[A*stralia's] Chief Medical Officer Professor Paul Kelly said while the Omicron variant does appear more likely to break through the protection provided by two doses compared to the Delta variant, it remains unlikely to bring on severe illness.

"We know that … the first two doses of the vaccine are not effective at stopping infection and mild illness as it was with previous variants of the virus," he said.

"The vaccine remains effective against severe disease, perhaps a little less than is the case with Delta, for example, but it does remain effective.

"And I think that's one of the reasons why we are seeing increases in cases, but not so much of an increase in hospitalisations or other forms of severe disease."

Professor Kelly said a booster shot can lift a person's immunity against Omicron to roughly the same level as two doses provide against the Delta variant.

"Boosters do indeed boost that protection — both against transmission and infection and against severe disease," he said.

"Two doses for Delta, three doses for Omicron is roughly equal."