and 1b, which focuses on frontline healthcare and quarantine workers, aged care residents and staff, older and vulnerable Australians.

The Prime Minister was asked whether the advice about the AstraZeneca vaccine would blow out the Commonwealth timeframe for all Australians to receive at least one dose of the COVID-19 jab by the end of the year.

"I think we have to take the time to assess the implications for the program," Mr Morrison said.

"When we've done that, we may be able to form a view, but I don't think anyone should expect that any time soon.

"But it won't stop the work that we're doing in rolling out the vaccination program right now with the doses that we have, particularly from Pfizer, but also rolling out from AstraZeneca, which are predominantly for older Australians above 50 in phases 1a and 1b."

Story By | Matthew Doran & Stephanie Dalzell