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 bepeople 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.
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“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 sub-variants BA.4 and BA.5 are better at escaping immunity than earlier variants,” she said.
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“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 theAustralian National Disease Surveillance Plan for COVID-19 which 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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