On Thursday evening, Prime Minister Scott Morrison held a surprise press conference where he advised the AstraZeneca COVID-19 vaccine should not be used by Australians under 50.
The recommendation comes after a link was confirmed between the AstraZeneca vaccine and rare blood clots.
European authorities have identified the link, prompting the United Kingdom to offer people aged under 30 an alternative vaccine due to the risk.
Watch: Thank you to masks. Post continues after video.
Morrison advised that the Pfizer vaccine should now be adopted as the preferred vaccine for people aged under 50, following recommendations from the Australian Technical Advisory Group on Immunisation.
"We've been taking the necessary precautions based on the best possible medical advice," said Morrison.
"It has not been our practice to jump at shadows."
The recommendations are made under an "abundance of caution" of the rare but serious side effects mostly associated with younger people.
"This is a very rare event," said Chief Medical Officer Professor Paul Kelly.
"At the moment, it seems to be around 4 to 6 per million doses of vaccine. It's only been found in the first dose of the AstraZeneca vaccine, usually within 4 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."
However, Morrison stressed, "The advice here today is not to NOT have the AstraZeneca vaccine - there is not a prohibition on the use of the AstraZeneca vaccine for persons under 50."