Nearly half of all adults in Australia show signs of having had COVID-19 within the past three to six months, according to a new national report.

The findings, released on Wednesday, revealed that by mid-June, 46.2 percent of people aged 18 to 89 had caught the virus in the preceding months, a huge leap since the previous study in late February when about 17 percent of adults appeared to have recently been infected.

The study, overseen by the Kirby Institute at the University of New South Wales and the National Center for Immunisation Research and Surveillance, was done in collaboration with an array of institutions, including Australian Red Cross Lifeblood, the Peter Doherty Institute for Infection and Immunity, and the Institute of Clinical Pathology and Medical Research.

The screening, known as a serosurvey, examined about 5,140 samples from blood donors. 

The researchers were looking for two types of antibodies - one type, the anti-spike antibodies, indicating a person has been vaccinated or has built up some natural immunity to the virus due to community ­exposure, while the other antibody, the anti-nucleocapsid protein, indicating a person has had a recent COVID-19 infection.

The findings revealed that young adults were the hardest hit, with 61.7 percent of 18 to 29-year-olds having evidence of a recent infection, in comparison to 25.7 percent of those aged 70 to 89.

Antibody prevalence was similar across the nation, even in states such as Western Australia, which had avoided high case numbers throughout much of the pandemic.