"We will all be exposed to it at some point, including our children," Dr Wark said as the Delta variant spread from Sydney to regional NSW and other states.

The John Hunter Hospital specialist and University of Newcastle conjoint professor said COVID-19 "will not vanish now" and at some point the virus will "get into our community and circulate".

Schools are emerging as hotbeds of Delta transmission in countries such as the United Kingdom and the US which have removed many COVID-19 restrictions.

Dr Wark said vaccinating teenagers and younger children would be a "population decision" to limit the virus' spread.

"At this point, given the very, very low risk to a 14-year-old ... it's difficult to see a great benefit for that individual to rush out and get vaccinated. For children under the age of 10, they're at even lower risk again."

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He said someone in their 20s was "probably 30 or 40 times" more likely to suffer serious COVID illness than someone aged 10 to 20.

"It's still that young adult group that are probably at greatest risk of both infection and transmissions."

Premier Gladys Berejiklian has raised the prospect of sending children back to school when NSW hits vaccination targets which could leave millions of people, including students themselves, unvaccinated when face-to-face lessons resumed.

The federal government has approved the Pfizer vaccine for about 220,000 at-risk children aged 12 to 15. It expects recommendations from the Australian Technical Advisory Group on Immunisation regarding Pfizer for those aged 12 to 15 in the "coming months".

Vaccination trials for under-12s are under way in the US, but Pfizer says it does not expect approval for this age group in America until at least the end of this year.

Dr Wark said it was unlikely Australia would approve the vaccines for young children until "the second half of next year easily".

This presents a dilemma for governments if COVID lingers in the community.

Opening up schools could expose the unvaccinated and vaccinated people with suppressed immune systems to the threat of serious disease.

"There is transmission occurring at that level, and it has a flow-on effect filtering through the rest of the communities," Dr Wark said. "It does appear that schools are featuring more highly as the source of transmission."

He said vaccinated people who did not derive the full benefit of the vaccines were "an important group".

"Some people, even if we vaccinate them, they are not going to necessarily get the efficacy reported in the trials.

"These are people particularly who have problems with their immune system or are on medications which are immunosuppressive.

"In this group, it is becoming very, very clear, things such as neutralising antibody levels are significantly less."

Allowing students to return to school would be "more reasonable" in areas with low transmission.

"At some point there clearly has to be a balance between opening up and risk, and where that appetite is going to be is as much a political decision as what it is a medical decision," he said.

"The Doherty modelling assumes ... that you've got to get that mark of 70 to 80 per cent of your population vaccinated, but even they're advocating you're going to need face masks, social distancing, limited lockdowns."

He said countries had become better at protecting the elderly and otherwise vulnerable, reducing the threat of a repeat of the March and April 2020 "nightmare".

In the UK, which opened up on July 19 as Delta cases were rising sharply, infections continued growing to about 45,000 a day then dipped to about 25,000 in early August. They have risen again to about 29,000 in recent days. The daily death rate has risen from about 50 to 90 since July 19.

Average daily deaths in the US have grown from about 200 in July to 650.

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