All state and territory governments in Australia have cut the so-called COVID immune period from 12 weeks to 28 days, meaning you are at risk of catching COVID just one month after recovering.
Driving that change is Omicron subvariants BA.4 and BA.5, which are causing more people to get reinfected with COVID.
But if you test positive soon after having COVID, how do you know if you are shedding or if you have caught the virus again?
Reinfection window shortened
Until recently, advice from the national Department of Health said Australians should only get tested for COVID-19, 12 weeks after completing their previous isolation, even if new symptoms showed or they were in close contact with a positive case.
However, Chief Medical Officer Paul Kelly said on Tuesday that advice was changing.
"So, it's important that anyone who develops symptoms again 28 days or more after they have had a previous COVID-19 infection to get tested and take the appropriate arrangements again in terms of isolation."
How long can you shed the virus for?
University of Sydney epidemiologist Associate Professor Meru Sheel said the virus could still be present inside your body past the infectious period.
"You might have bits of the virus sitting in your lungs and that can be detected when you do a PCR," she said.
Due to viral shedding, some people can continue to test positive weeks or even months after their infection.
The chair of epidemiology at Deakin University, Professor Catherine Bennett, said because of this it was harder to know if a positive PCR test was a new infection or viral shedding.
How do you know if it is round two?
Professor Bennett said positive rapid antigen tests were a good indication of a new infection.
"While they may miss a few more infections [because] they're not as sensitive, they're able, when they do pick up an infection, to pick up an active infection," she said.
Dr Sheel agreed that it was unlikely a person would test positive on a RAT if they were shedding.
Professor Bennett said it would be possible to tell if the infection was a new one on a PCR test if genomic testing was done, but that was not done on all results anymore because of how many cases there were.
But she said the risk of long-term shedding was reasonably low, particularly if someone had a mild infection, so if a person tested positive again and had symptoms it was a good indication of a new infection.
"If you did test positive on a PCR test, then that would really raise your suspicion that it could be both COVID infection and a new infection," Professor Bennett said.
Why is the period getting shorter?
Dr Sheel said the change reflected the current epidemiology, given more reinfections were happening sooner than 12 weeks.
Professor Bennett said the decision to cut the so-called immune period down to 28 days was about trying to manage risk.
She said if you had a positive COVID case in your household and if you got symptoms, your risk of having COVID was "very high" even if you tested negative on a RAT.
"Now sadly that's true even for someone four weeks after their own infection," she said.
Why was it 12 weeks beforehand?
Professor Bennett said it was previously 12 weeks because a Delta infection boosted your immunity against Delta.
She said even earlier, the immune period was six months, but it has all changed with Omicron.
"Omicron, strangely, when you have an infection, doesn't protect you as well from another infection from Omicron," she said.
"That's partly because there's so much diversity in the Omicron virus."
But she said new subvariants, including BA.4 and BA.5, were also causing more reinfections.
Woman reinfected for the third time
For the third time since March, Samantha Eccles is stuck at home with COVID-19 and she has some frank advice for anyone who thinks they cannot get quickly reinfected.
"Don't assume it's not COVID," she said.
"Even if someone tells you you're probably just shedding virus, go and check — do a couple of RATs.
"If you're in doubt, go and get a PCR test."
The 52-year-old Adelaide high school teacher caught her most recent infection just four weeks after her second, which was 13 weeks after the first.
"I'm sure I'm not the only person who's had it three times in such a short time period," Ms Eccles said.
When she went to get a PCR test for the third infection, she was told COVID-19 could stay in the system for some time, but she told staff her symptoms were new and different, so they tested her again.
She ended up going to hospital with chest and back pain, and a temperature above 39 degrees Celsius.
"One of the doctors was telling me it's very unlikely you've got COVID because it's only been four weeks, but my PCR came positive while I was in hospital," she said.
