Australians under the age of 50 will be warned off receiving the AstraZeneca vaccine, after a link was confirmed between the jab and rare blood clots.
European authorities have identified the link, prompting Australia's drug regulators to hold urgent meetings to consider their findings.
The Prime Minister received recommendations from the Australian Technical Advisory Group on Immunisation on Thursday night.
Chief among them is that the Pfizer vaccine should now be adopted as the preferred vaccine for people aged under 50.
The recommendations are made under an "abundance of caution" of the rare but serious side effects mostly associated with younger people, Prime Minister Scott Morrison said.
"We've been taking the necessary precautions based on the best possible medical advice," he said.
"It has not been our practice to jump at shadows."
One person in Australia so far has developed the clots after receiving the jab, a man in his 40s who was admitted to hospital in Melbourne.
ATAGI also recommended those under the age of 50 who have received their first AstraZeneca jab should proceed with their second, as the medical advice indicates the rare blood clots only develop after the first dose.
Only where the benefit clearly outweighs the risk should an initial AstraZeneca dose be administered to someone under the age of 50.
"Ultimately here, the choice is with individual Australians and their doctor," Mr Morrison said.
"This is not a directive. This is not an instruction."
Health care workers aged under 50 who were due to receive the AstraZeneca vaccine will now be prioritised for the Pfizer vaccine, which will likely delay phase 1b of the rollout.
"Important to remember, though, that we don't have community transmission," Health Department Secretary Brendan Murphy said.
"These healthcare workers are not at significant risk if it means they wait a few more weeks to get their vaccine."
ATAGI also considered action taken overseas in formulating their advice, with the United Kingdom to offer people aged under 30 an alternative vaccine, and other countries considering attaching warning labels.
Chief Medical Officer Professor Paul Kelly said the adverse reaction to the jab was so rare it was not picked up in clinical trials.
"At the moment, it seems to be around four to six per million doses of vaccine," he said.
"It's only been found in the first dose of the AstraZeneca vaccine, usually within four to 10 days after that vaccine.
"But it is serious, and it can cause up to a 25 per cent death rate when it occurs."
The health authorities stressed that people aged over 50 should still enthusiastically line up to get the AstraZeneca jab.
"They are the most vulnerable group in the country," the prime minister said, pointing out the majority of deaths from the virus in Australia were among the elderly.
"The health advice is that the AstraZeneca vaccine for persons aged over 50 is well advised because, should there be an outbreak of COVID-19, they would be very exposed to that risk."
"The risk is extraordinarily low. I've had my first dose. I'll have my second dose," Brendan Murphy added.
However, ATAGI's advice essentially relegates most of the government's vaccine rollout timetable to the waste bin.
The government had initially hoped to fully vaccinate all willing Australians by October, with the majority receiving the AstraZeneca jab.
Now it is unsure if every Australian will even be offered their first dose this year.
"We will have to replan... how we will deliver vaccines and we'll have to come back at a later stage with better estimates on when things in each phase will be completed," Dr Murphy said.
The use of the Pfizer vaccine will be ramped up to accommodate for the change.
Some 20 million doses of the Pfizer vaccine are due in Australia by the end of the year, and Health Minister Greg Hunt said he is confident they will be able to increase their supply to Australia in the near future.
The government also hopes deliveries of the 51 million doses of the Novovax jab Australia ordered will begin from October, despite the vaccine remaining in the clinical trial phase.
ATAGI's recommendations and the changes to the vaccine rollout will also be discussed among state and federal health ministers and at the next meeting of national cabinet on Friday.
Australian Associated Press