Australians under 50 are being told to avoid getting the AstraZeneca COVID-19 vaccine where possible.

Prime Minister Scott Morrison called a snap press conference on Thursday night to announce the news.

The Australian regulator has recommended people under 50 seek the Pfizer vaccine, where possible.

It says the AstraZeneca vaccine is incredibly safe for use, but the advice is being made out of an “abundance of caution”.

It comes following concerns over what’s causing extremely rare cases of blood clotting after having the injection.

People under 50 who have had the first dose of the AstraZeneca vaccine without side effects, are safe to receive the second dose.

The news means Australia’s vaccine roll-out will now fall well and truly behind schedule.