Official federal government modelling released in April said Australia’s testing program was catching 93 per cent of all symptomatic cases. However, the new study suggests 85 to 90 per cent of all cases were not being picked up.
The study has significant implications. If accurate, it means Australia has endured a far-higher number of infections than previously thought, without overwhelming the hospital system.
“We have had more infection than we thought. And hospitals have coped extraordinarily well,” said Professor Elizabeth Gardiner, head of the Australian National University’s cancer department and a co-author of the paper.
That put into question whether stricter lockdowns, such as Melbourne’s current stage four restrictions, were necessary, she said.
“Should we remain at that level of lockdown? No. I don’t think the numbers justify it.”
But the study also has significant limitations.
Its sample size is too small to make a confident estimate about the true level of infection around Australia: only 2991 people were tested, about two-thirds of them from Victoria and NSW. Of those 2991, 41 had antibodies to COVID-19.
Professor Brendan Crabb, an antibody-testing expert and director of the Burnet Institute who was not involved in the study, said the estimate of 71,400 infections was “on the high side of likelihood, but not out of the realms of possibility”.
The antibody test used was developed by a team of researchers at the Australian National University and has a false positive rate of 1.09 per cent.
“There is still much margin for error here,” Professor Crabb said.
The study’s lead author, Associate Professor Ian Cockburn, said it would “not be too surprising” that many infections had not been picked up.
In the early stages of Australia’s pandemic, many people were turned away from testing stations; it is estimated about 40 per cent of people with the coronavirus do not show any symptoms.
The low number of overall infections “tells you the restrictions work”, Professor Cockburn said. If Australia’s restrictions had been less strict, there would have been more confirmed cases but more unconfirmed cases too, stressing healthcare systems, he said.
Professor Kristine Macartney, director of the National Centre for Immunisation Research and Surveillance, said coronavirus cases were probably missed in the first weeks and months of the pandemic in Australia when there were greater restrictions on who could access testing.
“What we probably now appreciate is that the virus was infecting people who may not have been eligible for testing at that time,” she said. “It does indicate that Australia … has kept control of the spread of the virus.”
Other countries that have conducted similar antibody surveys, such as Iceland, also discovered their testing programs were missing many infections.
A separate antibody survey in NSW, being run by the Kirby Institute, is expected to release results later this week. Based on early data, NSW Chief Health Officer Kerry Chant estimated in July the true prevalence of coronavirus cases was between 1 and 2 per cent of the state’s population.
Infection control expert Professor Mary-Louise McLaws said most other countries had reported rates of coronavirus infections in their communities of between 1 and 10 per cent of the population much higher than the 0.28 per cent rate reported in the Australian survey.
Professor McLaws said while it was encouraging to see these low numbers of infection relative to the rest of the world, it would be great to see more testing.
“I would like to see both NSW and Victoria opening up their testing to anyone who wished to be tested, while at the same time prioritising people who had a contact or were feeling unwell,” she said.
The new study was uploaded to medical research website medRxiv on Tuesday but has not yet been peer reviewed or published in an academic journal.
Study volunteers were drawn from people entering hospital for elective surgery between June 2 and July 17.
Standard PCR testing can only tell if a person is infected with COVID-19 at the time of the test.
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Liam is The Age and Sydney Morning Herald’s science reporter
Aisha Dow reports on health for The Age and is a former city reporter.