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WA’s recent weak positive COVID case has infectious disease experts questioning whether the fly-in, fly-out worker was a false-positive or a very mild case unlikely capable of spreading to others.
The Greenwood man, aged in his 30s, returned two weak-positive rapid antigen tests on Monday as part of routine work-related testing.
He then had a PCR test, considered the best diagnosis tool for COVID-19, which returned a negative result.
The man had already recovered from coronavirus after contracting it overseas in early 2020, so has some immunity to contracting the disease.
On July 20, he attended Perth Airport at the same time as a Queensland man who would six days later test positive for the Alpha variant in Brisbane. In the only interaction between the pair discovered on CCTV, the Queensland man, who was wearing a mask, walked past the FIFO worker who had his back turned to him at the time.
The Queensland man has not yet been confirmed to have infected anyone, including those on his flight or the 62 people he then spent the following days residing with at a Brisbane backpackers.
Despite the epidemiology suggesting the Queensland man was not actively spreading the virus, WA Premier Mark McGowan said health authorities had analysed the unusual situation and determined there was a “serious prospect” the FIFO man was recently positive.
“It’s a very, very confusing and odd case because of all the circumstances involved,” he said.
“He returned positives and negatives so we were advised on [Wednesday] morning about the issue, and the chances were, even though his final test was negative, the chance was some time after the 20th he may have been positive so we’ve taken all precautions we can with the facts we have.”
Deakin University chair in epidemiology Catherine Bennett said it was rare but possible to miss a weak-positive case on a PCR test but pick it up on a rapid antigen test. But the circumstances of this case made it less likely the man was ever infected.
“He could have basically been fighting off an infection, so he had an infection but he was winning and so didn’t have symptoms and didn’t have a high enough viral load to even be detected in the PCR test,” she said.
“It would seem odd, it wouldn’t fit with the story though that the only crossover with this positive Queensland case was a fleeting meeting at the airport.
“It’s a very low probability that [the FIFO worker] is a case if that’s the only way he could have acquired it, the testing doesn’t rule it out though, it could be the case. And even if he is positive, he’s unlikely to be anyone who was very infectious.”
Peter Collignon, an infectious disease physician and professor at the Australian National University, said it was most likely the FIFO worker was a false-positive, explaining PCR testing is more sensitive than a rapid antigen test, which is often used as a screening tool, rather than a diagnostic tool.
“You can get infected twice, there’s data that shows that but it’s just very uncommon,” he said.
“The PCR test is really the gold standard, not the antigen tests, you get false-positives with antigen tests and also a lot of false-negatives. If he’s had two antigen tests and then a PCR test that’s negative, I would wonder whether the antigen test has given a false result.
“Nobody’s ever suggested antigen tests are more specific than the PCR test, the only advantage is they’re more rapid, but that’s at a price of less accuracy.”
Professor Collignon said there were tests which could help differentiate a recent infection from an old infection.
University of South Australia epidemiologist Adrian Esterman said it was unlikely the FIFO worker could have contracted the virus during fleeting contact at the airport with a case who has not spread it to anyone else.
“We know that 10 per cent of people cause 80 per cent of infections so that means the majority of people who are infectious infect no one or pass it on to one or two other people at most,” he said.
Professor Bennett said whether or not the man was a true case would likely come down to an academic argument.
“Whether he was a case or not, it almost becomes academic because they might go to all the trouble and prove he was a case, but if he didn’t infect anyone and wasn’t really capable of it based on his test results anyway, then it doesn’t really matter whether he’s a very low level infection or no infection – both can’t be ruled out, except if the only way he could have acquired the virus was through that very brief exchange at the airport, that makes it less likely,” she said.
In an ASX statement, FMG said it understood the man was not believed to have been infectious during the week he spent at Cloudbreak mine from July 20.
“As at 12pm, 5 August 2021, over 1,200 PCR tests and 850 rapid antigen tests have been completed across Fortescue’s operational sites,” the statement said.
“Everyone who has had a rapid antigen test has also been PCR tested. All rapid antigen tests have returned negative results and results are pending for the PCR tests, which are processed in Perth.”
The worker’s partner, housemate and three close work friends have so far all tested negative to COVID-19 and will be required to isolate for 14 days.
In total 76 close contacts, mostly workers on FMG’s Cloudbreak mine site, have been asked to get tested and isolate for 14 days regardless of the result.
A further 1554 casual contacts must get tested and isolate until they return a negative result.
WA Health did not respond to WAtoday’s request for details of the testing carried out on the FIFO worker, or whether any serology testing had been completed.