The novel coronavirus was spreading in Italy in September last year - months earlier than previously thought, a new study has found.
The shock findings change our understanding of the way Covid-19 spread around the world and raise questions about how long we have been living with the coronavirus.
The study, by the National Cancer Institute of Milan, looked at healthy volunteers who had enrolled in a lung cancer screening trial.
It found that 11.6 per cent of the 959 volunteers, who took part between September 2019 and March 2020, had developed coronavirus antibodies before February of this year.
Officially, Italy recorded its first Covid-19 case on February 21, in a small town near Milan.
Further testing by the University of Siena showing that four cases, which were samples taken in the first week of October, showed coronavirus antibodies.
That would mean the volunteers were infected in September, a co-author of the study has said.
Giovanni Apolone told Reuters: "This is the main finding: people with no symptoms not only were positive after the serological tests but also had antibodies able to kill the virus.
"It means that the new coronavirus can circulate among the population for a long time and with a low rate of lethality, not because it is disappearing, only to surge again."
The World Health Organisation has previously said that the novel coronavirus, SARS-CoV-2, was unknown before an outbreak in the Chinese city of Wuhan in late 2019.
But the WHO told Reuters: "The possibility that the virus may have silently circulated elsewhere cannot be ruled out."
It said it was reviewing the results of the study and was seeking clarification.
Researchers in Italy have previously reported a higher-than-normal rate of severe flu and pneumonia in the Lombardy region in late 2019, a sign that the new coronavirus may not have been as new as once thought.
The number of new coronavirus cases in Italy narrowed on Monday to 27,354, in keeping with weekend dips reflecting lower testing numbers, but the infection rate remained a stubborn 18 per cent.
Italy is still struggling to contain a second surge, with more than half of the country on partial lockdown.
Another 504 people died in the last 24 hours, according to Health Ministry figures, bringing the pandemic total of known deaths to 45,733, second in Europe behind Britain. Hospital admissions rose by nearly 500 while another 70 people were in intensive care.
Nationwide, more than half of the hospital beds were dedicated to Covid care, a level deemed critical by the government. The rates were highest in the north, where the health care system is generally more advanced.
- Additional reporting, AP