An unmitigated countrywide outbreak of the coronavirus would have completely overwhelmed intensive care beds with a peak daily demand of 35,000 beds, Chief Medical Officer Brendan Murphy revealed on Tuesday.
That was a "horrendous" scenario and the demand on intensive care beds would have been "completely beyond the realm of any country to create", Professor Murphy said.
But he stressed that it was not a realistic scenario since it was based on more than 23 million people being infected, or 89 per cent of the population. Authorities did not expect an infection rate that high.
In the unmitigated scenario, 38 per cent of people would need medical care. Demand would peak in week 18 of the outbreak.
The modelling, from the Doherty Institute, analyses different scenarios to bring the epidemic under control and ensure the health system doesn't get overrun.
Its report, by director of epidemiology Professor Jodie McVernon, says an unmitigated COVID-19 epidemic "would dramatically exceed the capacity of the Australian health system, over a prolonged period".
Isolating people and quarantining contacts would not be enough to bring it under control, with social restrictions also necessary.
With quarantine and isolation measures, the peak shifts to week 25, with a daily demand of more than 16,000 intensive care beds on the worst day, which was also outside Australia's capacity. Under that scenario 68 per cent would be infected, with 29 per cent needing medical care.
With quarantine, isolation and social distancing, authorities believe they can keep the peak demand on intensive care beds in the event of a countrywide outbreak to just under 5000 beds.
If transmission is cut 25 per cent from social distancing measures, 38 per cent of people would become infected, with 16 per cent needing medical care, still outside capacity.
But if transmission is cut by 33 per cent, 12 per cent of people are infected and 5 per cent need medical care, bringing numbers under control.
The peak is pushed out beyond week 41, which is near the end of the year.
On March 7, Australia had 2300 intensive care beds, with the ability to ramp up quickly to more like 5000 by using ventilators and spaces in other parts of hospitals, according to the Intensive Care Society.
The modelling, which has finally been released after weeks of pressure on the government, explains the increasingly strict measures the government has taken to limit the spread of the virus.
On Tuesday, case numbers reached 5844, with 44 dead. Professor Murphy said there were fewer than 100 people in intensive care and fewer than 40 on ventilators.
Professor Murphy stressed that the modelling was highly theoretical, not based on Australian data and was not a prediction of what would happen, but was designed to help authorities plan.
Professor Murphy said complacency was the biggest risk.
"The thing that concerns us most is the more than 500 people who have acquired this virus from someone in the community who doesn't know that they've had it," Professor Murphy said.
"If we in any way lose that rigour that the Australian community has embraced, particularly at Easter, it could all come undone. We've seen what the virus can do in a cruise ship [and] at a wedding."
Professor McVernon said it had been clear early on from Wuhan that "this disease was well beyond our experience of influenza pandemic preparedness".
"What we were seeing in terms of the rate of growth and early indicators of severity were really very marked and based on that we've since seen similar experiences in Europe and in the UK and in the US. It's been terrible to see those pandemics unfold," she said.
"When we went up with models early on it was very hard to comprehend the extent of severity but our government believed us."
She said it was clear very early that an unmitigated epidemic would be well beyond the capacity of any high-income country.
Professor of Mathematical Biology James McCaw said the next few weeks were crucial. While it would see an increase in hospitalisations, admissions to intensive care and deaths, that wave was driven by past cases and did not mean there was a loss of control, he said.
"We're in a very lucky position where we can think about the next steps and the very challenging questions ahead but from a position of relative calm as opposed to crisos we don't have an overwhelmed hospital system yet and we may never have one if we continue to base our responses on the best available evidence."
Prime Minister Scott Morrison said the modelling was based on a broad international data set, but didn't include Australian data because cases were still so low. As case numbers increased, Australian data would be used and the modelling would be broken down to state and territory level.
The data would help authorities "plan the way out", he said.
"For now and certainly over the weeks ahead the lesson is simple, we must continue to do what we are doing, that is how we get through this," he said.
For information on COVID-19, please go to the federal Health Department's website.
- You can also call the Coronavirus Health Information Line on 1800 020 080
- If you have serious symptoms, such as difficulty breathing, call Triple Zero (000)
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