EXPLAINER

Omicron drives COVID case explosion but data does not show full picture: experts

Official COVID-19 case numbers have not been accurate for "a long time" and the gap is only widening as Australia abandons PCR testing, experts warn.

Prime Minister Scott Morrison on Wednesday unveiled a range of changes to Australia's testing regime, including the 'gold standard' test no longer being required for those who return a positive result from a rapid antigen test.

It followed a significant loosening of testing requirements for close contacts, which once saw anyone who had spent 15 minutes with an infected person forced to get tested and isolate.

The shift to rapid tests as the norm was designed to ease pressure on Australia's buckling testing regime, after a pre-Christmas rush caused huge queues, delayed results, people being turned away and others simply deciding not to get tested.

But it raises the prospect of official case numbers, already questionable as Omicron runs rampant, being made all but redundant.

Biostatistics expert at the University of South Australia Adrian Esterman said rising positivity rates - the percentage of PCR tests returning a positive result - across Australia were no surprise given only those most likely to be infected were now being screened.

On December 5, just 0.43 per cent of PCR tests in NSW were coming back positive. By Wednesday, exactly one month later, that had climbed to almost one-third.

The positivity rate in the ACT was 0.07 per cent on December 4. It had exploded to 21.99 per cent one month later.

A look at the interactive graph compiled by The Canberra Times (below) shows each state and territory - potentially barring WA - on a similar trajectory.

Professor Esterman said those coming forward for PCR tests before the rule change were a mixture of the "worried well", those with symptoms but unlinked to a confirmed case, and those picked up by surveillance testing.

"It was a big mish-mash. But because of that, it was a genuine reflection of what was out there in the population, and we were having very low per cent positivity rates," Esterman said.

Canberrans line up for COVID-19 tests at Garran. Picture: Dion Georgopoulos

Canberrans line up for COVID-19 tests at Garran. Picture: Dion Georgopoulos

"That told us then that genuinely we didn't have much COVID around, because the denominator was representative of the infectious status of the population."

Official state and territory daily updates have consistently thrown up record increases, with Australia reporting its highest single-day increase for the third day running on Wednesday.

But Professor Esterman predicted the number of cases circulating in the community was now "way, way" higher than what showed up in official data.

"There's a huge pool of infected people out there that we haven't caught," he says.

"We're not picking up those thousands of people out in the community who are asymptomatic. How many there are, I couldn't tell you."

Deakin University epidemiology chair Catherine Bennett agreed the official data only showed an unknown "fraction" of cases.

But Dr Bennett said they were now reflecting a "different mix", with more people overall having COVID but their contacts now less likely to get tested.

While official data has not accurately reflected actual cases "for a long time", she says the gap is widening.

"Particularly as we came up to Christmas, more people [were] vaccinated [and] less likely to have symptomatic disease," she says.

"Equally, there was still an onus on people who did test positive, for their contacts to be quarantining. So that was probably a disincentive for some people."

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Dr Bennett said an effective way to accurately predict COVID levels in the community would be a large sample taken at random.

But in lieu of that, she argued the climbing positivity rate showed targeted testing "doing exactly what we need it to do": focusing limited resources on those most likely to be infected.

She said the rate had been accelerated by the arrival of summer, when flu and hay fever were less prevalent.

"We're getting rid of the background noise of symptoms from other causes," she said.

"If you've got any cold and flu symptoms, you're [now] more likely to have COVID than a winter cold, so symptoms at the moment have a bit of predictive value for COVID."

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This story Gap between listed and real COVID-19 cases grows first appeared on The Canberra Times.