Number of Australians who have had COVID-19 jumps in June survey of blood samples

At least 46 per cent of Australian adults had been infected with COVID-19 by early June, according to results from Australia’s most recent blood testing of relevant antibodies. 

Researchers have conducted their second “serosurvey”, testing blood donations for the presence of antibodies to SARS-CoV-2, to find the prevalence of infection had almost tripled since late February when it was estimated to be 17 per cent. 

Kirby Institute senior research fellow Dorothy Machalek said while the results revealed there was a “huge amount” of community transmission, it was not a surprise. 

“The international borders opened in April, we’ve had a further loosening of restrictions and for many people it’s been back to business as usual with mask-wearing reducing until now,” Dr Machalek said. 

“We’ve also gone into winter so the behaviour of people has changed— it’s gotten colder so we’re inside more. 

Low testing rates, asymptomatic cases and rapid antigen tests taken at home mean cases are likely to be under-reported to health authorities. 

Dr Machalek said these blood tests, while not a perfect representation of the population, provided a more complete picture of how many infections there were in the community. 

A woman takes blood from a man lying back in a chair.
Surveys testing for the prevalance of COVID-19 among the Australian population used residual blood donation samples from Lifeblood’s processing centres.(Supplied: Lifeblood)

Infections highest among young adults

Evidence of past infection was highest among donors in the 18-29-year age group, at 61.7 per cent, declining with increasing age to 25.7 per cent in donors aged 70-89 years across Victoria, NSW, Queensland and Western Australia.

In Western Australia, evidence of past infection in blood donors jumped from 0.5 per cent to 37.5 per cent over a three-month period.

The researchers examined 5,139 de-identified samples from Australian blood donors aged 18–89 years for evidence of COVID-19-related antibodies.

Two types of antibody to SARS-CoV-2 were tested for, with an antibody to the nucleocapsid protein of the virus providing an indication of infection in the past few months, while an antibody to the spike protein indicating past infection and/or vaccination.

The blood samples were collected from June 9 to 18 this year.

Dr Machalek said there were a number of limitations, and the surveys and tests weren’t 100 per cent sensitive.

“We know that we’re probably missing maybe up to 20 per cent of infections,” she said. 

Cases to rise further

The next round of the Australian Red Cross Lifeblood donor survey will begin around the end of August.

Dr Machalek expects to see a further rise of infection in the community then, with new Omicron variants circulating. 

“I suspect at some point those antibody levels will stabilise and they do wane [over time] as well,” she said.  

“We’re mindful that just because there is so much infection in the community it doesn’t mean that people are immune. 

“It’s just important to get your booster when you’re due or if you’re eligible, and also wear masks in settings where you may be at risk of getting sick.”

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