“Our epidemic is in decline,” said Professor Jodie McVernon, director of Doherty Epidemiology, which led the team that built the model for the federal government. “But it does not let us be complacent. If we were to release those measures now … those 10 cases today would produce 25 new cases over the course of their infection, not five.”
The number of new cases of COVID-19 diagnosed per day peaked at 460 on March 28 and has since fallen steeply. Just 38 new cases were diagnosed nationally on Wednesday.
The model is not considered accurate enough, however, to project when the virus would be eliminated in the community.
“And this virus has a habit of surprising us,” said Professor James McCaw, an infectious diseases epidemiologist and member of the modelling team.
Modelling of the situation in Tasmania suggests it has not reduced infection rates enough to eventually eliminate the virus. However, due to the small number of cases in Tasmania this estimate has a high degree of uncertainty.
The modelling is being used to guide the government’s response to the pandemic.
It focuses on Australia’s R0, or reproduction number – a measure of the average number of people an infected person infects. If that number drops below one, the virus will eventually drop out of circulation in the community. It is currently close to 0.5.
However, if social distancing measures are loosened, the number will rise.
“It’s the allure of life as normal,” Professor McVernon said. “Because we’re all susceptible to this, the virus will come back from somewhere. We cannot return to life as normal. We need to get that in our consciousness.”
It is still not known how many people are infected but display no symptoms. International estimates are as high as 60 per cent.
“It definitely makes you very cautious about relaxing measures,” Professor McCaw said.
Models can only reflect the assumptions modellers choose to put into them.
The Doherty model is still largely based on Chinese data. In particular, the model bases some of its estimates on the proportion of people who die from a COVID-19 infection.
That may be lower in Australia than China due to higher standards of healthcare, said Professor Catherine Bennett, chair of epidemiology at Deakin University.
If that is true, it would mean the model underestimates the spread of COVID-19 in Australia, says Professor Bennett, “especially the community transmissions that are now our greatest concern”.
Professor McCaw earlier described the model’s findings – based on three separate models, all which came to agreement – as very robust.
Liam is The Age and Sydney Morning Herald’s science reporter