More than half the population of the northern city of Bergamo have Covid-19 antibodies, Deuutsche Welle reports citing a new study.

Health authorities tested a sample of 9,965 residents for a blood test between April 23 and June 3. Some 57% of them had antibodies, which indicates they came into contact with the disease.

Authorities said the test was based on a random sample and sufficiently broad to draw these conclusions, however, they later said that most of those tested were residents of the worst-hit areas and many had already been placed under quarantine.

A test of 10,404 health workers — generally considered to be higher risk — found 30% of them had tested positive.

Bergamo was the worst-hit city in Italy, with deaths increasing 568% over the average for March. The province reported 13,609 cases.