A potential case of Covid-19 transmission between guests in Victoria's hotel quarantine system has authorities scrambling for answers.
Two separate groups of guests in adjacent rooms at Melbourne's Park Royal Hotel have tested positive for the more infectious B117 coronavirus variant, first detected in the United Kingdom.
Victoria's Police Minister Lisa Neville, responsible for overseeing the revamped hotel quarantine program, said genomics had shown the infections were identical.
"That means it's as if they have been in the same room together," she told reporters on Wednesday.
One of the groups, a family of five who are all now infected with the virus, arrived from Nigeria on January 20 and tested positive four days later.
A fellow returned traveller in an adjacent room, who restarted her 14-day quarantine stint after her husband arrived on January 16, twice tested negative before returning a positive result on January 28.
‘Opening door’ could have spread virus into corridor
Questions remain over how the woman in her 60s became infected, given her partner's day three and 11 swabs were negative.
Ms Neville said security footage outside the rooms had been reviewed, with no indication of any breach of protocols by the families or staff during their stay.
"The viral load in the room of the family of five ... was so high that just even opening the door to pick up your food has seen the virus get into the corridor," she said.
"That is the working assumption.
"There has been absolutely no kids running down corridors or movement between the rooms at all."
The infected woman remembers opening her door at the same time as the room next door but she has not been able to pinpoint the exact date or time.
Air vents ‘unlikely’ source of transmission
Deputy Chief Health Officer Melanie Van Twest said authorities believe the potential room leak stemmed from the family's collective infectiousness in combination with the potent UK strain.
"This might be a Swiss cheese line of holes where everything has lined up to create this particular event," she said.
"As far as we know, there's no community transmission. This has happened within the hotel. It's contained."
The hotel's ventilation system will be reviewed, although Ms Neville said an earlier report had found no air was being shared between rooms or into common spaces.
"It's probably unlikely to have been the ventilation system in this case," she said.
All positive cases have been moved to a health hotel and remain in isolation, while the husband of the infected woman has been moved to another room.
Some 100 hotel quarantine staff members and 37 returned travellers who have completed their 14 days on the impacted floor are now self-isolating at home. None have tested positive thus far.
The quarantine scare emerged on Thursday afternoon as Victoria reached 28 days without any new local cases, widely regarded as the milestone for community elimination of the virus.
There are 21 active cases of COVID-19 in Victoria, all in hotel quarantine.
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