Australians in their 30s are in intensive care with the coronavirus, Deputy Chief Medical Officer Paul Kelly said on Monday, driving home the fact that "this is not just an old person 's disease".
Professor Kelly did not say how many younger people were in intensive care, but said they did not have underlying risk factors.
"The more cases you get, the more likely it is that you will find people without any vulnerability to start with and relatively young people having this disease very severely, and this is a wake-up call for all of us," he said.
As of Monday, 448 Australians were in hospital with COVID-19, 96 in intensive care and 35 on ventilators. Thirty-nine people had died.
Professor Kelly urged all Australians to sign up to the "flu tracking" website to record respiratory symptoms each week, likening it to citizen science. The website records postcodes and health authorities are now looking to it as an early warning system for virus hotspots.
Australian cases have topped 5795, with close to 500 of them with no overseas links or links to other cases, with the source of their disease unknown.
"That is the component of the epidemic that we are most concerned about as we go forward," Professor Kelly said.
The government's latest epidemiology report shows people in their 20s are the biggest group to catch the virus, accounting for 22 per cent of cases to March 29.
Deaths so far are confined to the over-50s, with a Victorian man in his 50s becoming the youngest person to die in Australia to date.
People over 70 accounted for 13 per cent of cases. In this age group, men outnumber women.
Young people also recover faster, although it still takes people in their 20s a median time of 27 days to recover (from the time their symptoms started). People over 70 take a median time of 36 days to recover.
The government is preparing to finally release modelling on the spread of the virus on Tuesday, although it is unclear what level of detail will be made public.
Prime Minister Scott Morrison said last week the modelling was being reworked in light of the virus behaviour since the partial lockdown two weeks ago and the imposition since then of stringent measures to keep people apart and encourage them to stay home.
Cases continue to grow but at much lower rates than the daily increase of 20 per cent or more before the stepped-up response.
Economics professor at the Australian National University's Crawford School Quentin Grafton has called for the government to implement a "go hard and go early" strategy which he called "the Hammer".
The Hammer meant a lockdown for at least three cycles of infections, an initial six weeks, then the gradual relaxation of controls with intense testing and tracking.
"The Hammer is not just about "flattening the curve". It also means potentially shortening the curve," he said, writing in The Canberra Times on Tuesday.
"Not only is the total number and peak number of infections dramatically reduced, but so is the period in which the infection rate is unacceptably high. This saves a lot of lives. It also supports the economy."
Professor Grafton compared the strategy to "the Scythe", which is the herd immunity idea promoted by some people, allowing the virus to take its course.
But he said that amounted to "involuntary euthanasia of potentially tens of thousands of mainly elderly people", and was based on at least three false assumptions.
It assumed that the mortality rate is only a "bit worse" than an influenza epidemic, when in fact it appeared to be about 1 per cent, which was 10 times higher than the flu.
COVID-19 also had a much higher hospitalisation rate, with 1 per cent of flu cases hospitalised compared with 20 per cent of coronavirus cases.
The Scythe strategy also assumed people would become immune, which was likely but not established. And didn't take account of the long-term health impacts for people who had been ventilated.
Professor Grafton said the "Hammer" strategy would perhaps halve economic activity for six weeks, equating for a 6 per cent reduction over a year.
A "Slow Burn", which greatly reduced deaths but didn't fully control the crisis, would cost the economy more.
He said New Zealand was aiming for the Hammer, but it was too early to say whether Australia's current strategy would amount to a Slow Burn or a Hammer.
Professor Kelly urged people to sign up to www.flutracking.net/Join which has been used to date to track flu cases each year.
"It is very quick, it is on the web, when you join up you are asked some small details about where you live and so forth, just so we can have tracking across Australia and a map of hotspots of people with respiratory disease," he said.
The flu tracking website asks for personal health information when you register including month and year of birth, sex and education, along with postcode and an email contact.
Each week, people are asked whether they have had a fever or a cough in the previous week, and if so whether they had a sore throat, needed to take time off work, and had seen a doctor.
The website says the information is confidential, with emails kept separately to health information. Data but not contact information is shared for research.
The Health Department confirmed people would not be contacted via the website.
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