More than half of people in the epicentre of Italy's coronavirus outbreak have been infected, and the region has the highest rate of antibodies yet discovered, a study has found.
The city of Bergamo and its surrounding province were hit hard after the virus was first detected in Italy in late February, but it is believed to have been present since at least January.
Blood tests carried out on more than 20,000 people in the area between April 23 and June 3 revealed that 57 per cent had Covid-19 antibodies.
Among medical staff, it was 30 per cent.
The study suggests that the area may be approaching coronavirus herd immunity levels - experts suggest that may be around 60-65 per cent.
The theory is that if a certain percentage of the population is immune to an infectious disease, it will stop spreading.
In contrast, testing has shown that around 17 per cent of London's population now has Covid-19 antibodies, while the figure for the rest of the UK is 5 per cent.
Studies suggest that people who have been infected with Covid-19 gain some degree of immunity, although it is not clear for how long.
Bergamo is in the northern region of Lombardy, where more than 16,000 people have died from the virus - nearly half of Italy's 34,000 fatalities. Italy still has 35,000 people infected.
Bergamo became a chilling symbol of the nation's Covid-19 battle when army trucks had to be drafted in to remove more than 60 bodies in coffins from the city's overwhelmed cemetery.
In May, Italy's national statistics agency said Bergamo's overall number of deaths in March was up 568 per cent on previous years, making it by far the worst affected part of Italy.
Population density and atmospheric pollution have been posited for the high infection and death rates in Lombardy, as well as other factors, such as widespread manufacturing, with many businesses having commercial links with China, where the virus originated.
Despite Italy declaring a strict national lockdown in early March, mobile phone data suggests up to 40 per cent of people kept going out and about.
Lombardy is still discovering hundreds of new cases a week, but the virus is becoming less aggressive.
"Something has happened in terms of the aggressiveness," said Sergio Harari, an expert in pneumology from San Giuseppe Hospital in Milan, the capital of Lombardy.
"We don't know whether it's something in the viral load or whether it's a mutation."
Several of Italy's 20 regions are recording no new cases, including Sardinia, Umbria, Valle d'Aosta, Molise, Abruzzo, Calabria and Basilicata.
Italy has begun rolling out a contact-tracing app for users in four regions - Puglia, Abruzzo, Liguria and Marche - to find out whether they have come into contact with someone infected with Covid-19. Those who have would undergo tests and self-quarantine.