“Studies in the US and UK show the rate of antibodies is very steadily climbing. We are certainly under that proportion, but we suspect that, over time, as the virus continues to circulate, and we have limited restrictions we will reach those levels,” she said.
Ninety-eight per cent of samples surveyed had the types of antibodies gained from prior infection or vaccination.
The survey is the fifth serology survey conducted by the NCIRS. By testing blood samples for antibodies indicating a previous infection, researchers hope to gain a better understanding of how the virus has moved through the community.
“But we expect we will never capture the full extent of the virus in the community,” Macartney said.
More than 27 per cent those of aged 18 to 29 had antibodies from a prior infection, the highest rate of any age group.
Associate Professor Paul Griffin, an infectious diseases expert at the University of Queensland, said the undetected infections in his state were not a surprise.
In late 2021, Queensland reported a series of small coronavirus clusters and mystery singular cases, however none of the spot fires resulted in outbreaks the size seen in Victoria and NSW.
“We knew we were only catching a fraction of the cases [in Queensland],” he said, adding that the state had a high test positivity rate over its Omicron wave, struggled with PCR testing capacity and, like other states, been slow to provide a way for the public to report rapid antigen test results.
“We have been suspicious about the reported figures, assuming the rate was likely multiple times what had been reported.”
Data from NSW has shown the new BA.5 sub-variant of Omicron is overtaking BA.2 as the most prevalent variant in the state. International data indicates the newer sub-variant evades existing immune responses from prior infection and vaccination better than its predecessors.
The NCIRS will release the results of a study of paediatric COVID-19 infections, using data from children who had elective surgery, in the coming weeks.
Kaldor said the next quarter’s serosurvey will estimate the prevalence of antibodies following the spread of BA.2 and other Omicron sub-variants.
“It is likely to be a more complicated picture because the serology at the moment doesn’t distinguish between infections in different waves.
“We hope to develop more sophisticated serology methods that can identify recent infections more clearly.”
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