Australian health authorities have advised the Pfizer vaccine should be given to Australians aged under 50, amid concerns of rare blood clots potentially linked to the AstraZeneca vaccination.
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.
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," 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."