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The random nature of COVID-19 makes it impossible to know if Victoria’s small outbreak will lead to sustained infections or simply peter out, experts say.
That means government policy – restrictions and lockdowns – depend on how much risk-appetite Premier Daniel Andrews and his ministers have. On Thursday afternoon, reports suggested Mr Andrews had opted for some form of immediate lockdown after Victoria recorded 10 cases on Wednesday.
Melburnians wearing masks around South Melbourne Market in June.Credit:Wayne Taylor
Federal government tracking continues to suggest Victorians lead the nation in social distancing. But the Delta variant – perhaps 60 per cent more infectious than the virus that first left Wuhan – has upped the stakes.
It is not really possible to know if a lockdown is necessary at this stage, said Dr Jason Thompson, a University of Melbourne modeller who worked on the Victorian government’s epidemic modelling.
“What our models say is sometimes the infections burn out naturally, because people who are infected don’t come into contact with anyone or just stay in their house. And sometimes it takes off,” he said.
“People talk about the stochastic, or random, nature of these things, and that’s it. Occasionally it takes off, and occasionally it dies off.
“You don’t know. You simply don’t know. It might, it might not.”
This is driven by a key feature of COVID-19: many infected people simply do not pass the virus on, at least for earlier variants.
But a small number of infected people are super-spreaders who are responsible for most infections.
Victoria witnessed this when an infected person spent a night at several bars in Chapel Street, and did not pass the virus on at all.
“The wrong people have to get it. Like the removalist, who goes to several homes and spends lots of time there,” said Dr Michael Lydeamore, a Monash University researcher who is contributing to federal government epidemic forecasts.
Federal government data, based on mobility surveys, suggests Victoria’s “transmission potential” – how effectively we are social distancing – remains the lowest of any state in Australia other than locked-down NSW.
In earlier phases of the pandemic, this would give us confidence about the new cases. But Delta changes the game, experts said. Federal government modelling assumes it is about 60 per cent more transmissible than the virus that first came out of Wuhan.
When Dr Thompson places Delta in his models, “it just looks like a massive explosion if you don’t control it”.
La Trobe University epidemiologist Dr Joel Miller said he would be watching the identity of any cases found for clues as to whether more cases were likely. If the cases are found through contact tracing and no mystery cases show up, “I would not think lockdown is needed”.
“If we start seeing people show up who have been infectious for a few days without isolation, or people who have no clear exposure, that’s when we need to become nervous,” he said.
A study on Victoria’s second wave, funded by the Victorian Health Department and co-authored by Chief Health Officer Brett Sutton, found masks made the strongest contribution to bringing the outbreak under control.
Mandatory face-masks cut transmission by between 31 and 46 per cent while physical distancing cut transmission by between 6 and 28 per cent. On Wednesday night Victoria imposed new mask restrictions, requiring they be worn in all indoor areas.
“Face coverings was one of the most important interventions – maybe even more important than lockdown,” said Dr Romain Ragonnet, one of the co-authors of the study.
Liam Mannix’s Examine newsletter explains and analyses science with a rigorous focus on the evidence. Sign up to get it each week.
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