A new coronavirus case in international quarantine in Darwin this week shows the virus is still able to overcome hurdles designed to keep COVID-19 out of the country.
The 41-year-old man was diagnosed in the Howard Springs facility earlier in the week, becoming the first confirmed case among Australians repatriated from India since flights from the sub-continent resumed at the weekend.
And it is unique because all passengers were subject to a stricter testing regime that ultimately saw about 70 passengers barred from boarding.
To understand how the case flew under the radar, epidemiologists say it is important to consider two key factors: how long it takes for new infections to become detectable, and the uncomfortable fact that no testing and quarantine systems are completely foolproof.
How did two rounds of testing miss the case?
All passengers on the India flight were subject to new pre-flight testing and isolation measures unveiled when the federal government announced it would lift its temporary ban on travel from India.
NT Health has confirmed the man received negative results on the two rounds of pre-flight testing — a PCR test required up to 72 hours before departure, and a rapid antigen test conducted before boarding.
Deakin University epidemiologist Catherine Bennett said that could be because the virus generally takes several days from the point of exposure to establish itself at detectable levels.
"So if you were exposed at the airport, say, and that is a high risk in a country where there's a lot of people with the virus, then it's going to take you a couple of days to actually have enough virus in your throat to test positive," she said.
"There's always going to be that window between someone testing negative and potentially becoming positive that you can't test for, because the test isn't telling you the information you need.
LaTrobe University epidemiologist Hassan Vally said finding out precisely when and where the 41-year-old was exposed to the virus would be a "more challenging question to answer".
"Unless you sort of forensically go through the movements of that person in isolation, you won't know the answer to that," he said.
An NT Health spokeswoman said the man went on to return a positive result following routine testing at the Howard Springs facility on Sunday.
Supplied: Department of Defence
)Were health authorities expecting more cases from India?
The federal government's travel ban was implemented shortly after a spike in active cases at the Howard Springs facility, mainly among Australians repatriated from India.
It is important to note that the Northern Territory health authorities never said they expected the testing measures to stamp out incoming cases altogether once flights resumed.
Instead, they have set a target cap of 50 active COVID cases, before suspending flights again, to help keep infections at a manageable level.
Both epidemiologists the ABC spoke with said they were not surprised by the new diagnosis, despite the strengthened pre-departure testing regime.
"You've got to understand that even though we have some really rigorous processes and pre-flight testing, the whole reason we have quarantine is that they're not perfect," Dr Vally said.
Professor Bennett said the diagnosis also showed how prevalent the virus was in India.
"There is going to be an ongoing risk that people who've managed to stay safe might, in fact, in the very process of getting to the airport and getting on the plane, get that infecting dose and bring that with them on the plane."
Supplied
)Does this mean other passengers are at greater risk?
According to the experts, the fact that the 41-year-old didn't test positive for the virus prior to boarding the plane suggests he had a low viral load during the flight.
"You can only assume they don't have very much virus in their system at that point, so the risk they pose to others is less than someone who's actually tested positive," Dr Vally said.
"But at the same time, it's a reminder that you've got to assume that every person who boards a flight — even when they've tested negative — is potentially carrying the virus."
He said further cases from the same flight were possible.
"We have people wearing masks, there are very strict protocols in place to try to minimise risk, but that is the risk you take getting on the plane — that you will have someone who is positive on that plane," Professor Bennett said.
"The planes are very good, though — we haven't seen high records of transmission in recent times."
The NT Health spokeswoman said the man had no close contacts and remained in isolation.
The next repatriation flight from New Delhi is expected to touch down in Darwin within the next week.
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2021-05-18 19:09:38Z
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