Victoria is strongly considering extending the seven-day lockdown due to end at 11:59 on Thursday amid growing fears about rapid transmission in settings never seen before.
People exercise in Melbourne on Tuesday, the fifth day of a seven-day lockdown. Photo: AFP
The state reported six new locally acquired coronavirus cases on Wednesay taking the outbreak to 60 cases.
Chief Health Officer Brett Sutton said on Tuesday night that the virus was moving "faster than any other strain we've dealt with".
"We're seeing transmissions in settings and circumstances we've never seen before," he said.
His comments came as a new positive case was identified in a Victorian who had travelled to New South Wales and back again, prompting a whole raft of new exposure sites.
For the first time, health officials have seen four or five cases where people were infected by strangers by casual contacts.
Victoria's Covid-19 testing commander Jeroen Weimar said the interactions were "very fleeting contact" and quite different to what Australia is used to seeing.
Normally transmission had happened in workplaces, at big social events, or in the home, but in this outbreak people are getting sick just from breathing the same air as an infected person.
"What we're seeing now clearly is people who are, they're brushing past each other in a small shop, they're going around a display home, they're looking at phones in a Telstra shop," he said.
"This is, relatively speaking, relatively fleeting contact. They don't know each others' names. And that's very different to where we've been before."
Why the more rapid spread?
So what's changed? It's the threat posed by the highly infectious Covid-19 variant found in Melbourne.
The B.1.617 variant identified in India was first detected last October and has since spread to more than 40 other countries including Australia.
Experts have identified three subtypes. Cases in Victoria's latest cluster are infected with the first sublineage, B.1.617.1.
While there is still much to learn about the variants one thing is certain: it's much, much more infectious.
Dr Sumar Majumdar, an infectious diseases and public health expert at the Burnet Institute, said the Indian variant was able to spread more eaily through the air and people who don't know each other were becoming infected.
"They've gone into a space and then very quickly were breathing air from somebody else ... and got infected," he said.
"So the question is really, how good was the ventilation in these places."
And of course no-one was wearing masks at the time - so that crucial layer of protection was missing.
Early on in the pandemic, we were told you were at risk of being infected by Covid-19 after spending face-to-face time in a room with someone you knew.
But as evidence mounts about aerosol transmission of the virus, the settings of most risk have changed.
'Need that ring of steel'
The pandemic is worse today than it was a year ago and more variants are emerging on a regular basis, Professor Raina McIntyre, from the Kirby Institute said.
"So the stakes are much higher, and they (variants) will come in through quarantine," she told Afternoon Briefing.
MacIntyre agreed the biggest risk was breathing in contaminated air in poorly ventilated settings.
"Small things like keeping your car window open if you are travelling with others, or opening a window at home even though it's cold will make a big difference," she said.
Majumdar said the World Health Organisation has recently issued new advice about aerosol transmission - people should avoid the three C's: crowds, close-contact settings, and confined spaces.
"If you're in places where you think it's confined or poorly ventilated, it's not a safe setting," he said.
That advice will be especially important because it appears as though the serial interval - the time it takes between when a person becomes infected to then becoming infectious - is getting much shorter than the four days it used to be.
"I've noticed that the Victorian government have said it looks like a day, which is of great concern and suggests that of course, this lockdown was inevitable," said Professor Mary-Louise McLaws, an advisor to the WHO.
Majumdar said it was more important than ever for everyone to get vaccinated as soon as possible.
All these concerns will be weighing heavily on the minds of Victoria's public health officials as they try to decide which is the safest path forward to end the outbreak.
- ABC