A preliminary study, posted online this week by researchers at the Australian National University and elsewhere, estimates 71,000 Australians had COVID-19 by mid-July — 60,000 more than the official number of cases diagnosed by that stage.

The study involved testing 2991 elective surgery patients in 10 hospitals across four states to see whether they had antibodies against SARS-CoV-2, the virus that causes COVID-19.

The study initially found 41 positive patients (1.4%) but then adjusted for the false positives that would arise due to the imperfect specificity of the antibody test, which the researchers estimated would produce 11 false positives for every 1000 tests.

This yielded an estimated prevalence of 0.28% — or eight ‘true’ positives from the 2991 people sampled.