Almost one in every two Australians has had COVID-19

Almost one in two Australians had COVID-19 in the six months leading up to June, the results of a new blood survey indicate – and that figure is likely to be a significant understatement.
 
Researchers at UNSW’s Kirby Institute at UNSW Sydney and the National Centre for Immunisation Research and Surveillance (NCIRS) have released the results of their most recent study into the prevalence of SARS-CoV-2 antibodies in the wider community.
 
The research involved analysing 5139 de-identified blood donor specimens from around Australia from donations given from 9–18 June. Of those 46.2% showed signs of recent infection, up from around 17% the first time the study was conducted from 23 February – 3 March earlier this year.
 
The specimens were assessed for the presence of two types of antibodies: anti-spike antibodies, which indicate exposure to vaccination as well as natural infection, and anti-nucleocapsid protein antibodies, which point towards recent infection, particularly within the past three to six months.
 
Signs of previous infection were highest in NSW (49.8%), followed by Queensland (48.9%), then Victoria (46.5%).
 
While Western Australia had the lowest level of antibodies suggesting recent infection (38%), it showed the largest increase since the previous analysis when less than 0.5% of samples from the state show signs of COVID-19.
 
The study also confirmed the scale of COVID-19 spread in the 18–29 age range, where almost two in three (62%) showed signs of prior infection.
 
Among the older 70–89 cohort, the figure was much lower, with around one in four (25.7%) likely to have had COVID-19 according to the blood samples collected in June.
 
Dr Dorothy Machalek from the Kirby Institute said the survey was carried out to give a much clearer snapshot of the extent of infection.
 
‘While we know there is a lot of virus circulating in the community, we can’t rely on testing data alone to understand how many people have been infected, as some may experience only mild symptoms, or none at all, and they may not always have a test to detect the virus,’ she said.
 
‘There is also high use of rapid antigen tests, which don’t always get reported.’
 
Dr Machalek said the presence of long-lasting antibodies detected in the serosurveys allow a ‘more complete picture’ of how widespread COVID-19 is in the community.
 
However, given the testing took place last month, the figures are already likely to be significantly out of date with the spread of the highly infections BA.5 variant, which was confirmed as the dominant strain in Australia earlier this month.
 
There are possible confounders in the testing process, with people who give blood more likely to be vaccinated than the population as a whole and potentially less likely to be infected.
 
The researchers also suggest their testing could be missing more than one in five previous infections.
 
They reference evidence that anti-nucleocapsid antibodies could wane faster among vaccinated people and so are less likely to be detected.
 
‘Available local data shows that the sensitivity of the Roche assay to detect anti-nucleocapsid antibodies in vaccinated persons with breakthrough Omicron infections is 78%,’ the study reads.
 
‘This means that these seroprevalence estimates may miss approximately 20% of infections.
 
‘As vaccine coverage in Australia is high, measures of anti-nucleocapsid antibodies will underestimate the cumulative SARS-CoV2 attack rate in the population.’
 
The next survey, which involves collaboration with the Australian Red Cross Lifeblood, the Victorian Infectious Diseases Reference Laboratory, NSW Health Pathology and the Murdoch Children’s Research Institute, is due to take place in September this year.
 
It is likely to provide a clearer picture of the spread of the BA.5 variant as well as an indication of the impact of the decision to expand eligibility for a fourth dose of COVID-19 vaccine.
 
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BA.5 COVID-19 Kirby Institute Omicron