Nearly half of all adults in Australia had caught COVID-19 by early June and more than a quarter of the population was infected with the virus within the previous three months alone.
Latest results from a national survey detecting COVID-19 antibodies in blood donors found 46.2 per cent of the adult population – an estimated 9.5 million people – was infected with SARS-CoV-2 before the latest Omicron wave swelled.
A man donates plasma at the Lifeblood centre at Sydney’s Town Hall. Credit:Louie Douvis
The serology survey, conducted by the National Centre for Immunisation Research and Surveillance (NCIRS) and the Kirby Institute, found the prevalence of COVID-19 in the community had tripled since February when 17 per cent (almost 3.5 million people) had been infected.
The research team tested 5139 de-identified samples from Australian blood donors aged 18 to 89 and determined the prevalence of COVID-19 infections by detecting the presence of the nucleocapsid protein – an antibody present only in people infected with the virus, rather than antibodies produced by vaccines available in Australia.
Kirby Institute senior research fellow and epidemiologist Dr Dorothy Machalek said this rise in infections had been driven by Omicron BA.2 subvariants, which constituted 95 per cent of the spread by mid-April, up from 35 per cent in mid-February.
“This was also the blissful three months following the initiation peak of Omicron and this new wave where people relaxed. Restrictions eased, international borders reopened and people weren’t wearing masks,” Machalek said.
On Tuesday, a record 5571 COVID-19 cases were being treated in Australia’s hospitals, including 165 people in intensive care. An additional 46,007 cases and 100 deaths were also reported.
The survey published on Wednesday found younger people were most likely to be infected, and prevalence decreased in older age groups. Almost one-third of 18- to 29-year-olds had COVID-19 antibodies (61.7 per cent), compared with 25.7 per cent of 70- to 89 year-olds.
NSW had the highest prevalence of nucleocapsid protein antibodies (49.8 per cent), followed by Queensland (48.9 per cent) and Victoria (46.5 per cent). Western Australia had the lowest antibody prevalence (37.5 per cent), up from 0.5 per cent in late February.
The prevalence of the virus detected in the blood samples was two times higher than the number of COVID-19 case notifications from PCR and rapid tests in the two weeks before the blood samples were collected.
Machalek said the true prevalence of COVID-19 infections in the community was even higher, partly because the test used to detect the COVID-related antibodies picks up only 78 to 80 per cent, meaning more than 20 per cent could be missed.
“We also know that antibodies wane over time, so there might be people who were infected in 2020 or 2021 we’re not picking up,” she said.
Professor James McCaw, infectious disease epidemiologist and member of the Australian Health Protection Principal Committee said COVID-19 prevalence would probably rise again when the next serosurvey – due to be conducted in early August – picks up people infected during the current BA.4 and BA.5 Omicron wave.
“I don’t expect we’ll get to 100 per cent [infection prevalence], but it depends on how big this BA.4/BA.5 wave is,” McCaw said.
The blood samples were collected from Australian Red Cross Lifeblood donor centres between June 9 and 18 and were also tested for COVID-related spike antibodies, which are produced by both natural infection and vaccination. Ninety-nine per cent had these antibodies, reflecting Australia’s high vaccination rate.
“The message is not ‘everyone is going to get COVID so what’s the point of getting vaccinated’,” Machalek said.
“Vaccination is particularly important when we know the newer subvariants BA.4 and BA.5 are better at escaping immunity than earlier variants,” she said.
“Getting our boosters when eligible, following public health advice about wearing a mask where appropriate is still really important to get us through the next few months, especially to reduce the strain on our hospitals, nurses and doctors who have worked tirelessly over the last two years with no break,” she said.
Chief Medical Officer Professor Paul Kelly said the survey results were crucial for informing Australia’s pandemic response.
The surveys are part of the surveillance strategy outlined in the Australian National Disease Surveillance Plan for COVID-19 that includes case and mortality reporting, hospital surveillance, genomic sequencing and monitoring behaviour such as compliance with COVID-19 restrictions and public health recommendations.
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