Greg Dore from the UNSW's Kirby Institute says achieving zero community transmission is ideal but may not happen.

"I hope we do get back to zero transmission but it may not be feasible," he told Sydney radio 2GB on Wednesday.

The highly infectious nature of the Delta variant was a game changer, confronting health authorities with unprecedented challenges, Professor Dore said.

"Once it's very well established in the community, once you've got several hundred active cases in the community that will be tougher," he said.

Greater Sydney was recording 30 to 40 cases a day of COVID-positive people who had been circulating in the community unwittingly infecting others.

"Even though we're in lockdown ... there are opportunities for those people to pass the virus on," he said.

"No one in public health would be advocating for letting it rip ... but the problem with the Delta variant is once you get transmission heading upward it can escalate very, very rapidly," he said.

The only answer was for as many people to get vaccinated as soon as possible but Prof Dore lamented that confidence in the "highly effective safe" AstraZeneca vaccine had been undermined.

"We will look back at that as one of the most baffling public health disasters," he said.

Australia's expert immunisation panel ATAGI recommends the Pfizer vaccine for people under 40 because of an extremely rare side effect, with people in that age group advised to speak with their GP if they want to get the AstraZeneca jab.

Australian Associated Press