There's a new Omicron sub-variant - B.A5 - and it's threatening to become dominant.
Health Minister Mark Butler says it's proving to be a tricky adversary.
Data from Israel, which started administering fourth doses at the beginning of the year, shows the shot offers increased protection from catching COVID for about a month but still gives greater protection against severe disease after that.
Sydney University epidemiologist Professor Robert Booy says it's vital given Australia is now recording more than 300 COVId-19 deaths a week.
More than 95 per cent of Australians over 16 have currently had two doses of a vaccine.
But only 70 per cent have had their third dose, despite them being widely available since the start of the year.
As for fourth doses - up until now, only over 65s were eligible – and of that group, just over 60 per cent have had one.
The fourth dose is now available for everyone over 30, but it's only listed as specifically 'recommended' for people over 50.
