NSW recorded 30,062 COVID-19 cases yesterday and 16 deaths - the youngest in their 60s - while Hunter New England Health reported 2527 new cases to 8pm Saturday, with 106 people in hospital and nine in intensive care.

Australians have been told it's safer to open up because of the milder Omicron variant but NSW data shows Delta is still in the community and doing damage.

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Delta attacks the lungs but evidence is building to show Omicron is more of an upper-respiratory tract infection, or "URTI".

Data for the week to December 25 was published on Friday, showing 33 people in hospital either fully sequenced or PCR-indicated Omicron, compared to 101 people with Delta.

Another 620 hospital patients had not been sequenced.

"Approximately 80 per cent of confirmed and probable Omicron cases are fully vaccinated, while only 48.6 per cent confirmed Delta cases are fully vaccinated," the report says.

It also shows that 80 per cent of tested Omicron cases were in fully vaccinated people.

Questioned about this, NSW Health cautioned that the numbers of genome-tested cases were very small and could not be extrapolated.

A spokesperson later referred to yesterday's COVID press conference by Premier Dominic Perrottet, which finished with Nepean specialist Dr Nhi Nguyen saying the Newcastle situation showed Delta was still putting people in intensive care.

"We looked very carefully at the Newcastle group, and also anecdotally from our clinicians," Dr Nguyen said.

"Not all patients have been (geno)typed for Delta or Omicron, however there seems to be quite a different pattern, where a certain proportion of patients in ICU still are of the Delta strain and they seem to get the severe pneumonia and long intensive care stays, while those with Omicron, irrespective of needing to be in hospital, their hospital stays are much shorter and what we are seeing not needing to go onto ventilators."

Dr Nguyen said she did not have an "exact breakdown" but while ICU numbers were rising, the number on ventilators was "not rising as high and we think that might be a reflection of Omicron".

The differing disease patterns of Delta and Omicron have been picked up in recent academic papers.

Patients end up on ventilators with Delta because the disease it creates concentrates in the lungs, impeding their function and meaning ventilators are needed to keep people breathing, to oxygenate their blood.

Omicron, on the other hand, tends to inflame the airways leading to the lungs, creating symptoms that are more like well-known upper-respiratory tract infections or URTIs

NSW Health said yesterday that NSW had 1927 COVID patients in hospital, with 151 people in intensive care and 38 on ventilators.

NSW Health does not give a daily breakdown of Delta/Omicron cases, but most, if not all, of the people on ventilators are understood to be Delta.

In the week ending January 2, almost half - 49 per cent - of people in ICU were fully vaccinated, up from 33 per cent the previous week.

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On Wednesday, a World Health Organisation spokesperson said the shift from lungs to the respiratory tract was "good news", but more research was needed.

"We are seeing more and more studies pointing out that Omicron is infecting the upper part of the body. Unlike other ones, the lungs who would be causing severe pneumonia," WHO Incident Manager Abdi Mahamud said from Geneva.

Concern about political messaging that "everyone will get COVID" has been expressed by various health experts.

Professor Kathy Eagar, an author on 450 academic papers and director of the University of Wollongong's Australian Health Services Research Institute, warned about vascular disease from COVID yesterday.

"Australia needs to lift its game in keeping children safe.

"We genuinely do not yet know the long term health impacts of COVID.

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"Our child vaccination strategy is woeful and parents are rightly worried about return to school in just a few weeks. We need to err on the side of caution."

The federal Health Department COVID-19 home page of "case numbers and statistics" has close to 20 sections with various parameters of the pandemic in Australia but neither Omicron or Delta are mentioned directly.

A click-through to "variants" says the WHO is "responsible for tracking variants". No case numbers by variant were visible.

Nationally, the number of people in Australian hospitals with COVID on Saturday was put at 3955 with 291 in intensive care units.

NSW data from June 16 to December 25 - mostly before Omicron - shows that of 598 deaths, 96 were fully vaccinated, 76 partly and 420 "no effective dose", with six under investigation.

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