New analysis has estimated Australia's biggest COVID-19 wave might have been larger than first thought.
When December's Omicron wave struck, the country went from recording fewer than 1,500 cases per day to more than 100,000, in the space of one month.
Now, epidemiologists think the true number of cases was at least double what was recorded.
So, how could so many COVID-19 cases potentially go undetected, and are we underestimating our current caseload?
What study found
A study by the National Centre for Immunisation Research and Surveillance (NCIRS) and UNSW Sydney's Kirby Institute looked for COVID-19 antibodies in 5,185 samples taken from blood donors between late February and early March this year.
The researchers tested for two types of antibodies in the blood, including one that can only be found in the body after a COVID-19 infection, and not vaccination.
They estimated that, by the end of February 2022, at least 17 per cent of Australian adults — that's around 3.4 million people — had been infected, and that the vast majority occurred during the Omicron wave.
This means the number of people infected would have been at least double what had been officially reported.
For some, that could be a startling thought — but the director of the NCIRS, Kristine Macartney, said the data was not surprising.
Similar studies in other parts of the world have shown as many as five or 10 times the number of infections than officially reported, Professor Macartney said.
"[Antibody studies] give us a greater insight into how much the virus has spread in the community and also what the overall pattern of immunity to the virus looks like in the community," she said.
Queensland and Victoria had the highest proportion of adults with antibodies at 26 and 23 per cent respectively, according to the survey. Western Australia, which was yet to fully reopen, had the lowest at 0.5 per cent.
Adults aged 18 to 29 were more likely to have had an infection, with the researchers finding antibodies in 27 per cent of blood samples. Less than 7 per cent of people aged over 70 had antibodies.
How could so many COVID cases go undetected?
There's a few reasons for that.
Not everyone who gets COVID-19 will get symptoms or get tested, which means their cases may not be added to the official numbers.
Only cases detected by PCR tests, or self-reported positive rapid antigen tests, get included in official tallies.
"Up to around 30 to 40 per cent of infections can be asymptomatic, so people won't have any negative side effects or feelings that indicate to them they might be infected," Professor Macartney said.
"So they won't, logically, then go and get a test in most cases."
There are also other reasons why some people with COVID-19 may not get tested, such as not having easy access to rapid antigen tests or PCR testing.
"Some people who might have mild symptoms don't want to get tested because they might feel, 'Oh, it's a flu', or they might not have a rapid antigen test available. So they might not get tested and they might be missed," added Jaya Dantas, a professor with the Curtin School of Population Health.
At the time of the survey, about 80 per cent of the population was double vaccinated — another element which reduces the likelihood of developing symptoms.
Are we still vastly under-reporting the true number of COVID cases?
Possibly, but if so, it's unclear by exactly how much.
Researchers expect to have a better sense of this in the coming weeks when a second round of blood specimens is collected.
"It could be double. We won't know until we do the testing," Professor Macartney said.
"We've certainly had a continued transmission of the virus across the country and including in new parts of Australia that didn't have a lot of evidence of infection when we did this first study."
While COVID-19 restrictions and precautions have eased, many immunocompromised Australians are still very wary of the virus.
Australia officially reported 13 deaths and more than 22,500 new cases on Monday.
Professor Dantas said Australians should be careful, but not alarmed.
"Now that we have done away with the mask mandates and we are getting into our flu season, I think the next three or four months are testing times for us," she said.
"It's a matter of just being cautious and taking personal responsibility and also being mindful that there are those amongst us that are vulnerable and we need to look after each other in the community."
