The University of Melbourne’s modelling suggests that if the state government was to ease restrictions once the 14-day average reached 25,there would be a 64 per cent chance of the infection rate increasing to the extent that restrictions would have to be re-imposed by Christmas.
But at a press conference on Sunday,Premier Daniel Andrews said if restrictions were eased when the 14-day average dropped to five,there was a 3 per cent chance of case numbers increasing to the point that the state would have to go back into lockdown.
He called the analysis “one of the most comprehensive modelling exercises that we have ever seen in the state,arguably nationally,and a piece of work that has international significance as well”.
"You can't argue with this sort of data,you can't argue with science,you can't do anything but follow the best health advice,otherwise... we will just be beginning to lose control again of this virus and the consequences will be even greater than the difficulties -that I know and appreciate - many Victorians are experiencing today,"he said.
On Sunday,the Premier announced stage four restrictions would be extended for two weeks beyond the original end date of September 13. If case numbers remain around 30-50 per day on average by September 28,gatherings of up to five people from two households will be allowed,some workplaces will return to work and some children will start to return to school.
This graph shows how the state's 14-day average is tracking against the target. If the blue line showing the fortnightly average is within the green zone when it crosses the dotted line on September 28,restrictions will ease slightly:
From October 26,if the 14-day average has dropped below five,the curfew will end,there will be no restrictions on leaving home,public gatherings outside will increase to 10,outdoor dining will reopen at cafes and restaurants.
Victoria's Chief Health Officer Brett Sutton said the target of a 14-day average of five cases per day by that date,and none of those cases being from unknown transmission sources,was achievable.
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The model weighs up a lot of different factors such as the infectiousness of the virus,the percentage of cases that are asymptomatic,the number of people each new case infects on average,whether compliance to social distancing and mask-wearing directives drops over time and whether people with the virus fail to self-isolate.
But as its authors point out,there are some variables that the model can’t account for,such as the level of risk in particular areas,in particular industries and even the impact of weather conditions.
All these variables were fed into a supercomputer simulation,which played out the infection curve 1000 times. In 640 of those scenarios,it found cases rose to the point that restrictions had to be re-imposed before Christmas.
This is what the infection curve looked like in one of the simulations the modelling ran:
Deputy Chief Health Officer Allen Cheng said the modelling was not about predicting the future,but provided the most likely outcome based on current information and the impact of lockdown measures.
University of Melbourne professor of epidemiology Tony Blakely,one of the authors of the modelling,said there were two ways the model could be"beaten"- by improving contact tracing,and infectious diseases control protocols in hospitals and aged care settings.
"If we do our contact tracing better than we did three months ago,the contact tracers may be able to hold the case count without it going up again as badly as our model suggests,"he said.
Professor Blakely said the use of 14-day average as a metric of case numbers was a choice of the Health Department.
But Deakin University epidemiology chair Catherine Bennett said the modelling was of limited use because the research was based on international parameters and general principles and was not built around Victoria's infection data.
"We saw absolutely no evidence today there has been any analysis of Victoria's data - the modellers don't have access to it,"she said.
She said the modelling was most likely carried out scrupulously but"it is not the analysis we need".
“We’ve had 17,000 cases,that should have allowed the state government to make more evidence-based decisions about what restrictions should look like,” she said.
She said the tail of the epidemic - when numbers have started to drop - was uncertain and unpredictable,which meant it was difficult to predict whether the 14-day average would reach the targets set for the state to reopen at each step of the way.
You can read the full summary of the University of Melbourne analysis here: