Almost 1.5 million people in Italy or 2.5% of the population have developed coronavirus antibodies, a figure six times more than official numbers reported, according to a survey from statistics agency Istat on Monday.
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.
