Almost half of all Australian adults had been infected with the Covid-19 virus between January and June this year, with many of those infected in the last three months, 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.
Meanwhile, anti-spike antibodies were found in 99% of samples, consistent with Australia’s high vaccination rate.
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.
“Serosurveys give us a more accurate idea of the extent of infection and antibody prevalence in the community than cases reported to jurisdictions,” she said.
“Case notifications are always going to underestimate true infection rates because some cases are asymptomatic, some cases don’t get tested, and some cases don’t get reported. Having a more accurate picture of infection rates in the community can help inform and evaluate public health responses.”
Australia’s chief medical officer, Prof Paul Kelly, said data collected from the surveys was “crucial for informing our pandemic response”.
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.
Winkler said those donors with antibodies due to infection had likely caught the virus since January.
“In the case of anti-nucleocapsid protein antibodies, they’re produced at lower levels than other antibodies, and they wane faster in people who are infected after they’ve been vaccinated, compared to people who are unvaccinated who then become infected,” she said. “Because Australia has such high vaccination coverage, it’s likely that the anti-nucleocapsid protein antibodies that we’ve measured are mostly from recent infection, so from this year.”
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.
“We are seeing new variants of the Omicron version of the virus that causes Covid-19 in circulation in Australia, and that can lead to reinfection even in people who have been infected with an earlier variant,” she said.
“Vaccination reduces the risk of severe disease if you get Covid-19, whether it is your first time or a repeat infection”.
The next serosurvey will be conducted around the end of August.
