Almost 20% of Australian adults were infected with SARS-CoV-2 during the first Omicron wave, serology testing suggests — almost double the official case tally.

According to official government figures, some two million Australian adults contracted COVID-19 between December and February as Omicron swept through Australia.

But the scale of the increase also prompted a shift to rapid antigen testing, with most states relying on patients to self-report a positive test via contact-tracing apps.

During the last two weeks of February, researchers tested 5185 blood donations for anti-nucleocapsid protein antibodies to identify how many patients had been infected in the previous 3-6 months regardless of their vaccination status.