Experts believe a COVID-19 reinfection surge is under way across Australia, but the true picture may never be known as governments abandon attempts to track second cases.
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And elderly Australians are being warned to be especially cautious over a winter expected to come with another wave.
Australia is now edging towards six months since the highly-infectious Omicron variant ripped through Christmas. Immunity gained during that December explosion is beginning to wane.
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The strain's subvariants have also proved adept at avoiding immunity - from vaccination or prior infection - and Australians are increasingly testing positive to COVID-19 for the second time.
But the federal, state and territory governments have no system in place to track reinfections in real time, and infectious diseases analyst Adrian Esterman said Australians can only draw inferences from experiences overseas.
More transmissible, less lethal
Pre-Omicron data in Denmark showed reinfections accounted for around 4 per cent of new cases, but a British study in February showed that number rising to 10 per cent - the UK has now reported more than 800,000 reinfections.
"We can't say hand over heart the same thing's happening here, but there's no reason why we wouldn't be seeing a reasonably large number of reinfections in Australia as well," Professor Esterman said.
Two new Omicron subvariants - BA.4 and BA.5 - are driving a case surge in South Africa but, with just 2-3 per cent of PCR samples genomically sequenced in the ACT, the level of their spread within the capital is unclear.
ACT Health said it was unable to estimate the number of reinfections cases in the ACT. "Although evidence shows that reinfection occurs in Australia and around the world ... international data cannot be easily applied to the ACT," a spokesperson said.
Viruses typically become more transmissible but less lethal over time, and Omicron has supplanted the more deadly Delta strain as the most dominant in circulation.
And with second cases usually milder than an initial bout, Professor Esterman hoped the Christmas surge would help "dampen down" the impact on the hospital system, even as reinfections climb.
"We've got these more transmissible new subvariants coming along, which is not a good thing," he said.
"But at the same time, we're getting almost like herd immunity, where we have so many people infected now that it's more difficult for the virus to find people to go and attack.
"Obviously, with so many people infected already, those that it does attack will almost certainly have already had [COVID-19]."
'Herd protection'
Australian National University infectious diseases expert Peter Collignon was surprised by the level of circulation since Christmas.
But he drew a distinction between the current situation and herd immunity, which he said implied limited transmission.
"What we've got is herd protection. I think we're going to have ongoing transmission for years to come, but the effect will be markedly less," he said.
But he warned the true picture, including the reinfection rate and its long-term health impact, may not be known for a long time.
"[It's] still a presumption, and I'd like to see data ... [but] I think it will be difficult to do in Australia," he said.
Much of the protection was built up by young people, whose mobility saw them contract Omicron at higher rates over Christmas and New Year.
But amid concerns over an imminent winter surge, Professor Collignon said the "perverse downside" is that older people were more likely to bear the brunt of the next wave.
"Particularly people over the age of 70 need to be careful, and keep on taking the same sort of precautions they've been taking," he said.