Bergamo, the epicentre of Italy's COVID-19 outbreak, made the estimates based on data from a website where citizens log their health status.
More than a third of people in one of Italy's hardest-hit provinces have had coronavirus over the last two months, a new report estimates.
Authorities in Bergamo, Lombardy, made the estimation based on analysis of a COVID-19 tracking app released earlier this month.
The app allows locals to log and daily update their health status, giving authorities precious data to monitor the pandemic.
A report, based on data from April 12-20, found that 35 per cent of app users have or have had COVID-19.
The app only had data from 4 per cent of the province's 1.1 million inhabitants.
But it was extrapolated to estimate that 381,000 people in the region have been hit by COVID-19, of which 62,000 are still projected to have symptoms.
Having consulted virologists as part of the report, an estimated 10% of the population could be asymptomatic, leading to an estimated 45% of the population that could be affected by the virus, or nearly half a million people.
The report, however, outlines a key difficulty ahead, with the need for a larger sample size to provide increased accuracy. The municipality has launched a fresh appeal, urging for those who not only have experienced symptoms but those who have not, to register on the app, and log the status of their health.
In a statement to Euronews, the mayor's office said that they had eliminated clearly inconsistent answers from the database, for example, if a respondent declares that they had a fever but also had a temperature of 36 degrees.
They did acknowledge that the methodology may both overestimate and underestimate. If Bergamo citizens are seriously ill, for example, they are unlikely to fill out the forms, leading to a drop in figures. There is also no way to verify that the symptoms logged into the website are indeed confirmed cases of the virus which can lead to an apparent rise.
They did say that "response bias" is always present in questionnaires such as this and that it is limited due to checks or other methods.
They also said that the quota of symptoms is in line with other approaches, such as other demographic surveys and the Lombardy Region app.
Bergamo is the epicentre of Lombardy's COVID-19 outbreak, with 6,000 COVID-19 deaths, according to the report.
Lombardy is the worst-hit region in Italy with more than 13,000 fatalities.