Despite testing and checking against known databases, mystery still surrounds the source of the Delta COVID-19 variant outbreak affecting Melbourne and suspected exposure sites in New South Wales including Jervis Bay.
The Victorian Government says the state's lockdown is unlikely to be curtailed - and has announced a $32.2 million tourism package for struggling businesses on Sunday - as the state recorded four new locally acquired cases of coronavirus, bringing the current COVID-19 outbreak to 72 active cases.
Two people have recovered while there were six new infections recorded in hotel quarantine. Amid the current Victorian outbreak, 29,816 test results were returned over the previous 24 hours and 19,940 vaccinations were administered at state-run centres.
Two of the new COVID-19 cases are a 79-year-old resident of the Arcare Maidstone aged care centre and an agency nurse. Another positive case detailed on Sunday is an additional employee of the port Melbourne finance company, while a teacher who works at a school linked to the West Melbourne outbreak of the Delta variant has tested positive.
Authorities expect the Victorian lockdown will continue to Thursday with significant concerns about the now 10 known Delta variant cases and mystery surrounding where it came from. It is not known when, where and how the family from West Melbourne picked up the strain and possibly travelled with through Jervis Bay, Goulburn and Gundagai.
The Deputy Victorian Chief Health Officer Professor Allen Cheng says authorities are "neck and neck" with what they know about the virus, which he regards as the "downstream risk".
"Really most of the people that we're seeing at the moment are people that are already in quarantine and therefore there's no downstream risk from that," he said.
The "upstream", wider risk is another matter for everyone including contact tracers and genetic sequencers.
"So who, for these cases that we can't find, who gave them the infection? Particularly the family that returned from Jervis Bay," Professor Cheng said.
"We are concerned about who was it that might have given them infection, and therefore, you know, could there be other infections related to that."
The prevailing theory is that the Delta variant escaped from hotel quarantine. But where, what state and how is not known.
"I think it'd be fair to say just given, I think, five per cent of Victorians have been tested in the last seven days. If there was a big outbreak going on, I think, you know, we would expect to have picked it up," the deputy Chief Health Officer said.
Infectious diseases expert Professor Sharon Lewin from the Doherty Institute is heading sequencing efforts over the Delta spread. The highly infectious strain could have been picked up by the family in Jervis Bay, but she is leaning towards a possible leak from hotel quarantine.
"My strong hypothesis is that it's coming through an importation from overseas, through our quarantine hotels. That would be the most likely," Professor Lewin said on Saturday.
There has also been an unexpected wastewater detection of fragments of COVID-19 in the inner west of Melbourne which involve suburbs unrelated to the current outbreak. The suburbs include Aberfeldie, Essendon, Essendon West, Flemington, Footscray, Kensington, Maribyrnong, Moonee Ponds, Parkville and Travancore.
To assist struggling businesses, particularly regional alpine tourism businesses, the Victorian government has announced a $32.2 million tourism package, which will include a fourth release of regional tourism travel vouchers.
There will be 80,000 vouchers released under the next round of the travel voucher program.
The state assistance adds to the $460 million in support already announced for Victorian businesses as well as the Federal government's announcement of a temporary COVID-19 disaster payment for workers affected by extended lockdowns.
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