People reinfected with COVID-19 soon after recovering from an initial bout could be free to roam the community under current isolation rules, though health authorities insist the scenario is "unlikely".
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Canberrans who would otherwise be considered close contacts do not have to isolate or get tested after exposure to COVID-19, provided they have contracted the virus within the preceding month.
The guideline, which came into effect this month and signalled a loosening of the rules in the ACT, has been adopted by every state and territory bar Western Australia.
With both the Delta and Omicron strains circulating, it raises the prospect of a person being able to move freely through the community while infected.
But after Omicron swiftly became the dominant strain across Australia, a Health Department spokesperson played down the threat posed by reinfection within 30 days.
"This is in the current context of living with COVID, and based on current evidence, that reinfection within a month from any strain is unlikely," they said.
"The Communicable Diseases Network Australia is continuously reviewing the evidence and will update advice when more evidence is available."
But infectious diseases expert at the Australian National University Gaetan Burgio said "everything has changed" after the emergence of Omicron, which data from the UK shows results in "easily" five times more reinfections than Delta.
He said the highly-infectious new strain made it "not quite correct" to say reinfection was unlikely within a month, particularly before it supplants Delta entirely.
"If the first infection is Delta, the infection rate with Omicron would be high. However, if the first infection was Omicron, the infection rate will be lower," he said.
"A lot of the infection now is Delta, so I think the [re]infection rate is pretty high."
Delta accounted for more than 11 per cent of cases in NSW at the beginning of January, and a similar level would see Delta make up around 1500 of the 13,000 new cases reported in the state on Tuesday.
Although Delta likely accounted for a smaller percentage of cases almost a month later, Dr Burgio warned it was impossible to know without genome sequencing, which was largely abandoned when a December explosion rendered it impractical.
"It's really hard to tell unless you know exactly which strain you have been infected with ... but [Omicron] is spreading. It's taking over, for sure, until the next variant," he said.
Dr Burgio said a booster and previous infection significantly reduced the chances of severe illness from reinfection and, by extension, the likelihood of transmitting the virus.
But the value of allowing a 30-day isolation exemption was not "clear cut", he warned, and relied largely on the context; tighter rules in WA made more sense than in Victoria and NSW, where Omicron was already running rampant, he argued.
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In Victoria, a patient was exempt from isolating for 90 days after an initial infection, but that was slashed to 30 days as Omicron grew roots.
But the 30-day exemption is a loosening of the rules in the ACT, where COVID-19 clearance certificates previously told Canberrans to get tested if exposed again.
"It is important that recovered cases still get tested if they develop new symptoms that may be COVID-19," one read on December 31.
"Due to the emergence of the Omicron variant, this certificate does NOT exempt the individual from relevant testing and quarantine requirements in the situation of an exposure to a confirmed case of COVID-19."
Prime Minister Scott Morrison last week suggested Australia could follow the UK in slashing isolation from seven to five days if recommended by health authorities.
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