Epidemiologist Mary Louise McLaws thinks NSW and Victoria should bring the gap between Pfizer doses down from six to four weeks.
"There's only one small study that had nearly 300 healthcare workers who hadn't ever had infection and they looked at extending that dose," she told ABC TV on Monday.
"Mostly, it worked well during the Alpha (variant) period in England, but not Delta.
"I'd suggest that they go back to that 28-day period between first and second dose to try to get young people under the age of 40 vaccinated as soon as possible with two doses."
Victoria recorded 705 new local cases and one death on Monday.
In NSW, there were 961 infections and nine deaths on Sunday, while the ACT recorded 25 local infections.
Slightly more than 47 per cent of Victoria's over-16 population is double-dosed. That figure is 60 per cent in NSW.
Both states are progressively easing restrictions as they head towards vaccination thresholds of 70 and 80 per cent.
Nationally, 51.5 per cent of people aged 16 and older are fully vaccinated,
Children between the ages of 12 and 15 have been quick to roll up their sleeves. More than 27 per cent, or 340,000, have come forward for a jab.
The federal government has asked Pfizer to apply to Australia's Therapeutic Goods Administration for approval to vaccinate younger children.
It also wants states to open their borders in time for Christmas, but is receiving pushback from some, including Queensland.
Tasmania is also reticent to open its doors until all of its residents as young as 12 have been given the chance to get a jab.
The disability royal commission has expressed concern vulnerable people will be left behind when states and territories reopen.
It has recommended the federal government "use its best endeavours" to ensure no jurisdiction reopens until everyone with a disability can be vaccinated.
It comes as Australia works towards being able to manufacture mRNA vaccines so it doesn't have to rely on imports.
Health Minister Greg Hunt insists the government is making progress on that front after coming under fire for not securing more Pfizer jabs earlier.
AstraZeneca is made in Australia, but its take up has been hampered by changing medical advice and hesitancy.
Australia has 60 million doses of Pfizer ordered for 2022 and 50 million doses of Moderna.