The coronavirus was already present in two large cities in northern Italy in December, over two months before the first case was detected, a national health institute study of waste water has found.
Researchers discovered genetic traces of Sars-CoV-2 - as the virus is officially known - in samples of waste water collected in Milan and Turin at the end of last year, and Bologna in January, the ISS institute said in a statement.
Italy's first known native case was discovered mid-February. The results "help to understand the start of the circulation of the virus in Italy," the ISS said.
They also "confirm the by-now consolidated international evidence" as to the strategic function of sewer samples as an early detection tool, it added.