An extra week of lockdown in Victoria is unlikely to be enough to quash an outbreak of the Delta coronavirus variant, an epidemiologist says.
Based on modelling by Monash University and his own progression data, University of South Australia academic Adrian Esterman believes local Covid-19 cases will not drop to zero by the end of Victoria's seven-day lockdown extension.
"You've got at least another week and potentially two or three weeks," the veteran epidemiologist and biostatistician told AAP on Tuesday.
"It takes as long to get down as it does to take off. I'm expecting (Monash University) modelling forecasts of at least four weeks is probably correct."
In modelling published by The Conversation when the five-day lockdown was imposed last week, Monash experts predicted it would take at least 30 days of restrictions before Victoria would see three days of no transmission.
Victoria's five-day moving case average is up to 14.2 and the effective reproduction rate is beginning to settle at about three, meaning currently every Covid-19 case is infecting three others.
'Victoria outbreak is in early days'
Professor Esterman said that figure was not unexpected given the Delta strain's heightened infectiousness.
"The basic reproduction of Delta is six, whereas it was about three for the original Wuhan variant," he said.
"Victoria's outbreak is still in its very early days. It's only been a week."
Of the 85 cases in the current outbreak, only one is unlinked and nine of Tuesday's 13 local cases were isolating throughout their infectious period.
Prof Esterman said Victoria was in a "reasonably good position" because of the hard and fast lockdown.
"If they can get it down to just one or two cases a day and those cases are linked and they know where they've come from, then I think they might be willing to relax restrictions," he said.
"Certainly, the outbreak in Victoria will be over well before NSW. We know now that you go in there really fast and very hard to basically crush it."
With cases in Mildura, Phillip Island and Barwon Heads and about 5000 contacts isolating in regional Victoria, he said the Delta variant all but killed off the concept of localised lockdowns.
Most of the regional cases are due to the virus spreading at the MCG during an AFL game on July 10, raising the prospect of fans having to show proof of vaccination to attend major sports events in future.
'Lockdowns will always be in our armoury'
Premier Daniel Andrews argues it is too early to talk about what unvaccinated people can and can't do, given not all Australians are eligible for the jab.
Prof Esterman agrees that "privileges" for vaccinated Australians should not be floated until everyone has the opportunity to get immunised.
Even when that changes, he warned states and territories could still resort to lockdowns to combat new highly infectious strains that are yet to emerge.
"Lockdowns will always be in our armoury," Prof Esterman said.
"But as we get a higher and higher percentage of our population vaccinated, it's going to be less likely we'll have to use them."
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