Australia's health experts say even with the proposed reset of the COVID-19 vaccination program there is no way of putting a timetable on its completion.
The national cabinet - comprising the prime minister and state and territory leaders - is due to release details of a revised plan to roll out coronavirus vaccinations on Thursday.
The revision was needed after advice was received that the Pfizer shot should be the vaccine of preference for under-50s, and AstraZeneca used for over-50s due to blood clotting issues.
Federal health department secretary Brendan Murphy told a parliamentary inquiry on Tuesday he believed the first two phases of the vaccine program - focused on aged care, disability care and frontline workers - would be completed in May.
However, there was still much work to be done in terms of disability care residents.
So far of the 25,000 people in 6000 disability facilities, only 1640 have had a jab delivered.
The disability facility program had to be paused after the advice on AstraZeneca and blood clots, but would gear up shortly, the committee heard.
Professor Murphy said governments were committed to have all people vulnerable to severe COVID vaccinated by the middle of the year.
"At the moment we are not in a position to give an updated time on when vaccinations will be completed, but all first ministers want it completed as soon as possible," he said.
"There are still a number of uncertainties, even with a recalibrated plan."
He said these included hesitancy in the community and supplies of vaccine from overseas.
A third vaccine, made by Novavax, was now due in the third quarter of this year, he said, rather than mid-year as originally expected.
Health Minister Greg Hunt was also cautious when asked about a new timetable, with 1.7 million doses having been delivered so far nationally.
"We've had a six-fold, four-week improvement since GPs came on board," he said.
"The rollout is accelerating."
Australian Medical Association president Omar Khorshid said supply issues were no longer the main hurdle in the rollout.
"Our problem in the past was a lack of vaccines, but just in the last week or so, we've got a new problem and that is Australians have lost confidence in the vaccines," he told Nine.
"A real challenge for our governments, for our medical profession, is to actually get Australians to go into their GP, roll up their sleeve and get their vaccine."
Prime Minister Scott Morrison has flagged using mass vaccination centres later in the year as part of a "12-week sprint" before Christmas.
Deputy Labor leader Richard Marles said the coalition's complacency in focusing on just a handful of vaccine candidates risked Australia's economic recovery.
"If the prime minister has got one job this year, it is to vaccinate Australia. Right now, that seems in peril," he told Sky News.
"They really bet the house on the manufacture of AstraZeneca here and on it doing the lion's share of the work of vaccinating Australia."
Meanwhile, a vaccinated border staffer in New Zealand has tested positive for coronavirus just one day after the start of a two-way travel bubble with Australia.
Mr Hunt said Australian authorities had full confidence in New Zealand's containment system.
"They're on to this," he said.
