Almost 50% of Australians had been infected with Covid as of mid-June, blood survey shows

Almost half of all Australian adults had been infected with the Covid-19 virus as of June this year, according to a national report.

The finding comes from a serosurvey of antibodies to the virus detected in blood donations, conducted at the Kirby Institute and the National Centre for Immunisation Research and Surveillance (NCIRS).

Researchers examined 5,139 blood donations received from adults between 9 June and 18 June for anti-nucleocapsid protein antibodies, which indicate a recent infection. They also looked for anti-spike protein antibodies, which indicate immunisation.

Evidence of past infection was detected in 46.2% of samples. By comparison, the previous serosurvey estimated just 17% of Australians had been recently infected with the virus by late February. Compared with the previous survey, more than a quarter of the population was infected in the three months leading up to June. Infection rates were similar across all jurisdictions.

It comes as Australia records its highest level of Covid-related hospitalisations since the pandemic began.

Noni Winkler, an author of the findings and an epidemiologist at the NCIRS, said the sample size was large enough to reflect rates of the virus in the broader adult population.

It was likely to be an underestimate of infections, she said, since children were not included in the samples and because seroprevalence estimates may miss approximately 20% of infections.

The highest positivity rate was in the 18 to 29-year-old age group, at 62%, and the rate of infection declined with age.

In the 70-89 age group, the cohort with the highest number of deaths, prior infection was detected in just 25.7% of blood samples. The findings apply to Victoria, New South Wales, Queensland and Western Australia, the four jurisdictions with sample sizes large enough to calculate age-specific rates.

The study showed similar levels of prior infection across all states and territories, including Western Australia, which saw an increase from 0.5% to 37.5% in the three-month period since the last survey.

Winkler said those donors with antibodies due to infection had likely caught the virus since January.

The study could not distinguish repeat infections.

Director of NCIRS and investigator in the serosurvey program, Prof Kristine Macartney, said the best protection against serious illness from Covid is vaccinations and boosters, in line with current recommendations.

The next serosurvey will be conducted around the end of August.