First Omicron wave was really twice the size of recorded cases, study says

The number of COVID-19 infections in Australia was likely at least twice what was reported among adults during the first major Omicron wave, with Queensland recording the highest rate of any state.

Data from the National Centre for Immunisation Research and Surveillance (NCIRS) and the Kirby Institute’s latest serology survey of blood donors found that by the end of February, at least 17 per cent – or almost 3. 5 million – Australian adults had caught the virus.

The survey examined 5185 de-identified samples from Australian blood donors aged 18 to 89 to find evidence of COVID-19–related antibodies. Two types of antibody to SARS-CoV-2 were tested: the nucleocapsid protein, which is only present in someone who has been infected with COVID-19, and spike antibodies, which are produced by both natural infection and vaccination.

The highest proportion of adults with antibodies was in Queensland, where 26 per cent were shown to have had a COVID-19 infection by the end of February, despite only 10 per cent of Queenslanders having registered a test.

Twenty-three per cent of Victorian samples and 21 per cent of NSW samples showed antibodies gained by a prior infection. Western Australia was the outlier, with only 0. 5 per cent of samples suggesting a prior case, as the state kept its borders shut until early March.

This is the first in a series of four planned serosurveys throughout the year, and is an important opportunity to better understand the characteristics of the virus and the dynamics of the pandemic, John Kaldor from UNSW’s Kirby Institute said.

We found evidence of recent infection in roughly twice the proportion of the population that had recorded infections in first major Omicron wave. We expect the true number to be even higher because the serology doesn’t detect every case.

Professor Kristine Macartney, director of the NCIRS, said the results were expected, noting Australia had lower rates of antibodies in the community than similar surveys had found abroad.

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 , 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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