ROME (Reuters) – A sample survey has shown that more than half the residents of the northern Italian city of Bergamo have COVID-19 antibodies, city health authorities said on Monday.

Of 9,965 residents who had blood tests between April 23 and June 3, 57% of them had antibodies indicating they had come into contact with the coronavirus, the survey showed.

The city health authorities said the sample was “sufficiently broad” to be a reliable indicator of how many people had been infected in Bergamo, which became the epicentre of Italy’s outbreak.

Antibodies were found in just over 30% of the 10,404 health operators tested although they are generally considered more at risk than other people.

In a report released in early May, national statistics institute ISTAT said the number of deaths in Bergamo was up 568% in March compared with the 2015-2019 average, making it Italy’s worst-hit city in terms of deaths.

Its hospitals were soon overwhelmed by infected people and, with morgues unable to keep up, convoys of army trucks carrying away the dead became a chilling symbol of the global pandemic.

Bergamo province has reported 13,609 coronavirus cases, official data showed on Monday.

ISTAT and the health ministry have launched a nationwide blood testing campaign, aiming to test a representative sample of some 150,000 people to understand the extent of Italy’s COVID-19 epidemic.

Italy has reported almost 34,000 coronavirus deaths, with some 235,000 confirmed cases. The Lombardy region, which includes Bergamo, has registered over 16,000 deaths.