The survey by Istat and the health ministry, was based on antibody tests conducted on 64,660 people.
Official figures show 248,229 confirmed cases of COVID-19 patients in Italy, with 35,166 deaths.
The survey found marked local differences with the northern region of Lombardy, where the epidemic first broke out in February, showing 7.5% of the population had tested positive for coronavirus antibodies compared to just 0.3% in the southern region of Sicily.
The survey found almost 30% of people with antibodies were asymptomatic, pointing to the risk of the illness being spread by people unaware they were carriers. (Reporting by Emilio Parodi; Writing by James Mackenzie; Editing by Aurora Ellis)
Identical twins can be distinguished by their fingerprints.